The Astrophysical Journal, 885:106 (24pp), 2019 November 10 https://doi.org/10.3847/1538-4357/ab43c2 © 2019. The American Astronomical Society. Characterizing Magnetic Field Morphologies in Three Serpens Protostellar Cores with ALMA Valentin J. M. Le Gouellec1,2 , Charles L. H. Hull3,4,14 , Anaëlle J. Maury2,5, Josep M. Girart6,7 , Łukasz Tychoniec8 , Lars1 E. Kristensen9, Zhi-Yun Li10, Fabien Louvet11, Paulo C. Cortes4,12 , and Ramprasad Rao13 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile; Valentin.LeGouellec@eso.org 2 AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette, France 3 National Astronomical Observatory of Japan, NAOJ Chile, Alonso de Córdova 3788, Office 61B, 7630422, Vitacura, Santiago, Chile 4 Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile 5 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA 6 Institut de Ciències de l’Espai (ICE-CSIC), Campus UAB, Carrer de Can Magrans S/N, E-08193 Cerdanyola del Vallès, Catalonia, Spain 7 Institut d’Estudis Espacials de Catalunya, E-08030 Barcelona, Catalonia, Spain 8 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands 9 Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark 10 Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA 11 Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile 12 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 13 Submillimeter Array, Academia Sinica Institute of Astronomy and Astrophysics, 645 N. A’ohoku Place, Hilo, HI 96720, USA Received 2019 May 16; revised 2019 August 27; accepted 2019 August 29; published 2019 November 5 Abstract With the aim of characterizing the dynamical processes involved in the formation of young protostars, we present high- angular-resolution ALMA dust polarization observations of the Class 0 protostellar cores SerpensSMM1, Emb8(N), and Emb8. With spatial resolutions ranging from 150 to 40 au at 870 μm, we find unexpectedly high values of the polarization fraction along the outflow cavity walls in Serpens Emb 8(N). We use 3 mm and 1 mm molecular tracers to investigate outflow and dense-gas properties and their correlation with the polarization. These observations allow us to investigate the physical processes involved in the radiative alignment torques (RATs) acting on dust grains along the outflow cavity walls, which experience irradiation from accretion processes and outflow shocks. The inner core of SMM1-a presents a polarization pattern with a poloidal magnetic field at the bases of the two lobes of the bipolar outflow. To the south of SMM1-a we see two polarized filaments, one of which seems to trace the redshifted outflow cavity wall. The other may be an accretion streamer of material infalling onto the central protostar. We propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present at <500 au scales in Class 0 envelopes have grown larger than the 0.1 μm size of dust grains in the interstellar medium. Our observations allow us to constrain the magnetic field morphologies of star- forming sources within the central cores, along the outflow cavity walls, and in possible accretion streamers. Key words: ISM: jets and outflows – ISM: magnetic fields – polarization – radiation mechanisms: thermal – stars: protostars Supporting material: data behind figures 1. Introduction which impact protostellar disk formation (Wurster & Li 2018), accretion and ejection processes, effects of turbulence (Offner & Protostellar cores are forming within the densest parts of Chaban 2017), and core fragmentation (Machida et al. 2005). molecular clouds, where star formation mostly occurs along We can better understand these phenomena by observing the organized filamentary structures (André et al. 2000, 2014). Within polarization of thermal dust emission, which is the most these dense regions, prestellar cores, which are stellar precursors, commonly used tracer of magnetic fields in the interstellar are collapsing under their own gravitational field and form Class 0 medium (ISM), from the scales of molecular clouds down to the protostellar cores. At this evolutionary stage, the protostar is ∼100 au spatial scales of Class 0 disks. Dust grains are assumed accreting material from the surrounding envelope, where most of to produce polarized thermal emission thanks to the alignment of the source’s mass is still located. The accretion is known to be their angular momentum (aligned along their minor axis) with ruled by a variety of physical processes, of which the main the ambient magnetic field, via the actions of radiative alignment observational signature is the vigorous ejection of material in the torques (RATs, Lazarian 2007; Andersson et al. 2015). Thus, the form of a bipolar outflow. The evolution of these young accreting emission we detect is polarized orthogonal to the magnetic field objects is well known to be strongly regulated by magnetic fields, component projected on the plane of the sky. 14 Single-dish observations of the magnetized ISM have NAOJ Fellow. revealed organized magnetic field lines toward dense star- forming filamentary structures, unveiling the role of the Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further magnetic field on 0.1 to 10 pc scales (Alves et al. 2008; Planck distribution of this work must maintain attribution to the author(s) and the title Collaboration et al. 2016; Pattle et al. 2017). Interferometric of the work, journal citation and DOI. observatories such as the Submillimeter Array (SMA) and the 1 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Combined Array for Research in Millimeter-wave Astronomy Table 1 (CARMA) probed magnetic field morphologies at the scales of Serpens Source Information the protostellar envelopes (∼1000 au). More recently, observa- Name αJ2000 δJ2000 Menv Lbol tions with the Atacama Large Millimeter/submillimeter Array (Me) (Le) (ALMA) are able to resolve the tiniest features of protostellar Serpens SMM1-a 18:29:49.81 +1:15:20.41 cores (see Li et al. 2014; Hull & Zhang 2019 for reviews). Serpens SMM1-b1 18:29:49.68 +1:15:21.09 At the core-envelope scale, dust polarization observations Serpens SMM1-b2 18:29:49.66 +1:15:21.20 20 100 have unveiled interesting results about the relative orientation Serpens SMM1-c 18:29:49.93 +1:15:22.00 of the magnetic field with respect to the bipolar outflow. The Serpens SMM1-d 18:29:49.99 +1:15:22.98 outflow of a protostellar core is considered to be closely linked Serpens Emb 8 18:29:48.09 +1:16:43.30 with the core rotation axis, because the outflow launching Serpens Emb 8-b 18:29:48.13 +1:16:44.57 9.4 5.4 mechanisms can consist of magnetohydrodynamic (MHD) disk Serpens Emb 8-c 18:29:48.03 +1:16:42.70 winds triggered within the rotating circumstellar disk (Pudritz Serpens Emb 8(N) 18:29:48.73 +1:16:55.61 et al. 2006; Frank et al. 2014; Bally 2016). Consequently, the study of the magnetic field orientation with respect to the Note.Values of envelope mass and bolometric luminosity are from bipolar outflow axis is an important proxy to understand observations that encompass the whole core of SMM1 (Enoch et al. 2011; the role played by the magnetic field in the regulation of the Kristensen et al. 2012), as well as Emb 8 and Emb 8(N) together (Enoch et al. angular momentum of a protostellar core. At envelope scales of 2009, 2011). ∼1000 au, Hull et al. (2014) and Hull & Zhang (2019) (with twice the sample) found that the magnetic field is randomly Main. The position of the peak dust continuum emission, aligned compared with the outflow axis, suggesting that the associated with these three protostellar cores and their magnetic field at envelope scales is not affecting the surrounding core fragments (possibly containing protostars), magnetically driven winds at disk scales. However, a more can be found in Table 1. recent work by Galametz et al. (2018) using a smaller sample, The intermediate-mass protostellar source Serpens SMM115 suggested a bimodal distribution, exhibiting magnetic fields is the most luminous source in the cloud (Lee et al. 2014), with that are preferentially aligned either parallel or perpendicular to a luminosity of Lbol=100 Le (Kristensen et al. 2012). The the outflow orientation. In addition, they noticed that there is protostellar envelope was found to have a mass of about more large-scale rotation and multiple systems in cores where Menv∼20 Me (Enoch et al. 2011) and is surrounded by a disk- there is a large angle between the main core-scale field and the like structure with Mdisk∼1.0 Me and Rdisk∼300 au (Enoch outflow axis. Simulations have shown that these results could et al. 2009). ALMA observations from Hull et al. (2016) show depend strongly on the relative strengths of the magnetic field, a one-sided, high-velocity, highly collimated molecular jet turbulence, and rotation (e.g., Machida et al. 2006; Offner et al. (∼80 km s−1) from the central source SMM1-a. The base of the 2016; Hull et al. 2017b; Lee et al. 2017). narrow jet is surrounded by a wide-angle outflow cavity, whose The orientation of the magnetic field in protostellar cores has walls were observed in free–free emission by the Karl G. been the focus of studies investigating the formation of disks, Jansky Very Large Array (VLA) (Rodríguez-Kamenetzky et al. which is strongly impacted by the phenomenon of magnetic 2016). Hull et al. (2016) showed an extremely high-velocity braking. This is the case because the magnetic field can remove (EHV), one-sided redshifted molecular jet from the protobinary enough angular momentum from the envelope material to system SMM1-b located to the NW of the central source. These impede the formation of large protostellar disks at early times three sources were observed in full polarization by CARMA in (Hennebelle et al. 2016). In this respect, Maury et al. (2019) the TADPOL survey (Hull et al. 2014). Hull et al. (2016) characterized the disk size distribution in a sample of Class 0 attributed the ionization of the outflow cavity walls to UV protostars and suggested that the magnetic field may indeed radiation escaping from the accreting central protostar or to the play an important role in the formation of disk structure at the precession of the high-velocity jet, which would impact the earliest protostellar evolutionary stage. surrounding envelope. Goicoechea et al. (2012) proposed an The magnetic field morphologies seen in small-scale (i.e., a alternative scenario, where the ionizing radiation is caused by fewhundred au) observations from CARMA, SMA, and ALMA distributed shocks throughout the outflow. Interferometric dust have unveiled a variety of scenarios. These include a few results polarization observations of this source have suggested that the showing that the magnetic field seems to follow the edges of the SE redshifted lobe of the bipolar outflow from SMM1-a is outflow cavity (Hull et al. 2017a; Maury et al. 2018; Hull et al. shaping the magnetic field (Hull et al. 2017a). SMM1-a is also 2019), as well as magnetic field morphologies in young known to host a hot corino-like central region, because a few embedded disk structures that seem to exhibit both poloidal complex organic molecules (COMs) have previously been and toroidally wrapped field components (Stephens et al. 2013; detected, including methanol, methyl formate, dimethyl ether, Rao et al. 2014; Segura-Cox et al. 2015; Alves et al. 2018; Harris vinyl cyanide, and ethylene glycol (Kristensen et al. 2010; et al. 2018; Ohashi et al. 2018; Sadavoy et al. 2018a). Öberg et al. 2011; Tychoniec et al. 2018). Hot corinos are The Serpens region exhibits a filamentary structure with two thought to correspond to the central region of the protostar, compact star-forming clumps, Serpens Main and Serpens where the temperature is high enough to sublimate icy grain South, which are located at a distance of 436±9 pc mantles, which release COMs into the gas phase (Maury et al. (Ortiz-León et al. 2017). The recent star formation episode 2014; Walsh et al. 2014). observed in this region has been tentatively interpreted as resulting from a collision of two molecular clouds (Duarte- 15 Serpens SMM1 has been called by many other names, such as Serpens- Cabral et al. 2010, 2011). Serpens SMM1, Emb 8(N), and Emb Emb6, Serpens FIRS1, Serpens-FIR1, IRAS 18273+0113, S68 FIR, S68 8 are three Class 0 protostars in the NW subcluster of Serpens FIRS1, and S68-1b. 2 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. 16 Serpens Emb 8 and 8(N) are two low-mass protostellar Table 2 sources separated by 15 7, i.e., ∼7000 au. These sources were ALMA Observation Details observed in Enoch et al. (2009, 2011) with Bolocam and have a Data Set Baselines Calibrators combined envelope mass of Menv∼9.4 Me and a bolometric (m) luminosity of Lbol=5.4 Le. These observations have a spatial bandpass J1751+0939 resolution of ∼13,500 au, thus encompassing the two proto- A 16.5–763 phase J1751+0939 stellar sources. ALMA dust polarization observations of flux Titan Serpens Emb 8 exhibited a chaotic magnetic field morphology; the authors concluded that the magnetic field is most likely bandpass J1751+0939 weak with respect to the cloud-scale turbulence (Hull et al. B 12.4–3042 phase J1751+0939 2017b). flux J1751+0939 Regarding the relative age of Emb 8 and 8(N), the bandpass differences between the two bipolar outflows of both sources C 11.7–3320 phase See note offer a clue. Unlike Emb 8, Emb 8(N) exhibits a pristine EHV flux jet on both sides, which has not propagated as far as the outflow from Emb 8 (Dionatos et al. 2010; Tychoniec et al. 2019). As Note.In data set C, all three calibrators were J1751+0939 in one execution, molecular jets are generally an indication of the young age of a and J1924-2914 in the other. protostar (Bally 2016), we propose that Emb 8(N) may be younger. Moreover the outflow opening angles of these sources which of them produced the best images at our multiple desired are quite different, which can be related to age (Arce & spatial resolutions. Sargent 2006; Velusamy et al. 2014; Hsieh et al. 2017). The The polarized dust continuum images were produced by opening angle of Emb 8(N) is smaller, again suggesting a using the CASA task clean, applying four rounds of younger age for Emb 8(N). consecutive phase-only self-calibration, using the total intensity In this paper we present ALMA 870 μm polarization (Stokes I) solutions as a model for the Stokes Q and U, with a observations toward the three Class 0 protostars Emb 8(N), Briggs weighting parameter of robust=1. The three Stokes Serpens SMM1, and Serpens Emb 8. We describe in Section 2 parameters I, Q, and U were cleaned separately after the last the different observational data and the data reduction. In round of self-calibration using an appropriate residual threshold Section 3 we present the dust polarization and total intensity and number of iterations. The linear polarization properties of maps, as well as a few molecular line observations. Finally, we the radiation field from the thermal dust emission are given by discuss in Section 4 the different polarization patterns and the the Stokes parameters Q and U, whereas the Stokes I parameter potential grain alignment mechanisms implied, as well as the gives the total intensity of the dust continuum emission. The relations between the bipolar outflow and the magnetic field quantities derived from the combined use of the three Stokes morphology. Finally, we draw our conclusions in Section 5. maps are the polarized intensity P, the polarization fraction Pfrac, and the polarization position angle χ: 2. ALMA Observations and Data Reduction P= Q2 + U 2 (1 ) We present three 870 μm ALMA dust polarization observa- P Pfrac = (2 ) tions of our three sources in Serpens. Each of the data sets A, I B, and C targeted all three sources, and they were taken on ⎛ 1 U⎞ 2015 June 3 and 7, 2016 September 12 and 13, and 2017 c = arctan ⎜ ⎟. (3) July 31 (ALMA projects: 2013.1.00726.S, 2015.1.00768.S, 2 ⎝Q⎠ 2016.1.00710.S; PI: C. Hull). The synthesized beam of our Although the Stokes parameters Q and U can be positive or observations varies from 0 33 to 0 11, corresponding to a negative, the polarized intensity P is always positive. This spatial resolution varying from ∼144 au in data set A up to introduces a bias in the measurement of the polarized intensity, ∼48 au in data set C, at a distance of 436 pc. Each data set consists of four spectral windows of 2 GHz each, ranging in especially for emission with a low signal-to-noise ratio (S/N), frequency from 336.5 to 350.5 GHz. The details of the where P is below the 3σP threshold, σP being the noise in the P observations can be found in Table 2. In data sets A, B, and map. We corrected this bias in our P maps in order to arrive at C the polarization calibrators were respectively J1751+0939, the corrected polarized intensity, following the method J1751+0939, and J1924-2914, chosen for their high polariza- described in Vaillancourt (2006) and Hull & Plambeck tion fraction. The ALMA flux calibration accuracy in Band 7 (2015). Note that in Case-3, the Stokes parameters I and Q & (870 μm) is 10%. See Nagai et al. (2016) for a complete U come from different combinations of data sets (see Table 3). description of the ALMA polarization system. Therefore, before debiasing P and making the P and I images, We faced some issues when imaging data sets B and C, since we used the CASA task imsmooth to smooth the three Stokes they were “semi-pass,” because the requested resolution and parameters I, Q, and U to have the same reconstructed beam sensitivity were not reached. To improve our image quality, the (by convolving the map with a 2D Gaussian kernel). The data sets were combined together following three different resulting beam was chosen in such a way that it encompasses schemes during the production of the Stokes images (see perfectly the two beams resulting from the different combina- Table 3). The choices of which data sets to merge depended on tions. In addition, we performed a primary beam correction on 16 Serpens Emb 8 has also been called S68N, and Serpens Emb 8(N) also has all the maps of total intensity and polarized intensity presented the name S68Nb. in this article. 3 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Table 3 Imaging Details Case Source I Q and U θres Ipeak Irms Qrms Urms Figures (arcsec) ( ) mJy beam ( ) mJy beam ( )mJy beam ( ) mJy beam Data set A Ser-Emb 8 A A 0.35×0.32 102 0.060 0.024 0.024 8–10 Ser-Emb 8(N) A A 0.35×0.32 55 0.066 0.024 0.025 1 Case-1 Ser-SMM1 ABC ABC 0.15×0.14 203 0.57 0.033 0.030 5–7, 14 Case-2 Ser-Emb 8 AC AC 0.20×0.16 75 0.18 0.024 0.024 8 Ser-Emb 8(N) AC AC 0.26×0.22 44 55 0.025 0.025 1–4 Ser-SMM1 C B 0.13×0.13 182 0.7 0.066 0.058 5, 11, 15 Case-3 Ser-Emb 8 C BC 0.12×0.11 53 0.2 0.033 0.033 8 Ser-Emb 8(N) C BC 0.14×0.11 53 150 0.035 0.035 1 Note.Case-1, Case-2, and Case-3 are different combinations of the data sets A, B, and C. θres is the angular resolution of the observations. Ipeak is the peak total intensity of the Stokes I total intensity map. Irms, Qrms, and Urms are the noise values in the Stokes I, Q, and U maps, respectively. The values are calculated as flux density per unit of synthesized beam θres. The maps of Serpens SMM1 and Emb 8 from data set A were previously published in Hull et al. (2017a, 2017b). Finally, we present 1.3 mm (Band 6) and 3 mm (Band 3) map, which may be because the emission is too faint, resulting ALMA spectral-line data (ALMA projects: 2013.1.00726.S in a loss of signal due to a lack of sensitivity in the higher and 2016.1.00710.S; PI: C. Hull), which were taken on 2014 resolution beam. Note that as the polarized intensity is less August 18 and 2016 October 4, respectively, and have angular limited in dynamic range than the total intensity, dust resolutions of approximately 0 45×0 55 and 0 56×0 6. polarization appears where there is no detection of Stokes I, The data include the following transitions: CO (J=2→1) especially in the highest resolution maps we present of all three (used to trace the outflow, shown in Hull et al. 2016, 2017a for sources. the case of Serpens SMM1), 13CS (J=5→4), C18O (J= Apart from the dust emission seen along the outflow walls, a 2→1), and DCO+ (J=3→2). filament is seen in the low- and mid-resolution maps in the dust continuum, orientated NW to SE, which has no obvious 3. Results relation with the protostellar outflow and appears unpolarized (see Figure 1 top left and right panels). The mid-resolution map Below, we discuss our results from the dust polarization, clearly allows us to disentangle the outflow cavity walls (traced continuum, and spectral-line observations of the three proto- by the high-resolution polarized emission) from this large stars Serpens Emb 8(N), SMM1, and Emb 8. In Figures 1, 5, unpolarized filament. We see a clear asymmetry in the and 8 we describe the magnetic field morphology recovered at polarization of the cavity walls, which may be linked to this several spatial scales probed by the ALMA data (for example, filament. On each side of the bipolar outflow, one side of the data set C recovered angular scales from 0 11 to ∼1 3). In outflow cavity walls is depolarized: the northern wall on the Figures 2, 6, and 9 we discuss the spatial correlation of the dust redshifted side, and the southern wall on the blueshifted side. continuum emission, magnetic field orientation, and the These depolarized zones overlap with the large-scale filament, molecular outflows. In Figures 3, 7, and 10 we present maps suggesting that, for example, the polarized emission from the of polarization fraction and polarized intensity. Finally, in cavity wall and from the large filamentary structure could have Figure 4 we compare the dust polarization with the emission of blended together, resulting in the depolarization we see. molecular species tracing the dense gas (13CS (J=5→4), Indeed, the emission from infalling envelope material is known C18O (J=2→1), and DCO+ (J=3→2)) toward Serpens to be polarized (e.g., in B335: Maury et al. 2018; BHR 71 Emb 8(N). IRS1: Hull et al. 2019; and in SMM1-a, see below). Given the average flux of this filamentary structure, a detection of 3.1. Serpens Emb 8(N) polarized emission at 3σP would imply a polarization fraction Serpens Emb 8(N) has never been observed at such high of 20%. We address the question of the local conditions angular resolution. In Figure 1 we show multi-scale observa- necessary to enhance the alignment of dust grains in tions of the magnetic field and thermal dust continuum Section 4.4; in light of the fact that grain alignment in the emission around the protostar, with spatial resolutions of 146, filament is not likely to be strongly enhanced, it is possible that 105, and 55 au (from data set A, Case-2, and Case-3: see the filament appears unpolarized because the recovered Table 3). We resolve progressively enhanced dust continuum continuum is simply too faint to detect polarization. Therefore, emission along the outflow cavity walls, although this becomes the filament may alter the polarized emission at places where it faint at the highest angular resolution (Figure 1, bottom left is in the same line of sight as the outflow cavity walls. panel). This type of structure is created by the outflow, which This filament does not appear in the highest resolution map clears the cavity and causes material to accumulate beside the (Figure 1 bottom left panel). If we broadly calculate what its outflow, resulting in a high-density, compact feature that is flux would have been at the highest angular resolution given enhanced because of how the emission is spatially filtered by the flux measured in the mid-resolution map (Figure 1 right the ALMA interferometer. The intermediate-resolution map panel), we obtain a value below the 3σI threshold, suggesting a (Figure 1, right panel) is the one that recovers the highest flux lack of sensitivity rather than a filtering effect. Moreover, the density in both total intensity and polarized dust emission. fact that the data sets B and C (Tables 2, 3) have significantly Most of the features are resolved out in the highest resolution less integration time strengthens this hypothesis. 4 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 1. Magnetic field around Serpens Emb 8(N). Line segments represent the magnetic field orientation, rotated by 90° from the dust polarization angle χ (the length of the segments does not represent any quantity). They are plotted where the polarized intensity P>3σP. The color scale is the total intensity (Stokes I) of the thermal dust emission, shown from 3σI. The gray contour indicates the 3σI level. Top left: data set A, see Table 3. σP=35 μJy beam−1, σI=66 μJy beam−1. The peak polarized and total intensities are 0.51 mJy beam−1 and 55 mJy beam−1, respectively. The red ellipse in the lower left corner represents the synthesized beam of ALMA from data set A only. The beam size is 0 35×0 32, with a position angle of −61°. 5. Right: combination of the data sets A and C, see Table 3 Case-2. σP=35 μJy beam−1, σI=55 μJy beam−1. The peak polarized and total intensities are 0.38 mJy beam−1 and 44 mJy beam−1, respectively. The blue and red arrows represent the directions of the blueshifted and redshifted lobes of the bipolar outflow, respectively. The beam size is 0 26×0 22, with a position angle of −64°. Bottom left: combination of the data sets A, B, and C differently for Stokes I, Q, and U, see Table 3 Case-3. σP=43 μJy beam−1, σI=150 μJy beam−1. The peak polarized and total intensities are 0.23 mJy beam−1 and 27 mJy beam−1, respectively. The beam size is 0 14×0 11, with a position angle of −60°. 8. The Stokes I, Q, and U maps for the high-, mid-, and low-resolution observations are available in FITS format as the data behind the figure. (The data used to create this figure are available.) Figure 2 presents the integrated blue- and redshifted The polarized dust intensity and polarization fraction in CO (J=2→1) emission around the protostar. CO faithfully Serpens Emb 8(N) are shown in Figure 3. As the polarization traces the outflowing gas in Class 0 protostars because it is still fraction comes from the ratio of the polarized intensity P to the mostly molecular at this stage of protostellar evolution (Arce total intensity I, it is important to consider the S/N of both P et al. 2007; Panoglou et al. 2012). The results exhibit a pristine, (color scale) and I (contours) in order to determine whether the very high-velocity,17 highly collimated molecular jet, strengthen- corresponding polarization fraction is reliable. Therefore, in order ing the above assumption that the structures seen in the dust to derive the polarization fraction, we considered only emission continuum trace the cavity walls, because they perfectly embrace within the zones of 3σP and 5σI, where σP and σI are the rms the CO outflow. It is worth noting, however, that some vectors noise levels in polarized and total intensity, respectively. Figure 3 show up within the CO emission, particularly on the blueshifted shows a significant amount of dust polarization along the outflow side of the outflow. This polarized emission might be still linked to cavity walls, whereas the central region is unpolarized, at this the outflow cavity wall, and might simply overlap with the CO resolution, where the Stokes I emission peaks. The lower- emission because of projection effects. We shall also keep in mind resolution map (Figure 1, top left panel), however, shows a that the spatial resolution of the CO emission is twice as coarse as detection of polarized emission associated with the Stokes I peak, the resolution of the dust continuum. A kinematic study by corresponding to a polarization fraction of ∼0.4%. Tychoniec et al. (2019) found that this collimated jet is consistent It is common to see a “polarization fraction hole” where the with the young age of the source, considering the narrow opening dust continuum emission peaks. This can be due to collisional angle and the small dynamical age of the jet (the relative age of dust de-alignment in high-density regimes (Lazarian 2005; Emb 8(N) and Emb 8 is discussed in Section 4.3). Bethell et al. 2007; Pelkonen et al. 2009), or higher magnetic field dispersion in zones of high column density, because we 17 know the degree of organization of the magnetic field is a Note that we integrated the emission in such a way that we probe both the high-velocity and EHV components of the outflow: see Tychoniec et al. (2019) key point to allow the detection of polarized dust emission for details. (Maury et al. 2018). In our case, we might observe this 5 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 2. Moment 0 map of CO (J=2→1) on a color scale overlaid with the total intensity contours and magnetic field orientations around Serpens Emb 8(N). The moment 0 map is constructed by integrating emission from −53 to 0 km s−1 (blue) and from 15 to 40 km s−1 (red). vLSR is ∼8.5 km s−1. The peaks of the red- and blueshifted moment 0 maps are 2.10 Jy beam−1 km s−1 and 2.52 Jy beam−1 km s−1, respectively. Same as Figure 1 (right) for the line segments. The black contours trace the dust continuum from Case-2 (see Table 3) at levels of 11, 16, 24, 44, 74, 128, 256×σI, where σI= 55 μJy beam−1. The red ellipse in the lower left corner represents the synthesized beam of ALMA continuum observations. The beam size is 0 26×0 22, with a position angle of −64°. The green ellipse represents the resolution from the molecular line maps. Its size is 0 53×0 45. Note that the two lobes of the outflow have been plotted on different flux scales to enhance their visibility; as a result, relative fluxes should not be inferred from this image. The red and blueshifted moment 0 maps are available in FITS format as the data behind the figure. (The data used to create this figure are available.) Figure 3. Dust polarization intensity (top) and polarization fraction (bottom) in Serpens Emb 8(N) from Case-2. Same as Figure 1 (right) for the line segments. The color scale in the top panel is the polarized intensity P, shown where phenomenon in the inner core of Emb 8 (N). However, line-of- P>3σP. The color scale in the bottom panel is the polarized fraction Pfrac, shown where P>3σP and I>5σI. The peak polarized intensity is sight effects can result in a depolarized signal in the center of 0.38 mJy beam−1. The black contours represent the total intensity (Stokes I) the protostar as we see through zones where the redshifted and at the following levels: 11, 16, 24, 44, 74, 128, 256×σI, where σI= blueshifted counterparts have both affected the polarized dust 55 μJy beam−1. emission. This effect can decrease the polarized intensity in the equatorial plane, as has been seen in synthetic observations by Frau et al. (2011), under the threshold of the 3σP level. In wall is less polarized with a maximum in polarization fraction addition, at our resolution, a sharp change in magnetic field of 25% at 790 au from the protostar. orientation can produce cancellation of the polarized signal Finally, Figure 4 presents the integrated intensity maps for within the beam lying in the equatorial plane, leading to a the emission of the three dense-gas tracers 13CS (J=5→4), beam-sized depolarization zone (Kataoka et al. 2012; Lee et al. C18O (J=2→1), and DCO+ (J=3→2), which we com- pare with the polarized intensity. It is striking to notice how 2017; Kwon et al. 2019). However, almost no polarized both 13CS and C18O show up roughly where we see polarized emission is detected at this location in the map with the highest continuum emission in an E–W orientation, aligned with the angular resolution (Figure 1 bottom left panel), and thus the outflow. C18O is typically optically thin in protostellar cores, lack of polarization detection is likely due to low sensitivity at and thus traces high-density material that is warm enough to higher resolution (see Table 3). Thus, there is probably trigger the sublimation of C18O that was frozen onto dust polarization toward the peak of the dust continuum emission, grains. The spatial extent of this molecule has been used as a and more sensitive, high-angular-resolution observations tracer of protostellar accretion (Jørgensen et al. 2015; Visser should be able to recover it. In contrast, the polarization et al. 2015). 13CS peaks at the same place as C18O, but is less fraction along the outflow cavity walls, within the 5σI and 3σP spatially extended, and seems to be very well coupled with the zones, reaches 30% at 690 au and 36% at 1150 au from the dust emission in the SW outflow cavity wall. The kinematics of center of the protostar, along the northern edge of the these two lines did not reveal any evidence of rotation in the blueshifted outflow. The southern redshifted outflow cavity inner core, which has been seen previously in molecular line 6 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 4. Moment 0 maps of 13CS (J=5→4), C18O (J=2→1), and DCO+ (J=3→2) around Serpens Emb 8(N). The black contours represent the total intensity (Stokes I) at the following levels: 7, 11, 16, 24, 44, 74, 128, 256×σI, where σI=55 μJy beam−1, from Case-2. vLSR is about ∼8.5 km s−1. Left: moment 0 map of 13CS (J=5→4) in grayscale constructed by integrating emission from 5 to 12 km s−1. The rms noise level of the moment 0 map is 16 mJy beam−1 km s−1. The peak of the moment 0 map is 0.12 Jy beam−1 km s−1. Middle: moment 0 map of C18O (J=2→1) in grayscale, constructed by integrating emission from 6 to 11.25 km s−1. The rms noise level of the moment 0 map is 12 mJy beam−1 km s−1. The peak of the moment 0 map is 0.11 Jy beam−1 km s−1. Right: moment 0 map of DCO+ (J=3→2) in grayscale, constructed by integrating emission from 8 to 10 km s−1. The rms noise level of the moment 0 map is 13 mJy beam−1 km s−1. The peak of the moment 0 map is 0.067 Jy beam−1 km s−1. Same as Figure 1 (right) for the line segments. The red and blue arrows represent the bipolar outflow directions. The beam size of the continuum emission (red ellipse) is 0 26×0 22, with a position angle of −64°. The green ellipse represents the resolution from the molecular line maps. Its size is 0 53×0 45. observations of Class 0 disk–envelope systems (Ohashi et al. angular resolution in Figures 5 and 7 present a more complex 2014; Yen et al. 2015; Lee et al. 2016; Jacobsen et al. 2019; picture of the dust emission and magnetic field morphology, Hsieh et al. 2019). Rather, the kinematic information suggests with an angular resolution reaching 0 15×0 14 (∼57 au). that the gas is linked with the outflow motion. Finally, we SMM1-c, SMM1-d, and the two binary components of SMM1- present the integrated moment 0 map of the DCO+ emission. b are now totally resolved (see Appendix A for a scheme of the This molecule is formed from a reaction between H2D+ and the SMM1 core). The polarization to the west of SMM1-a and to the remnant CO in the gas phase. As low temperatures are essential south of SMM1-b has been mostly resolved out compared with for deuterium fractionation, DCO+ is known to be a good the lower resolution observations. Just a few remaining polariza- tracer of the cold, dense material located at the disk–envelope tion detections are seen toward the binary and just to the south of interface (Jørgensen et al. 2004, 2011; Murillo et al. it, consistent with the results from Hull et al. (2017a). To the east 2015, 2018). Its emission appears anticorrelated with the of SMM1-a we see weak polarization toward the E–W cavity polarization, and rather seems to trace the filamentary structure edge, which was already relatively faint in the lower angular mentioned above that is crossing over the protostar. This resolution data. Our results, however, show a clear filamentary anticorrelation with dust polarization has also been seen in structure to the south of SMM1-a, visible in polarization and total emission of N2D+ in BHR 71 (Hull et al. 2019), suggesting that intensity. This structure consists of two highly polarized filaments tracers of cold, dense material such as DCO+, N2D+, and (hereafter designated as the Eastern and Western filaments, see N2H+ are good proxies for probing the conditions necessary for Appendix A for a schematic presentation of the different features dust-grain alignment in protostellar cores. Finally, we do not in SMM1), which have magnetic fields that clearly lie along their detect an organized velocity gradient in the DCO+ at the major axes. These two filaments observed to the south of SMM1- 0.5 km s−1 spectral resolution of our data, and thus the role of a appear to be connected to the central core, i.e., the resolved hot the aforementioned large-scale filamentary structure in the corino of SMM1-a, which exhibits a complex polarization pattern. formation of Serpens Emb 8(N) remains to be determined. We discuss the possible physical origin of these filaments in Section 4.3. In Figure 7 we present the dust polarization intensity and 3.2. Serpens SMM1 polarization fraction around SMM1-a. It is immediately apparent We now present the results of our second source, Serpens that the two filaments to the south of SMM1-a exhibit high SMM1, an intermediate-mass Class 0 protostellar core. In polarization fractions, reaching values of 10% or higher (the Figure 5 we present the magnetic field orientations and thermal Eastern filament reaches a maximum of 20%). In the zone where dust continuum emission. The CO (J=2→1) integrated the two filaments appear to cross, the polarization intensity and emission tracing the low-velocity bipolar outflow around the orientation suggest that the superposition in the plane of the sky protostar is shown in Figure 6. Lower resolution polarization of the emission emanating from the two filaments has caused the observations of this source (as well as the CO map presented polarization to cancel. Indeed, where the two filaments cross here) were first published in Hull et al. (2017a). They found there is a clearly depolarized zone the size of the beam that the dust along the edges of the cavity of the wide-angle, (Figure 7). This strengthens the idea that these filaments are two low-velocity redshifted outflow was highly polarized. How- separate structures. Moreover, to the east of the depolarized zone ever, they did not detect any polarization that was clearly the magnetic field orientation is a bit offset from the major axis of related to the redshifted EHV jet of SMM1, reported in Hull the Eastern filament. This suggests that the polarization in this et al. (2016). The results from our observations at higher location is coming from both filaments, resulting in an average 7 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 5. Magnetic field around Serpens SMM1. Line segments represent the magnetic field orientation, rotated by 90° from the dust polarization angle χ (the length of the segments does not represent any quantity). They are plotted where the polarized intensity P>3σP. The color scale is the total intensity (Stokes I) of the thermal dust emission, which is shown when I>3σI. Left: combination of data sets A, B, and C, see Table 3 Case-1. σP=53 μJy beam−1 and σI=0.57 mJy beam−1. The peak polarized and total intensities are 6.28 mJy beam−1 and 203 mJy beam−1, respectively. The red ellipse in the lower left corner represents the beam size, i.e., 0 15×0 14, with a position angle of −48°. 5. Right: combination of the data sets B and C, see Table 3 Case-3. σP=80 μJy beam−1 and σI=0.7 mJy beam−1. The peak polarized and total intensities are 5.6 mJy beam−1 and 182 mJy beam−1, respectively. The red ellipse in the lower left corner represents the beam size, i.e., 0 13×0 13, with a position angle of −58°. 8. The Stokes I, Q, and U maps for the high- and mid-resolution observations are available in FITS format as the data behind the figure. (The data used to create this figure are available.) magnetic field orientation that is not perfectly aligned with either This source was the focus of Hull et al. (2017b), where they of them. Finally, to the south of the curved Western filament, the compare the observed magnetic field morphology of data set A polarization is orientated perfectly N–S along the straight Eastern with turbulent MHD simulations. They found that Serpens Emb 8 filament. The potential causes of these highly polarized filaments may have formed in a weakly magnetized environment, because are discussed in Section 4.3. no obvious hourglass morphology was detected. Additionally, As introduced in Section 3.1, we observe the “polarization they found no correlation between the magnetic morphology and hole” phenomenon in the inner core of SMM1-a, where we see a the gradient of the dust emission, suggesting that the field is not clear difference in polarization fraction between the hot corino strong enough to shape the structure of the dust. The magnetic and the two southern filaments. However, inside this central field morphology observed does not present major changes as we region (within the ∼32σI level, i.e., above the 9% level of the increase in resolution from data set A (Hull et al. 2017b) to Case-3 peak total intensity), the polarization appears quite inhomoge- (Figure 8 right panel). However, we now begin to resolve the neous. Both the polarized intensity and the polarization fraction magnetic field orientation in the central core Emb 8, which (achieving a maximum of 6%) exhibit strong peaks to the SE of exhibits an homogeneous E–W pattern. As for the dust the Stokes I peak. This highly polarized spot clearly contrasts polarization in the envelope, it mainly shows up around Emb with the remaining area within this central zone, which on 8-b and to the south of Emb 8, in a large arc-shaped structure that average has a polarization fraction of ∼1%. At first glance, the is not strongly correlated with the outflow (like Emb 8(N), for inferred magnetic field orientation in this central region appears example). As for Emb 8-c, this source appears unpolarized. quite radial. We discuss in Sections 4.1 and 4.2 the potential In Figure 9 we present the magnetic field orientations from causes of this polarization pattern in SMM1-a, which we data set A overlaid with Stokes I contours and the integrated attribute primarily to a poloidal magnetic field morphology. redshifted and blueshifted emission from CO (J=2→1) on a color scale. Contrary to the two other protostars presented above, the dust polarization is not clearly correlated with the 3.3. Serpens Emb 8 molecular outflow. It is worth noting some hints of outflow Finally, we present the ALMA dust polarization data of the cavity wall around the base of the blueshifted outflow, where protostar Serpens Emb 8, located at ∼7000 au to the SW of the dust emission seems to follow the wide-angle outflow; Serpens Emb 8(N). In Figure 8 we present two high-angular- however, there is almost no polarized dust emission in this area. resolution maps of this source, with spatial resolutions of 80 On the redshifted side, there are no obvious correlations, and 50 au. Apart from the central source Emb 8 we now clearly because the CO emission is far from the detected dust emission. resolve the two companions, which we deem Emb 8-b and Emb Nevertheless, all the magnetic field orientations to the south of 8-c (see Table 1 and the right panel of Figure 8). As was the Emb 8 are quite aligned with the outflow axis, suggesting that case for Serpens Emb 8(N), we attribute the loss of signal at the magnetic field may not be totally uncorrelated with the high resolution to a lack of sensitivity. bipolar outflow. Indeed, to the south of the central core, about 8 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 6. Moment 0 map of CO (J=2→1) on a color scale overlaid with the total intensity contours and magnetic field orientations around Serpens SMM1. Same as Figure 5 (left) for the line segments. The moment 0 on the color scale is constructed by integrating emission from −13 to 4 km s−1 (blue) and from 10.5 to 30 km s−1 (red). vLSR is ∼8.5 km s−1. The peaks of the red- and blueshifted moment 0 maps are 5.40 Jy beam−1 km s−1 and 3.90 Jy beam−1 km s−1, respectively. The black contours tracing the dust continuum are 8, 12, 20, 32, 64×the rms noise σI in the Stokes I map, where σI=0.57 mJy beam−1. The red ellipse in the lower left corner represents the synthesized beam after combining data sets A, B, and C. The beam size is 0 15×0 14, with a position angle of −48°. 5. The green ellipse represents the resolution from the molecular line map, and measures 0 53×0 45. Note that the two lobes of the outflow have been plotted on different flux scales to enhance their visibility; as a result, relative fluxes should not be inferred from this image. The redshifted and blueshifted moment 0 maps are available in FITS format as the data behind the figure. (The data used to create this figure are available.) Figure 7. Dust polarization intensity (top) and polarization fraction (bottom) in 70% of the magnetic field line segments are aligned with the SMM1a, from Case-1. Same as Figure 5 (left) for the line segments. The black outflow axis within an offset of ±20°. contours tracing the dust continuum are 8, 12, 20, 32, 64, 128, 220, 300×the Finally in Figure 10 we present the maps of polarized intensity rms noise level σI in the Stokes I map, where σI=0.57 mJy beam−1. The color and polarization fraction of Emb 8 from data set A. The polarized scale in the top panel is the polarized intensity P, which is shown where P>3σP. The color scale in the bottom panel is the polarization fraction Pfrac, intensity peaks in the central core, which has a polarization fraction which is shown where P>3σP and I>5σI. The peak polarized intensity is of 0.7%. The polarization fraction within the core increases 203 mJy beam−1. The red ellipse in the lower left corner represents the progressively to the north and south of the central core, achieving synthesized beam after combining data sets A, B, and C and measures values of up to 30% inside the regions of 5σI and 3σP. 0 15×0 14, with a position angle of −48°. 5. correlation between the outflow morphology and the magnetic 4. Discussion field, especially around the outflow cavity walls (Section 4.3). Our high-resolution polarimetric results from the three Finally, we investigate the cause of the enhancement of the protostars Serpens Emb 8(N), SMM1, and Emb 8 lead us to polarization along the cavity walls, focusing particularly on the discuss the causes of the polarization patterns in each of these role played by the radiation field (Section 4.4). sources. We investigate the different environmental conditions and try to cautiously infer the possible physical processes that 4.1. Investigating Possible Grain-alignment Mechanisms would lead to the different behaviors (e.g., polarization fraction Causing Polarization in Inner Envelopes at Scales of and spatial distribution) of the polarized thermal dust emission. r<200 au We start by focusing on the polarization pattern seen in the inner cores of our sources, investigating first the optical thickness and In protostellar envelopes, the long axes of dust grains are the correlation between the structure of the continuum emission expected to be oriented orthogonal to the surrounding magnetic and the polarization (Section 4.1). Second, under the hypothesis field (Andersson et al. 2015). This effect has been the target of that the polarization reflects the magnetic field morphology, we many observations in both low- and high-mass star-forming discuss the poloidal pattern of the magnetic field visible in the regions (see Hull & Zhang 2019 for a review of interferometric inner core of two of our sources (Section 4.2). Third, we study the polarization observations). B-RATs (designating dust grains 9 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 8. Magnetic field around Serpens Emb 8 from Case-1. Line segments represent the magnetic field orientation, rotated by 90° from the dust polarization angle χ (the length of the segments does not represent any quantity). They are plotted where the polarized intensity P>3σP. The color scale is the total intensity (Stokes I) of the thermal dust emission, which is shown where I>3σI. Left: combination of data sets A and C, see Table 3 Case-2. σP=30 μJy beam−1 and σI=0.18 mJy beam−1. The peak polarized and total intensities are 0.48 mJy beam−1 and 75 mJy beam−1, respectively. The red ellipse in the lower left corner represents the beam size, i.e., 0 20×0 16, with a position angle of −67°. Right: combination of data sets B and C, see Table 3 Case-3. σP=30 μJy beam−1 and σI=0.2 mJy beam−1. The peak polarized and total intensities are 0.32 mJy beam−1 and 53 mJy beam−1, respectively. The red ellipse in the lower left corner represents the beam size, i.e., 0 12×0 11, with a position angle of −62°. 7. The Stokes I, Q, and U maps for the high- and mid-resolution observations are available in FITS format as the data behind the figure. (The data used to create this figure are available.) magnetically aligned via radiative alignment torques) is the precession timescale of large dust grains ( 100 μm) can be favored mechanism to explain the polarization at core/ISM longer than the gaseous damping timescale, which causes the scales, although an improved version of the theory including grains to be aligned via RATs with respect to the gradient in the paramagnetic inclusions in dust grains had to be developed in radiation field instead of with respect to the magnetic field. In order to reproduce the high (∼20%) polarization fractions this case, sometimes known as “k-RAT” alignment, the long observed by Planck in the diffuse ISM; see Hoang & Lazarian axes of the dust grains may be aligned orthogonal to the (2016). Guillet et al. (2018) also proposed an explanation for gradient in the radiation emanating from the central protostar. the high values of polarization fraction encountered at ISM In our high-angular-resolution observations of SMM1-a, we scales. Their models revealed that, when there is a high enough compare the relative orientation between the polarization and the mass fraction (∼0.8–1) of aligned grains, a combined radiation field in the center of the protostar. To do so, we use the population of silicate and amorphous carbon grains can map of Stokes I gradient as a proxy for the radiation field. In this reproduce these high polarization fractions. While this grain way, we can test whether the polarization orientation is alignment mechanism most likely continues to operate at the perpendicular to the Stokes I gradient, which would be an high column densities typical of inner envelopes, the different argument in favor of the k-RAT solution. A caveat of this local conditions of radiation, temperature, opacity, and density comparison is that inhomogeneous (i.e., aspherical) conditions of in these regions might cause other mechanisms to contribute to temperature, density, and optical thickness may alter this the millimeter and submillimeter (hereafter “(sub)millimeter”) correlation. In addition, the photons that are primarily responsible polarization signal from dust. These effects include dust self- for the radiative torque acting on grains at a given location are scattering and alignment of dust grains with respect to the those with the largest energy density, i.e., those near the peak of radiation direction, both of which we explore below. the spectral energy distribution (SED) at that location. These may or may not be the (sub)millimeter photons that we detect with ALMA. Nevertheless, we think this is a reasonable assumption, 4.1.1. Grain Alignment via Radiative Torques which has been discussed before in the interpretation of high- The polarization orientations in SMM1-a (where no addi- resolution ALMA results (Sadavoy et al. 2018a). tional rotation had been performed on the polarization angle χ) The bottom left panel of Figure 12 shows the distribution of from the highest resolution observation (Case-3 of Table 3) are the differences between the inferred magnetic field position plotted in Figure 11. The central region of SMM1-a, inside the angles18 and the Stokes I gradient in the inner core of SMM1-a 32σI contour, exhibits a generally azimuthal polarization pattern, which could be characteristic of dust grains that are 18 While here our aim is to test the k-RAT mechanisms by comparing the aligned with respect to the local radiation field, rather than with relationship between the radiation field and the polarization (which is unrelated the local magnetic field. In the theoretical study led by Lazarian to the magnetic field in k-RAT models), Figure 12 shows HROs using the inferred magnetic field, because we ultimately conclude that the polarization in & Hoang (2007) and Tazaki et al. (2017), they found that in all of our sources is most likely caused by magnetically aligned grains (see environments such as protoplanetary disks, the Larmor Section 4.2). 10 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 9. Moment 0 map of CO (J=2→1) on a color scale overlaid with the total intensity contours and magnetic field orientations around Serpens Emb 8. Line segments represent the magnetic field orientation, rotated by 90° from the dust polarization angle χ (the length of the segments does not represent any quantity). They are plotted where the polarized intensity P>3σP, where σP=25 μJy beam−1. The moment 0 on the color scale is constructed by integrating emission from −10 to 6.5 km s−1 (blue) and from 11 to 21 km s−1 (red). vLSR is ∼8.5 km s−1. The peaks of the red- and blueshifted moment 0 maps are 1.65 Jy beam−1 km s−1 and 1.68 Jy beam−1 km s−1, respectively. The black contours, which are tracing the dust continuum from data set A (see Table 3), are 3, 6, 9, 13, 16, 24, 44, 74, 128×the rms noise σI in the Stokes I map, where σI=60 μJy beam−1. The red ellipse in the lower left corner represents the synthesized beam of ALMA continuum observations. The beam size is 0 35×0 32, with a position angle of −63°. The green ellipse represents the resolution from the molecular line maps, measuring 0 53×0 45. Note that the two lobes of the outflow have been plotted on different flux scales to enhance their visibility; as a result, relative fluxes should not be inferred from this image. The redshifted and blueshifted moment 0 maps Figure 10. Dust polarization intensity (top) and polarization fraction (bottom) are available in FITS format as the data behind the figure. in Serpens Emb 8 from data set A. Same as Figure 9 for the line segments and (The data used to create this figure are available.) the Stokes I contours. The color scale in the top panel is the polarized intensity P, which is shown where P>3σP. The color scale in the bottom panel is the polarization fraction Pfrac, shown where P>3σP and I>5σI. The peak of the polarized intensity is 0.69 mJy beam−1. The red ellipse in the lower left corner (details of the calculations can be found in Appendix B). The represents the synthesized beam of ALMA continuum observations. The beam resulting distribution is single-peaked, with a maximum at 0°, size is 0 35×0 32, with a position angle of −63°. which suggests a polarization orientation perpendicular to the inferred radiation field. This implies that it is possible that dust inferred in a few young stellar objects (Chiang et al. 2012; grains have been aligned with their minor axes along the Sadavoy et al. 2016; Chacón-Tanarro et al. 2017; Agurto- radiation gradient. However, several caveats remain in this Gangas et al. 2019) and protoplanetary disks (e.g., Pérez et al. hypothesis: the distribution is quite broad, suggesting some 2012, 2015; Trotta et al. 2013; Testi et al. 2014; Tazzari et al. imperfections in this alignment solution. In addition, the peak 2016; Liu et al. 2017; Harsono et al. 2018; Huang et al. 2018). in polarized intensity yields a strong deviation from the perfect However, based on the aforementioned results, the dust grain azimuthal pattern. size of 100 μm invoked in Tazaki et al. (2017) is at the upper This type of azimuthal polarization pattern caused by large limit of the grain sizes inferred to-date in Class 0 protostars. dust grains has been seen in the HL Tau disk at 3 mm In order to constrain the physical conditions (and possibly wavelengths (Kataoka et al. 2017; Stephens et al. 2017). dust-grain growth) within the central cores and to discuss the However, some caveats were raised by Yang et al. (2019), who possible grain-alignment mechanisms, we investigated the explained that k-RATs should enhance the polarized emission optical thickness of our sources by measuring the spectral along the major axis of an inclined disk instead of exhibiting index α of the observed flux densities, given by the azimuthally symmetric polarized intensity seen in HL Tau. In the case of Class 0 protostars, the age of the system is a ⎛ n ⎞a Fn » Fn 0 ⎜ ⎟ . (4 ) determining factor for the amount of grain growth that has ⎝ n0 ⎠ occurred. However, the typical duration of the Class 0 stage of ∼0.2 Myr (Dunham et al. 2015; Kristensen & Dunham 2018) We use the continuum observations from our ALMA Band 3 seems to be long enough, because grain growth has been (3 mm) data set as well as our Band 7 (870 μm) data set B 11 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. SMM1-a. However, two strong contradictions lead us to discard k-RATs as the dominant polarization mechanism occurring here: the first one is the breadth of the distribution in the HRO histogram presented above. The second is that we do not observe the polarized intensity predicted in Yang et al. (2019), where their model predicts that the k-RAT mechanism should enhance the polarized intensity along the major axis of the protoplanetary disk structure, as mentioned above. This last point is not straightforward for SMM1-a, because we do not detect any hints of a flattened, rotationally supported structure in any of our molecular line observations. 4.1.2. Dust Self-scattering Polarized dust emission can also arise from self-scattering of dust grains. If the dust grain sizes have reached ∼100 μm, the self-scattering effect is expected to be the dominant polariza- tion pattern in dense environments such as those found in protoplanetary disks (Kataoka et al. 2015). Self-scattering is a Figure 11. Polarization orientations in SMM1-a from Case-3. Same as Figure 5 highly frequency-dependent phenomenon (Stephens et al. (right panel) for the line segments, except that in this case, they are not rotated by 90°, and instead represent the actual polarization orientations. The color 2017), reaching a maximum efficiency for dust grains with scale is the dust polarization intensity P. Both the color scale and the line sizes of ∼λ/2π (Kataoka et al. 2015). Observations of dust segments are shown where P>3σP, where σP=80 μJy beam−1. The black scattering in protoplanetary disks have found several polariza- contours represent the total intensity at 16, 38, 72, 130, 200×σI, where tion patterns, which are highly dependent on both the optical σI=0.7 mJy beam−1. The beam size of the continuum emission (red ellipse) is thickness and inclination of the observed disks (Kataoka et al. 0 13×0 13. 2016; Bacciotti et al. 2018; Girart et al. 2018; Hull et al. 2018; Ohashi et al. 2018; Dent et al. 2019). (Table 2), which are separated by a period of three weeks. To SMM1-a is most likely inclined (Yıldız et al. 2015 estimate measure the flux of SMM1-a, Emb 8(N), and Emb 8, we fit a an inclination of 50° with respect to the line of sight), and its single 2D Gaussian component model to the visibilities of our hot corino, as seen above, might be optically thick in the center; data sets using the UVMULTIFIT tool (Martí-Vidal et al. 2014). thus, the polarization might arise from the outer layer of this Figure 13 shows the fit results and the spectral index α for the central core. Sahu et al. (2019) proposed two scenarios to describe the geometry of a different hot corino, namely that of inner core (∼200 au) of our sources. An optically thick source NGC 1333 IRAS 4A1: either an optically thick circumstellar would have a spectral index of α≈2 (blackbody case, in the disk or a temperature-stratified dense envelope. If the structure Rayleigh–Jeans regime), whereas a source considered to be we see in SMM1-a is flattened by rotation, it is conceivable that optically thin would have α 3. We were not able to spatially the asymmetry in the polarized intensity (which is brighter on resolve the spectral index in the inner cores of our sources because the near side, which we know thanks to the location of the the beam of the 3 mm data set is significantly larger than the beam redshifted jet) is due to dust self-scattering. However, several of our Band 7 observations. We are thus averaging the optical points fail to corroborate this hypothesis of self-scattering in depth over the hot corino of SMM1-a with this Gaussian fitting. SMM1-a. First, the typical level of polarization fraction In Figure 14 we present the brightness temperature over the expected from dust self-scattering is ∼1%, whereas we observe center of SMM1-a, overlaid with the Stokes I and polarized a highly polarized spot in SMM1-a, which exhibits a intensity contours. The brightness temperature peaks at 101 K polarization fraction of 6%. Second, the polarization orienta- and is as low as ∼20 K at the outer edges of the hot corino. tions (see Figure 11) do not fit the prediction of self-scattering Moreover, we notice that inside the hot corino, the peaks in the theory, especially around the highly polarized zone on the polarized emission identified above are located on either side of redshifted side. Finally, as mentioned above, the current lack of the Stokes I peak (i.e., the horseshoe-shaped zone, which is a detected disk structure toward SMM1-a makes it difficult to likely to be an optical depth effect), which suggests that the interpret our results in the context of the self-scattering theory, which requires a disk-like structure. polarized emission is not originating in the regions of highest In their models, Yang et al. (2017) characterized the optical depth in the central core of SMM1-a. polarization emanating from inclined disks where the scattering The joint consideration of the brightness temperature map grains had not yet settled to the disk mid-plane. In such disks, and the derived spectral index value suggests that the very the polarized intensity becomes more asymmetric (i.e., the inner ∼100 au zone is optically thick and/or has dust polarized emission gets brighter on the near side of the disk) as emissivity properties that are significantly different from the the optical depth increases. We do see an asymmetry in the rest of the hot corino. Dust-grain growth may have begun in the polarized intensity in SMM1-a. However, unlike the models of very center of the hot corino; however, to confirm this we will Yang et al., which show polarization primarily along the minor need to further investigate the spatial distribution of the dust axes of the disks, we see primarily azimuthal polarization, emissivity index and the dust temperature (Bracco et al. 2017). inconsistent with dust self-scattering in an inclined disk. Tazaki et al. (2017) predict that the k-RAT mechanism requires In Serpens Emb 8, we do see polarization in the inner core large dust grains in order to operate, so this grain-alignment (∼200 au) at a level of 0.7%. The self-scattering phenomena hypothesis may indeed be relevant in the inner core of could indeed create this level of polarization fraction; however, 12 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 12. Calculated histograms of relative orientation (HROs) between the inferred magnetic field orientation and the density gradients in the total intensity maps (in gray), or in the blueshifted (in blue) and redshifted (in red) moment 0 maps of the CO (J=2→1) low-velocity outflow in Emb 8(N) (top) and Serpens SMM1 (bottom). In SMM1-a, the dust emission is separated into the central hot corino and the two southern filaments. Furthermore, concerning the redshifted outflow lobe of SMM1, we did not calculate the gradient in the central zone of SMM1-a, in order to focus more clearly on the correlation between the outflow cavity walls and the magnetic field orientation. See Appendix B for the gradient maps and a detailed explanation of how the HROs were produced. as mentioned above, we would expect the polarization orientation to be along the minor axis of the source, which can be inferred from the bipolar outflow axis (Cox et al. 2018). In the case of Emb 8, the orientation of the polarization is not aligned with its inferred minor axis. Moreover, given the spectral index of its inner core (α≈3.36), it is most likely optically thin. Consequently, we discard this hypothesis for Emb 8 as well. In these sources, it is not always straightforward to determine which polarization mechanism causes the polarization from the innermost regions of our sources. Overall, however, we conclude that neither self-scattering nor grain alignment via k-RATs is occurring in our sources. Moreover, a final caveat regarding the potential occurrence of these two polarization mechanisms is how the environmental conditions of the hot Figure 13. Flux evolution with frequency from the best-fit visibility models. corino would change the dust grain size distribution. For The y-axis is the integrated flux in jansky obtained fitting multiple 2D Gaussian example, the temperature and radiation from this high-column- components to the source visibilities. The x-axis is the frequency in gigahertz. density zone may be adequate to trigger radiative torque Two data sets were used here, containing observations at 870 μm (Band 7) and disruption (RATD), recently introduced in Hoang et al. (2019) 3 mm (Band 3). Each point is a fit to the visibilities from one spectral window, displayed with an error bar of ±σ, where σ is the error that takes into account and Hoang et al. (2019). Via the RATD phenomenon, large both the error of the fitting algorithm on each point and the 10% uncertainty in aggregates can be spun-up to suprathermal rotation speeds and the ALMA flux calibration system. disrupted into individual icy grains, which would lower the 13 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. region and the base of the EHV jet. Another point that corroborates the poloidal field hypothesis is the structure of the polarized intensity map. Between the two highly polarized zones in SMM1-a, respectively linked with the red- and blueshifted sides of the outflow, is a depolarized delimitation line that clearly divides the two sides of the poloidal magnetic field in SMM1-a (see the thin white region in the center of the color scale in Figure 15). This effect is also seen in Emb 8(N) (see Section 3.1), and can be explained by the sharp change of polarization orientation, which causes a line of depolarization with a width equal to the beam size (Kwon et al. 2019). Consequently, having such a depolarized line supports our hypothesis that a poloidal magnetic field configuration is the cause of the polarization in the central core of SMM1-a. The case of the central 200 au in Emb 8(N) is less obvious, because we do not detect a large amount of polarized intensity. However, the observations at the lowest angular resolution (Figure 1 top left panel) show a few detections in the center, suggesting the presence of a poloidal magnetic field in the Figure 14. Brightness temperature map of the inner core of SMM1-a from central core of Emb 8(N). Case-1. The color scale represents the brightness temperature calculated in the Rayleigh–Jeans approximation at a wavelength of 870 μm. The brightness We plotted the distribution of the difference in angle between temperature peaks at 101 K. The black contours represent the total intensity at the magnetic field orientation and the gradient from the moment 12, 20, 32, 64, 128, 220, 300×σI, where σI=0.57 mJy beam−1. The red 0 map of the low-velocity red- and blueshifted outflow of contours represent the polarized intensity at 3, 10, 17, 24, 32, 50, 68×σP, SMM1-a in Figure 12 (see the two bottom right panels). We where σP=53 μJy beam−1. The red ellipse in the lower left corner represents the synthesized beam after combining data sets A, B, and C. The beam size is compare CO and polarization data with different angular 0 15×0 14, with a position angle of −48°. 5. resolutions, which explains the small number of points displayed in the histograms. To derive the gradient in the HRO histograms, maximum dust grain size encountered in these kinds of we select regions with strong gradients in the integrated blue- environments. If this is indeed the case, the resulting (smaller) and redshifted CO maps in order to pick up only the polarization dust grain size distribution would make it likely that B-RATs orientations associated with the edge of the outflow cavities are the dominant grain-alignment mechanism even in the (see Appendix B for the gradient maps and details of the bright, dense hot corino regions that we observe. calculations). In the case of the blueshifted outflow of SMM1-a, We did not discuss the case of Emb 8(N), because it exhibits the few beams contributing to the histogram are located to the almost no detection in the center. In summary, we assume that NW of the inner core of SMM1-a (see Figure 15). These few B-RATs are the dominant grain-alignment mechanism respon- points, while not statistically significant, still suggest that the sible for the polarization detected in our three sources. Under magnetic field is tracing the blueshifted outflow cavity. The few this assumption, we continue below by discussing the inferred points in the histogram on the redshifted side seem randomly magnetic field maps we obtain in our three sources. distributed, because the low-velocity redshifted outflow emission is spatially extended and does not exclusively overlap with the polarization in the central zone of SMM1-a. 4.2. Poloidal Magnetic Field at Outflow Launching Points The question remains, why is the polarized intensity so Since our analysis presented in Section 4.1 suggests that different between the red- and blueshifted sides of the outflow most of the polarization detected toward the continuum peaks in SMM1-a? An asymmetry is clearly visible in the maps of of our sources cannot be entirely due to self-scattering or polarized intensity and polarization fraction (Figure 7). We see alignment with the radiation field, here we explore the an intense peak of 6% at the base of the one-sided, redshifted properties of the magnetic field as inferred from the detected molecular jet, which may be the cause of the enhanced polarization patterns. We present maps where the polarization polarization efficiency. However, Rodríguez-Kamenetzky et al. orientations have been rotated by 90° to show the orientation of (2016, and references therein) and Dionatos et al. (2014) the magnetic field projected on the plane of the sky. detected ionized and atomic jets on the blueshifted side of Both Serpens SMM1-a and Emb 8(N) seem to exhibit SMM1-a, where we see no signs of enhanced polarization. poloidal magnetic field morphologies in the central 200 au of Hoang et al. (2018) developed a new theory of grain their cores. In Figure 15 we present the magnetic field alignment by mechanical alignment torques (MATs). Unlike orientation in SMM1-a overlaid with the low-velocity blue- the mechanical alignment theorized by Gold (1952), who and redshifted outflow and the EHV redshifted molecular jet, proposed that the major axes of dust grains could become revealing significant correlations between the outflowing aligned parallel to the gas–dust flow, Hoang et al. found that material and the magnetic field. Indeed, the magnetic field dust grains can become aligned with respect to the magnetic seems to present a bipolar structure that follows the molecular field through mechanical torques induced by supersonic gas– emission on both sides of the low-velocity CO outflow. dust drift (in an outflow or a wind from an asymptotic giant Moreover, the redshifted EHV jet axis appears perfectly branch star, for example), yielding a polarization orientation aligned with the magnetic field orientation within the highly that is consistent with a poloidal magnetic field aligned with the polarized zone, i.e., the SE part of the central core of SMM1-a. bipolar outflow axis. Consequently, the EHV redshifted jet may This suggests a connection between the magnetic field in this trigger a significant amount of grain alignment efficiency via 14 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 15. Magnetic field and outflow around SMM1-a from Case-3. Same as Figure 5 (right panel) for the line segments. Same as Figure 11 for the polarized intensity on the color scale and Stokes I contours. The blue contours represent the moment 0 map of the low-velocity blueshifted outflow, constructed by integrating the emission of the CO (J=2→1) from −13 to 4 km s−1. The levels are 5, 6, 7, 8, 10, 12×0.4 Jy beam−1 km s−1, the rms noise level of the moment 0 map. The red contours in the left panel represent the moment 0 map of the low-velocity redshifted outflow constructed by integrating the emission of the CO (J=2→1) from 16 to 21 km s−1. The levels are 6, 7, 8, 9, 10, 11, 12×0.14 Jy beam−1 km s−1, the rms noise level of the moment 0 map. Finally, the red contours in the right panel represent the moment 0 map of the EHV redshifted jet constructed by integrating the emission of the CO (J=2→1) from 40 to 80 km s−1. The levels are 8, 10, 12, 14, 16×0.03 Jy beam−1 km s−1, the rms noise level of the moment 0 map. No blueshifted emission in shown in the right panel, because we do not see any trace of the EHV jet on the blueshifted side. The beam size of the continuum emission (red ellipse) is 0 13×0 13. The green ellipse represents the resolution from the molecular line maps, and measures 0 53×0 45. the MAT mechanism, which can contribute, along with RAT the poloidal component of the magnetic field is dominant over alignment, to the observed intense polarization at the base of any rotation in SMM1-a. At the scales we resolve, we probe the the redshifted molecular jet. According to the RAT theory, material at the disk–envelope interface, where we do not see magnetic grain alignment can easily produce this 6% level of any evidence of a rotationally supported disk. In such a case, a polarization; however, such a high peak in polarization fraction poloidal field aligned with the bipolar outflow axis is an within the otherwise weakly polarized “polarization hole” is expected result from MHD simulations of jets and outflows quite unexpected, and has never been seen before so clearly, to (Fendt 2006; Pudritz et al. 2007; Ramsey & Clarke 2019). A our knowledge. In summary, MATs may be occurring at the possible proof of this scenario is the work by Lee et al. (2018), same time as RATs, but it is not trivial to disentangle the who report linear spectral-line polarization in the SiO relative roles played by these two alignment mechanisms. molecular jet emanating from the Class 0 protostar HH 212. In their observations of the intermediate-mass Class 0 source The shear between the wind from the disk and the ambient OMC-3 MMS 6 with ∼60 au resolution, Takahashi et al. medium may produce a poloidal field at or near the interface, (2019) reported a similar asymmetry in the polarized emission which would be consistent with what is observed here, at the base of the redshifted outflow from that source, where provided that the interface region dominates the polarized they argued that grains are most likely magnetically aligned. It emission. is therefore possible that this type of asymmetry in the Given the ∼50° inclination of the system, the magnetic field polarized intensity could be caused by inclination effects; component orthogonal to the mid-plane Bz is likely to be SMM1-a and OMC-3 MMS 6 have similar estimates for their dominant over the radial component Br of the field, and will inclination angles. While Frau et al. (2011) did see symmetric, be reflected in the projected field morphology in the plane of double-peaked polarization profiles along the minor axes of the sky (Harris et al. 2018). This scenario is consistent with the their models of collapsing, magnetized protostellar cores magnetic field morphology we see in the center of the SMM1-a (which did not take into account optical depth effects), they core, which is predominantly poloidal, but does have a did not see the same types of asymmetries seen here and in significant radial component, leading us to consider whether Takahashi et al. (2019). SMM1 is actually more pole-on than previously thought. The Outflow launching theories have predicted that magnetic estimation of the source inclination made in Yıldız et al. (2015) field lines can be wrapped by the rotation of the inner envelope may not be particularly robust, because they simply examined and the disk (Hennebelle & Ciardi 2009; Kataoka et al. 2012), the shape of the outflow (i.e., whether the outflow lobes were resulting in a toroidal magnetic field morphology that has overlapping, and what is the spatial extent of the low-velocity tentatively been seen in a few observations of protostellar cores line wings) in the red- and blueshifted lobes, in order to infer and disks (e.g., Rao et al. 2014; Alves et al. 2018; Ohashi et al. inclination values within bins of 10°, 30°, and 70°, with an 2018). We do not detect this morphology here, suggesting that uncertainty of 30°. In addition, if SMM1 were close to edge-on, 15 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. the fragments of the SMM1 core, SMM1-b, -c, and -d, should Eastern filament may correspond to a polarized accretion lie close to the equatorial plane of the core, as inferred from the streamer that is infalling onto the central protostar. Filamentary bipolar outflow of SMM1-a. However, SMM1-b lies along the structures seem to be common at scales 1000 au in outflow cavity, which favors a source configuration closer to protostellar cores (Tobin et al. 2010; Lee et al. 2012; Cox pole-on. The two other fragments SMM1-c and SMM1-d, et al. 2018; Sadavoy et al. 2018b; Hull et al. 2019; Takahashi however, are aligned with the equatorial plane projected on the et al. 2019), and many cases exhibit magnetic fields that lie plane of the sky. The fact remains that, assuming SMM1 has an along the structures’ major axes. Accretion-related filamentary intrinsically poloidal magnetic field, a configuration closer to structures have been seen in MHD simulations of magnetized, pole-on could yield the radial component of the magnetic field collapsing cores (e.g., Seifried et al. 2015; Väisälä et al. 2019); that we see projected on the plane of the sky toward the hot detailed comparisons of these types of simulations with high- corino of SMM1-a. resolution observations such as those we present here will paint Finally, the presumed poloidal magnetic field morphology a clearer picture of the nature of these magnetized structures can also result from the magnetic field lines being pinched by and their possible link to protostellar accretion processes. the gravitational collapse of the core, typically known as the Serpens Emb 8(N) presents a very pristine case of a magnetic “hourglass” magnetic field morphology (Girart et al. 2006). field aligned along the outflow cavity walls: indeed, the HRO Magnetic field orientations exhibiting a radial pattern similar to distributions in the top three panels of Figure 12, comparing the what we see in the central core of SMM1-a have been seen magnetic field and both dust and CO moment 0 emission toward the high-mass star-forming region W51 (albeit at larger gradients, all peak at ∼90°. Note that the distribution from the scales and lower, ∼1000 au resolution) in the observations of gradient of the continuum dust emission map (top left panel of Koch et al. (2018), who attribute the pattern to gravitational Figure 12) has some points between 15° and 45° that correspond infall. While the first observations of hourglass-shaped fields to the magnetic field orientations located where the dust-emission were seen at the ∼1000 au scales of protostellar envelopes, it is gradient is no longer dominated by the outflow cavity walls, but not impossible that the detected poloidal magnetic field in the rather by the central core emission (on the redshifted side). The inner ∼200 au core of SMM1-a is affected by this gravitational polarization patterns observed recently in B335 (Maury et al. pinching effect. This effect has been seen, for example, in the 2018) and in BHR 71 IRS2 (Hull et al. 2019) appear quite similar magnetic field morphology probed in the inner few hundred au to the case of Emb 8(N), and have magnetic field lines that are of the B335 protostellar core (Maury et al. 2018). pinched in the equatorial plane and that clearly follow the outflow Note that the collapse of magnetized material can also form a cavity walls. In B335, comparisons with MHD simulations flattened structure known as a “pseudodisk” (Galli & (Maury et al. 2018; Yen et al. 2019) have led to the conclusion Shu 1993). While the curved Western filament in SMM1-a is that this source most likely formed in a magnetically dominated most likely tracing one of the redshifted outflow cavity walls, it environment. Future observations at higher angular resolution of is possible that the N–S oriented Eastern filament is linked to the kinematic properties and polarized emission in the central zone this pseudodisk and traces infalling material. MHD simulations of Emb 8(N) will allow us to better understand the physical showed enhanced infall of material at 45° between the conditions responsible for this magnetic field structure. equatorial plane and the outflow axis of a protostar (Kölligan The large-scale polarization data of this region, observed & Kuiper 2018); this is similar to the orientation of the Eastern filament. Projection effects could explain why these two with the SCUBA polarimeter on the James Clerk Maxwell filaments might be of different natures—such as accretion telescope (Davis et al. 2000; Matthews et al. 2009), indicate an streamers or structures associated with outflow cavity walls overall E–W orientation of the magnetic field in the Serpens (see Section 4.3)—despite being so close to one another. Main molecular cloud. It is interesting to examine the potential influence of this large-scale magnetic field on our results at high spatial resolution. The bipolar outflow of Emb 8(N) has an 4.3. Magnetic Field Lines along Outflow Cavity Walls orientation that is 20° different from purely E–W. Even in the At scales larger than the central 200 au, it is clear that CARMA polarization data (Hull et al. 2014), the recovered outflow activity is strongly linked with the magnetic field in the magnetic field is purely E–W. The results we present here show two protostellar sources SMM1-a and Emb 8(N), because the an overall E–W magnetic field lying along the outflow cavity magnetic field morphology seems to follow the outflow cavity walls (see Figures 1 and 16 for the HRO); however, in some walls from the central 200 au up to ∼1000 au. places the magnetic field deviates from the overall E–W In SMM1, our results at the highest angular resolution orientation and clearly follows the cavity edges, which resolve out most of the polarized emission except the two very indicates that the magnetic field at this scale has been affected bright filaments to the south of SMM1-a. The distribution in the by the outflow activity. As for the redshifted outflow of HROs comparing the dust continuum emission with the SMM1-a, it is relevant to discuss the potential impact of the magnetic field from both filaments (calculated after removing outflow on the magnetic field. We previously identified the central core emission, see Appendix B for details; see the Eastern filament as a potential accretion streamer, whereas Figure 12, bottom row, second from the left) exhibits a the Western filament is very likely the redshifted cavity wall, distribution roughly peaking at 90°, which confirms that the which therefore exhibits an associated magnetic field com- magnetic field lies along the two filamentary structures. As pletely different from the large-scale field. Hull et al. (2017a) mentioned in Section 4.2, the highly polarized Western did not find in their energetic comparisons an obvious filament follows the edge of redshifted integrated CO emission, difference between the magnetic and the outflow energies; indicating that it most likely corresponds to the outflow cavity however, we think this hypothesis of having an outflow-shaped wall. The Eastern filament is clearly shifted from the outflow magnetic field is reasonable, considering how the magnetic axis, and does not overlap with any structure seen in the field orientations at small scales have deviated from the large- moment 0 map of CO tracing the outflow. We propose that the scale field morphology. 16 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 16. Distribution of the magnetic field position angles in our three protostars. We selected the position angles in the central ∼4″ zone around each protostar, and where the polarized intensity was above the 3σP level, where σP is the rms noise level of the polarized intensity map. Serpens Emb 8 seems dominated by polarized emission 4.4. Addressing the Cause of Enhanced Polarization along mainly emanating from the envelope of the protostar, and thus Cavity Walls there is not as clear a correlation with the outflow. Still, the Our results lead us to the question, what are the physical thermal dust emission on the blueshifted side shows some hints conditions necessary to achieve enhanced polarization in the of outflow cavity walls, but with polarization that is a bit offset walls of an outflow cavity? to the east of this zone. To the SE of the Stokes I peak, there is One local condition along outflow cavities that clearly a large area of polarization corresponding to a magnetic field distinguishes them from the rest of the envelope is that they are that is perfectly aligned along the outflow axis, indicating that exposed to a strong, high-energy radiation field emanating from the outflow might have influenced the magnetic field in this the central protostar. Indeed, energetic photons are expected to zone. Emb 8 is similar in some ways to the protostar BHR 71 be generated by the accretion processes in the central protostar, IRS1 (Hull et al. 2019), in the sense that the magnetic field leading to a high flux of UV and X-ray photons that heat the seems to be affected by both the outflow activity and the envelope via the photoelectric effect (Spaans et al. 1995; envelope-based polarized emission, as can be seen in the Stäuber et al. 2004, 2005). As outflow cavities are cleared of northern outflow cavity of BHR 71 IRS1. envelope material by the outflow itself, these energetic photons As mentioned in Section 4.2, the high values of polarization can escape from the central protostar and travel throughout the fraction we encounter near fast-moving material could be cavity. The shape of the cavity edges (e.g., parabolic) allows explained by a contribution from the MAT mechanism to the this high-energy radiation to impinge upon the cavity walls observed polarization, especially on the redshifted side of the (Visser et al. 2012). Indeed, ionized cavity walls have been central core of SMM1-a and in Emb 8(N), which both exhibit very seen toward SMM1-a by Hull et al. (2016), who analyzed free– high-velocity, highly collimated jets. Indeed, if we look at the free emission seen in the VLA observations of Rodríguez- values of polarization fraction in Emb 8(N), which reach 36% on Kamenetzky et al. (2016). the blueshifted side, we see that these detections are located on the The inner surfaces of the cavity walls also see their chemical edge of the CO emission (Figure 2), indicating a possible composition evolve, thanks to molecular photodissociation by UV interaction between the molecular jet and the aligned dust grains photons (Drozdovskaya et al. 2015). Observations of light emitting this polarized emission. With RAT theory it is difficult to hydrides, water, and high-J transitions of CO have led to a better reproduce these high polarization fractions, even with very understanding of the effect of the UV field on the chemistry in elongated aligned dust grains; consequently, it is conceivable that highly irradiated protostellar outflows (van Kempen et al. 2009; both MATs and RATs are occurring in the type of environment Yıldız et al. 2012; Lee et al. 2015; Benz et al. 2016). The physics that we are seeing in Emb 8(N). This hypothesis is reinforced by of UV-irradiated shocks in these same regions has also been the fact that the polarization in the southern cavity wall of the addressed in a number of studies focusing on Herschel observations (Goicoechea et al. 2012; Kristensen et al. redshifted jet of Emb 8(N) is fainter and has a lower polarization 2013, 2017; Mottram et al. 2014, 2017; Karska et al. 2018). fraction precisely where the redshifted CO emission does not Note that Tychoniec et al. (2019) report that the redshifted overlap completely with the polarized dust emission. This suggests molecular jet of Emb 8(N) has a bow shock visible in SiO that we may need to consider outflow evolution (for example, (J = 5  4) emission ∼3000 au from the central source; this precession with respect to the outflow cavity walls) in order to type of shock could contribute to the irradiation of the outflow better understand our observations. Finally, we note that we detect cavity walls and thus to the enhanced polarization we see. values of high polarization fraction toward Serpens Emb 8 where UV-irradiated cavity walls toward Class 0 protostars have there is no intense gas–dust flow, i.e., far away from the bipolar been inferred from observations of UV-tracing molecules outflow (see Figure 10). However, Emb 8 has two little such as c-C3H2 and CCH, which are expected to be seen in companions, Emb 8-b and Emb 8-c, that may play a role in photon-dominated regions (PDRs; Murillo et al. 2018). enhancing the polarization observed in the core. In summary, the These two molecules have been seen toward other highly local environmental conditions in protostars—particularly around irradiated regions such as the Orion bar (Cuadrado et al. their molecular outflows and jets—may be conducive to alignment 2015), the Horsehead nebula (Guzmán et al. 2015), and from both MATs and RATs, thus yielding enhanced levels of dust protoplanetary disks (Kastner et al. 2015; Bergin et al. 2016). polarization. However, our observations are not yet able to Of the Class 0 sources whose cavity walls show both UV- disentangle the roles played by each of these two grain alignment tracing molecules and dust polarization, there are two so far mechanisms. that exhibit an excellent correlation between the polarization 17 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. −2 10 cm (Bohlin et al. 1978). We calculate the gas mass 22 inside a circular area of 300 au in diameter located at a distance of 600 au from the protostar along the SW cavity wall of Emb 8(N), using the following relation between the flux density and the gas mass: Sn d 2 Mgas = , (5 ) kn Bn (Td) where Bν(Td) is the Planck function at the 343.5 GHz frequency of our observations, the distance d=440 pc, the estimated dust temperature Td≈20 K (Hull et al. 2017a), and the opacity κν at a wavelength of 1 mm is 2.74 cm2 g−1 (Ossenkopf & Henning 1994). We assume a gas-to-dust ratio of 100. The flux density along the cavity walls of Emb 8(N) is not constant and slowly decreases the further we look from the central protostar. We selected the SW outflow cavity wall because it is not just the brightest cavity wall, it also presents resolved polarized dust emission that matches very well the integrated intensity of the CCH emission line, as shown in Figure 17. Note, however, that the CCH emission is even brighter at >600 au distances from the central source, suggesting that UV Figure 17. Moment 0 map of CCH (N = 3  2, J = radiation efficiently drives UV-sensitive chemistry even where the 7 2  5 2, F = 4  3) around Serpens Emb 8(N). The black contours column density of the cavity wall has decreased. represent the total intensity (Stokes I) at the following levels: 11, 16, 24, 44, 74, 128, 256×the rms level in the Stokes I dust emission map, where σI=55 We derive the column density in the cavity wall of Emb 8(N) μJy beam−1, from Case-2. vLSR is ∼8.5 km s−1. The grayscale is the moment 0 as follows: map of CCH constructed by integrating emission from 5.4 to 12.8 km s−1. The rms noise level of the moment 0 map is 0.32 mJy beam−1 km s−1. Same as Mgas NH2 = , (6 ) Figure 1 (right) for the line segments, which represent the magnetic field. The m H2 m H A red and blue arrows represent the bipolar outflow directions. The beam size of the continuum emission (red ellipse) is 0 26×0 22, with a position angle of where mH is the mass of a hydrogen atom, μH2 is the mean −64°. The green ellipse represents the resolution of the molecular line maps, and measures 0 46×0 41. molecular weight per hydrogen molecule (μH2=2.8 for gas composed of 71% hydrogen, 27% helium, and 2% metal mass; and the molecular emission. The first is B335; Imai et al. Kauffmann et al. 2008), and A is the area over which we (2016) found CCH and c-C3H2 emission along the cavity derived the flux density. Given a flux density of ∼5.5 mJy walls, which also show enhanced polarization (Maury et al. measured in a 200 au diameter circle located at 600 au from the 2018). The second is Ser-Emb 8(N), discussed above and dust continuum peak of Emb 8(N), we obtain values of column shown in Figure 17.19 density between ∼3.4×1022 cm−2 and ∼2.5×1023 cm−2, The UV radiation field emanating from the material accreting using dust temperatures of 100 K and 20 K, respectively. Note onto the central protostar is a good candidate to explain that the gas temperature is usually much higher than the dust enhanced polarization in the cavity walls via the RAT grain- temperature, as has been seen in models of UV-irradiated alignment mechanism. We see what appear to be thick layers of polarization from magnetically aligned dust grains in the outflow cavity walls (Visser et al. 2012; Drozdovskaya et al. outflow cavity walls of Emb 8(N) (∼200 au thick) and BHR 71 2015); however, the two temperatures are largely decoupled. IRS2 (∼300 au thick). In an attempt to reveal the nature of the Note that these values of the envelope column density at small radiation responsible for aligning the grains in outflow cavity scales are at best lower limits, as a result of the interferometric walls, Hull et al. (2019) calculated the depth to which UV and filtering of the optically thin dust continuum emission. As a longer-wavelength photons could penetrate in order to align result, these values strongly suggest that UV radiation cannot grains to the depth that they see in their observations, and also penetrate deep inside the cavity walls. to align a sufficient quantity of grains to produce the typical Finally, we estimate the penetration depth of longer- polarization fractions that they (and we) detect. wavelength photons impinging on the cavity walls. Ossenkopf Following the procedure of Hull et al. (2019), we estimate & Henning (1994) calculated opacity values κν at different the depth to which UV and longer-wavelength photons can wavelengths for gas number densities of 106 and 108 cm−3,20 penetrate into the walls of the outflow cavities of Serpens Emb and thus we use the κν values from Ossenkopf & Henning 8(N). From their PDR model results, Girart et al. (2005) found (1994) corresponding to gas densities of 106 cm−3. The optical that UV radiation is fully extincted at a visual magnitude of depth is given by the following equation: ∼1, which corresponds a column density between 1021 and s 19 Note that Serpens Emb 8 also shows CCH (M. van Gelder et al. 2019, in tn = ò0 kn rdust ds. (7 ) preparation; ALMA project 2017.1.01174.S, PI E. van Dishoeck) toward much of the polarized emission that is published in Hull et al. (2017b) and shown at higher resolution in Figure 10. However, in this source, which has a chaotic magnetic field morphology, the outflow cavities are not clear. Further work is 20 necessary to tell the full story of the relationship between UV-sensitive Our two derived gas densities for the SW cavity wall of Emb 8(N) are chemistry and the enhanced polarization in Emb 8. 1.24×108 cm−3 at 20 K and 1.74×107 cm−3 at 100 K. 18 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. If we assume constant values of dust opacity κν and dust Eastern filament in SMM1-a, the polarization to the north of mass density ρdust throughout the thickness of the cavity wall, the central core of Emb 8, the polarization detection in the we can derive the path length s at an optical depth of τ=1 by equatorial plane in B335 (Maury et al. 2018)), the “arm-like” using s=1/κνρdust. Note the values of κν in Ossenkopf & structure in OMC-3 MMS 6 (Takahashi et al. 2019), the Henning (1994) are normalized by the dust mass density, and magnetized bridge in IRAS 16293 (Sadavoy et al. 2018b), and thus we multiply these values by ρdust in our calculations.21 For the sharp filamentary structure to the NE of BHR 71 IRS1 (Hull 1 μm photons, the penetration depths assuming temperatures of et al. 2019) are unlikely to be strongly illuminated by UV 20 K and 100 K are ∼1 au and ∼7 au, respectively. For 10 μm radiation from the central protostar, because these structures are photons, the depths are ∼5 au and ∼35 au. For 100 μm not associated with the outflow cavity walls. This leaves us photons, the depths are ∼200 au and ∼1400 au. Finally, for with the question, where does the irradiation and/or anisotropic 1 mm photons, the depths are ∼4200 au and ∼30,000 au. radiation field necessary to align these grains via RATs come RAT theory suggests that dust grains can only be spun-up from? While the answer is not yet clear, comparing dust efficiently by photons whose wavelengths are comparable to polarization observations with synthetic observations of MHD the dust grain size (Lazarian & Hoang 2007). Assuming this is models (see Valdivia et al. 2019) will enable a deeper true, and given that most short-wavelength (10 μm) photons understanding of the dust grain populations in the innermost do not penetrate to depths larger than a few tens of au, our regions of Class 0 envelopes. observations suggest that a population of large, >10 μm sized dust grains should be present in the cavity walls of Serpens Emb 8(N) to explain the ∼300 au thickness of the polarized 5. Conclusions emission. Note that, in their study of the polarized outflow We have presented ALMA dust polarization observations of cavities of BHR 71 IRS2, Hull et al. (2019) also find that only the three Class 0 protostars Serpens SMM1, Emb 8(N), and long-wavelength (i.e., mid- to far-infrared) photons can Emb 8, at spatial scales from ∼150 au down to ∼40 au. The penetrate to depths similar to those where we see polarization conclusions we draw are as follows. in the cavity walls of Serpens Emb 8(N). Hull et al. conclude that this scenario of a “thick” layer of 1. Serpens Emb 8(N) exhibits strong polarization along its aligned dust grains in BHR 71 is more likely than the scenario outflow cavity walls, with magnetic field orientations where the polarization is produced by an extremely thin layer aligned with the major axis of the dust emission. of grains aligned only by UV/optical photons. However, even 2. In Emb 8(N), there is an obvious anticorrelation between given the likelihood of the thick-wall scenario, the numbers we areas where we see polarized dust emission and emission calculate above are still an upper limit to the penetration depth. from warm dense-gas tracers such as C18O and 13CS, and This is because the “thick” wall we see will have its thickness regions of cold, dense gas traced by DCO+. increased, at least to a small degree, by the projection of the 3. Serpens SMM1-a presents several interesting polarized curved cavity wall onto the plane of the sky. emission zones. To the south of the central ∼200 au core Beyond outflow cavity walls, the high polarization fractions are two polarized filaments with magnetic field orienta- observed at small radii, where dense protostellar envelopes are tions along the filamentary structure. We identified one optically thick to short-wavelength photons, require that grains filament as being the cavity wall of the redshifted be large (>10 μm) in order to be to aligned via B-RATs with outflow; we speculate that the other one may be a the local (sub)millimeter radiation field, as shown for the first potential infalling accretion streamer. time in Valdivia et al. (2019). While dust is known to grow to 4. The inner cores of SMM1-a and Emb 8(N) exhibit such sizes in circumstellar disks (e.g., Pérez et al. 2012; Testi poloidal magnetic fields that are perfectly aligned with et al. 2014; Stephens et al. 2017; Hull et al. 2018), this degree the bipolar outflows; no hints of toroidal magnetic fields of grain growth at large (∼500–1000 au) distances from the wrapped by core rotation have been found. central source is unexpected, although a recent work by 5. The polarized intensity map of the inner core of SMM1-a Galametz et al. (2019) has found that large grains (millimeter- presents a clear asymmetry, exhibiting an intense sized) might be present in Class 0 protostellar envelopes. polarized spot that peaks at 6% in polarization fraction. However, as was proposed in Wong et al. (2016), outflow This asymmetry in the polarized emission may be linked activity may be sufficient to transport large dust grains from the to the fact we only see an EHV molecular jet on the innermost part of the core (where the large grains are most redshifted side of the bipolar outflow, which could likely formed) to the envelope, where we can detect their contribute to the high efficiency of the grain alignment in presence via the dust polarization we observe in outflow cavity this zone via a combination of the MAT and RAT walls (in the case of our calculations, at 600 au from the center alignment mechanisms. of the protostar). 6. We propose that the enhanced polarization seen along Because the average size of the dust grains probed changes outflow cavity walls in Class 0 protostars is caused by with observing wavelength, it would be interesting to perform irradiation of the cavities, which have been cleared of longer-wavelength measurements of the polarized features we material by the outflow activity. Several studies (includ- detect that are presumably not being irradiated by a strong UV ing the spectral-line observations of Emb 8(N) that we field. Such observations would allow us to characterize the show here) have reported UV-tracing molecules along environmental conditions (in terms of dust grain population and outflow cavity walls, indicating chemistry driven by UV radiation field) necessary for RATs to occur. Indeed, the irradiation in these regions. However, to align dust grains deep within the walls of the cavity and to cause the high 21 The chosen κ values from Ossenkopf & Henning (1994) are the following: polarization fraction that we see, longer-wavelength (i.e., κ1 μm=1.18×104 cm2 g−1, κ10 μm=2.35×103 cm2 g−1, κ100 μm= mid- to far-infrared) photons may be necessary, which 5.92× 101 cm2 g−1, and κ1 mm=2.74 cm2 g−1. would most effectively align large (>10 μm sized) grains. 19 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. This challenges our current understanding of grain growth in Class 0 sources. 7. High polarization fractions are seen in highly embedded areas of Class 0 sources that are unlikely to be irradiated by the central protostar. These regions include the possible accretion streamer to the south of SMM1-a, the polarized emission we see to the north of the central core of Emb 8, and magnetized filamentary structures detected toward several other sources. The question of what would trigger the enhanced polarization in these regions—which are highly embedded, and far away from any obvious source of strong irradiation—remains open. More work is needed to better understand the relationships among the radiation field, chemistry, and dust-grain alignment in young protostellar sources. Understanding better the environmental conditions that trigger enhanced polarization in embedded protostars will require working with next-generation full-polarization radiative transfer such as POLARIS (Brauer et al. 2016; Reissl et al. 2016, 2017); this future work will Figure 18. Schematic view of Serpens SMM1. The color scale is the total ultimately improve our understanding of the role played by the intensity (Stokes I) of the thermal dust emission from Case-1, shown when I>3σI, from the combination of data sets A, B, and C; see Table 3 Case-1. magnetic field in protostellar evolution. σI=0.57 mJy beam−1. The peak in the total intensity is 203 mJy beam−1. The red ellipse in the lower left corner represents the beam size, measuring The authors acknowledge the support of the North American 0 15×0 14, with a position angle of −48°. 5. ALMA Science Center. V.J.M.L.G. and C.L.H.H. acknowl- edge the ESO Studentship Program, and the guidance and Appendix B support of Eric Villard. C.L.H.H. acknowledges the support of Gradient Maps Derived for the Histograms in SMM1 and both the NAOJ Fellowship as well as JSPS KAKENHI grant Emb 8(N) 18K13586. This work has received support from the European Research Council (ERC Starting Grant MagneticYSOs with The histograms presented in Figure 12 show the distributions grant agreement no. 679937). J.M.G. acknowledges the support of relative orientations between the magnetic field and the of the Spanish MINECO AYA2017-84390-C2-R grant, and the intensity gradients of both the continuum dust emission and the Joint ALMA Observatory Visitor Program. Astrochemistry in integrated CO (J=2→1) moment 0 emission, within the Leiden is supported by the Netherlands Research School for Serpens SMM1-a and Emb 8(N) protostellar cores. Astronomy (NOVA), by a Royal Netherlands Academy of Arts To produce the histograms, we select the magnetic field and Sciences (KNAW) professor prize, and by the European orientations in pixels with emission over the 3σP threshold, Union A-ERC grant 291141 CHEMPLAN. L.E.K. acknowl- where σP is the rms noise value of the polarized intensity map. edges a research grant (19127) from VILLUM FONDEN. In the case of the SMM1-a core, the emission from its Z.-Y.L. is supported in part by NASA 80NSSC18K1095 and neighbors, i.e., SMM1-b, -c, and -d, is removed by hand in NSF AST-1716259 and AST-1815784. This paper makes order to take into account only the emission related to SMM1- use of the following ALMA data: ADS/JAO.ALMA# a. We consider separately the emission emanating from the 2016.1.00710.S, ADS/JAO.ALMA#2013.1.00726.S, ADS/ central inner core of SMM1-a (applying a threshold of 25σI, JAO.ALMA#2015.1.00768.S, and ADS/JAO.ALMA#2017. where σI is the rms noise level of the Stokes I map) and the 1.01174.S. ALMA is a partnership of ESO (representing its emission emanating from the dust cavity walls and filaments member states), NSF (USA) and NINS (Japan), together with (where the selected emission is between 7σI and 25σI). As for NRC (Canada), MOST and ASIAA (Taiwan), and KASI Emb 8(N), we just select the value above 11σI in the total (Republic of Korea), in cooperation with the Republic of Chile. intensity map, in order to focus on the central core and to avoid The Joint ALMA Observatory is operated by ESO, AUI/ considering any emission from the large-scale dense, cold NRAO, and NAOJ. The National Radio Astronomy Observa- filament described in Section 3.1. Concerning the blue- and tory is a facility of the National Science Foundation operated redshifted moment 0 maps, we use the same ranges of under cooperative agreement by Associated Universities, Inc. integration as the those presented in Figures 2 and 6, applying Facility: ALMA. a threshold of three times the rms noise level of the moment Software:APLpy, an open-source plotting package for 0 maps. Python hosted athttp://aplpy.github.com (Robitaille & After calculating the gradient, we apply a selection on the Bressert 2012). CASA (McMullin et al. 2007). Astropy gradient values in order to focus on the relationship between (Astropy Collaboration et al. 2018). the magnetic field and the emission gradients in regions of high gradient. This selects, for example, the clear outflow cavity walls in the dust emission and moment 0 maps, the filamentary Appendix A structures of SMM1-a, and the central inner cores of our Scheme of the SMM1 Core protostars. In order to select regions of high gradient, we A schematic view of the Serpens SMM1 core is shown in consider only gradients >1/100×(peak – rms) for the Figure 18. moment 0 maps, and >1/1000×(peak – rms) for the dust 20 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 19. Gradient maps in Serpens Emb 8(N). The black contours trace the dust continuum from Case-2 at 11, 16, 24, 44, 74, 128, 256×σI in the total intensity map of dust emission, where σI=55 μJy beam−1. Top: the color scale represents the gradient within the central zone of Emb 8(N), derived using the total intensity map of dust emission. The gradients are computed where the emission is >11σI in this central region. Bottom left: gradient derived within the moment 0 map of the blueshifted CO emission integrated from −53 to 0 km s−1. Bottom right: gradient derived within the moment 0 map of the redshifted CO emission integrated from 15 to 40 km s−1. The noise value in both moment 0 maps is 0.1 Jy beam−1 km s−1; the gradients are calculated where the integrated CO emission is >3×this value. All gradients are displayed where where the gradient is >1/100×(peak – rms) in the moment 0 maps, and >1/1000×(peak – rms) in the dust emission maps. 21 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. Figure 20. Gradient maps in Serpens SMM1-a. The black contours trace the dust continuum from Case-1 at 8, 12, 20, 32, 64, 128, 256×σI in the total intensity map of dust emission, where σI=0.57 mJy beam−1. Top left: the color scale represents the gradient derived within the central zone of SMM1-a and using the total intensity map of dust emission. The gradients are computed where the emission is >25σI in this central region. Top right: gradient derived within the filaments and outflow cavities of SMM1-a and using the total intensity map. The gradients are computed where the emission is between 7σI and 25σI; we also remove the emission from the other cores SMM1-b, -c, and -d. Bottom left: gradient derived within the moment 0 map of the blueshifted CO emission integrated from −13 to 4 km s−1. Bottom right: gradient derived within the moment 0 map of the redshifted CO emission integrated from 10.5 to 30 km s−1. The noise values in the blue- and redshifted CO moment 0 maps are 0.58 and 0.43 Jy beam−1 km s−1, respectively. The gradients are calculated where the integrated CO emission is >3×these values. All gradients are displayed where the gradient is >1/100×(peak – rms) in the moment 0 maps, and >1/1000×(peak – rms) in the dust emission maps. Emission in the central zone of SMM1-a and from SMM1-b has been removed in order to focus on the outflow cavities of SMM1-a. 22 The Astrophysical Journal, 885:106 (24pp), 2019 November 10 Le Gouellec et al. continuum maps. 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