A&A 664, A171 (2022) Astronomy https://doi.org/10.1051/0004-6361/202243414 & © C. Vastel et al. 2022 Astrophysics FAUST V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin C. Vastel1 , F. Alves2 , C. Ceccarelli3 , M. Bouvier3 , I. Jiménez-Serra4 , T. Sakai5 , P. Caselli2 , L. Evans1,6 , F. Fontani6 , R. Le Gal3,12 , C. J. Chandler7 , B. Svoboda7 , L. Maud8 , C. Codella3,6 , N. Sakai9 , A. López-Sepulcre3,12 , G. Moellenbrock7 , Y. Aikawa13 , N. Balucani14 , E. Bianchi3 , G. Busquet33 , E. Caux1 , S. Charnley15 , N. Cuello3 , M. De Simone3 , F. Dulieu16 , A. Durán17 , D. Fedele6 , S. Feng18 , L. Francis20,21 , T. Hama22 , T. Hanawa23 , E. Herbst24 , T. Hirota19 , M. Imai10 , A. Isella25 , D. Johnstone20,21 , B. Lefloch3 , L. Loinard17,26 , M. Maureira2 , N. M. Murillo9 , S. Mercimek6 , S. Mori34 , F. Menard3 , A. Miotello8 , R. Nakatani9 , H. Nomura19 , Y. Oba27 , S. Ohashi9 , Y. Okoda10 , J. Ospina-Zamudio3 , Y. Oya10,11 , J. E. Pineda2 , L. Podio6 , A. Rimola28 , D. Segura Cox2 , Y. Shirley29 , L. Testi6,8 , S. Viti30,31 , N. Watanabe27 , Y. Watanabe32 , A. Witzel3 , C. Xue24 , Y. Zhang9 , B. Zhao2 , and S. Yamamoto10,11 (Affiliations can be found after the references) Received 24 February 2022 / Accepted 17 June 2022 ABSTRACT Aims. Methanol is a ubiquitous species commonly found in the molecular interstellar medium. It is also a crucial seed species for the build-up of chemical complexity in star forming regions. Thus, understanding how its abundance evolves during the star formation process and whether it enriches the emerging planetary system is of paramount importance. Methods. We used new data from the ALMA Large Program FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar protostars) to study the methanol line emission towards the [BHB2007] 11 protobinary system (sources A and B), where a complex structure of filaments connecting the two sources with a larger circumbinary disc has previously been detected. Results. Twelve methanol lines have been detected with upper energies in the [45–537] K range along with one 13 CH3 OH transi- tion and one methyl formate (CH3 OCHO) line blended with one of the methanol transitions. The methanol emission is compact (FWHM ∼ 0.500 ) and encompasses both protostars, which are separated by only 0.200 (28 au). In addition, the overall methanol line emission presents three velocity components, which are not spatially resolved by our observations. Nonetheless, a detailed analysis of the spatial origin of these three components suggests that they are associated with three different spatial regions, with two of them close to 11B and the third one associated with 11A. A radiative transfer analysis of the methanol lines gives a kinetic temperature of [100–140] K, an H2 volume density of 106 –107 cm−3 and column density of a few 1018 cm−2 in all three components with a source size of ∼0.1500 . Thus, this hot and dense gas is highly enriched in methanol with an abundance as high as 10−5 . Using previous continuum data, we show that dust opacity can potentially completely absorb the methanol line emission from the two binary objects. Conclusions. Although we cannot firmly exclude other possibilities, we suggest that the detected hot methanol is resulting from the shocked gas from the incoming filaments streaming towards [BHB2007] 11A and B, respectively. Higher spatial resolution observations are necessary to confirm this hypothesis. Key words. astrochemistry – radiative transfer – techniques: interferometric – line: identification – ISM: abundances – ISM: molecules 1. Introduction their presence in the gas has been explained by non-thermal desorption processes such as photodesorption from UV pho- The rich chemistry observed in solar-type protostars is triggered tons (Bertin et al. 2016), chemical desorption (Minissale et al. at earlier stages of the protostellar evolution (e.g. Ceccarelli 2016), sputtering caused by a gentle shock (Flower & Pineau des et al. 2007; Caselli & Ceccarelli 2012). Methanol (CH3 OH) is Forets 1995), or cosmic-ray induced chemistry (Shingledecker a key ingredient in the enhancement of this chemical complex- et al. 2018; Wakelam et al. 2021) and the reactive desorption of ity from prestellar cores to protoplanetary discs (Vastel et al. methanol followed by gas-phase reactions (Vasyunin et al. 2017). 2014; Bizzocchi et al. 2014; Maret et al. 2005; Walsh et al. iCOMs have also been found at a later stage: the Class 0/I proto- 2016; Spezzano et al. 2020, 2022; Punanova et al. 2022). It stars (André et al. 1993). Here, the central protostar is embedded is the seed from which interstellar complex organic molecules in a dense envelope as the parental dense core undergoes gravita- (iCOMs: Ceccarelli et al. 2017) sprout (e.g. Charnley et al. 1992; tional collapse. The iCOMs are observed in the lukewarm region Garrod et al. 2008; Balucani et al. 2015; Taquet et al. 2016; where the dust temperature is between the CO and H2 O subli- Aikawa et al. 2020; Jin & Garrod 2020). They have been detected mation temperatures (∼30–100 K (e.g. Jaber et al. 2014) and in towards prestellar cores (Bacmann et al. 2012; Vastel et al. 2014; the compact hot corino region where the dust temperatures are Jiménez-Serra et al. 2016, 2021; Scibelli & Shirley 2020), where higher than ∼100 K and densities are larger than ∼107 cm−3 (e.g. A171, page 1 of 17 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is published in open access under the Subscribe-to-Open model. Subscribe to A&A to support open access publication. A&A 664, A171 (2022) Ceccarelli et al. 2000b; Cazaux et al. 2003; Bottinelli et al. 2004; surrounded by a fainter, elongated structure tracing the inner Jørgensen et al. 2016; Yang et al. 2021; Chahine et al. 2022). component of the core envelope. The spectral line data reveal a Finally, iCOMs are also detected in protoplanetary discs (Öberg bipolar outflow launched at the edge of the inner disc at a radial et al. 2015, 2021; Favre et al. 2018; Lee et al. 2019). distance of 90–130 au from the central source. The disc is rota- The gas phase is therefore chemically enriched in iCOMs, tionally supported, with Keplerian rotation revealed by H2 CO due to either direct release from dust mantles or gas phase for- lines (Alves et al. 2017, 2019). At much higher angular reso- mation from simpler molecules released from the mantles. Two lution (∼0.0400 ), Alves et al. (2019) uncovered a young binary possibly distinct classes of solar Class 0/I protostars have been protostar system embedded in circumstellar discs that have radii found: the hot corinos (Ceccarelli et al. 2007) and the warm car- of 2–3 au. The protostars are named [BHB2007] 11A (hereafter bon chain chemistry sources (WCCC: Sakai & Yamamoto 2013), 11A) for the northern source (α (2000) = 17h11m23.097s, δ although intermediate situations are also possible (e.g. Yang (2000) = –27◦ 240 32.8500 ) and [BHB2007] 11B (hereafter 11B) et al. 2021). An intermediate (hybrid) source was also found by for the southern source (α (2000) = 17h11m23.09s, δ (2000) = Oya et al. (2017). –27◦ 240 3300 ). These systems are surrounded by a complex fil- Since the chemical composition of the planetary systems amentary structure (the so-called streamers) connecting to the depends on the chemical evolution starting from the earliest larger circumbinary disc. We note that streamers have recently phases of the protostar formation, it is crucial to understand been seen in other more evolved objects (Alves et al. 2020) and how the molecular complexity is transferred from the large-scale modelled as mostly free-falling (Pineda et al. 2020; see Pineda envelope (a few thousand au) to the small-scale structures of the et al. 2022, for a review). The binary mass ratio is &1, since the protostellar system and how it is maintained in the inner disc 11A disc is slightly more massive (of the order of a Jupiter mass) system (≤50 au). than the 11B disc and the system has a projected separation of In this context, the ALMA (Atacama Large Millime- 28 au (∼0.200 ). Source 11B is located between high-velocity com- ter/submillimeter Array) Large Program FAUST (Fifty AU ponents of the CO emission (VLSR < −1.5 km s−1 and VLSR > STudy of the chemistry in the disc/envelope system of Solar-like 9 km s−1 ), which is interpreted as enhanced gas accretion towards protostars)1 is designed to survey the chemical composition of a 11B from the circumbinary disc. Both protostars are accreting sample of 13 Class 0/I protostars on a scale of about 50–1000 au from their individual circumstellar discs as inferred from cen- (all sources have a distance lower than 250 pc). The description timetric continuum emission observed from both sources. The of the project is reported in Codella et al. (2021). The selected radio data are consistent with free-free emission from ionised sources represent the protostellar chemical diversity observed at jets launched by the disc-star system (Alves et al. 2019). The large scales and have been observed in three frequency setups dynamical mass of the protobinary system has been estimated chosen to study both continuum and line emission from specific as an upper limit from the Keplerian model of the H2 CO data molecules: 85.0–89.0 GHz, 97.0–101.0 GHz, 214.0–219.0 GHz, with a value of 2.25 ± 0.13 M . 229.0–234.0 GHz, 242.5–247.5 GHz, and 257.5–262.5 GHz. The In this paper, we report high-J methanol emission FAUST survey provides a uniform sample in terms of frequency (45 K < Eup < 537 K ) from [BHB2007] 11 obtained in the setting, angular resolution, and sensitivity. We report the first framework of the FAUST collaboration observed in a ∼0.400 results obtained towards the [BHB2007] 11 Class 0/I protostar beam. These data unveil the hot corino nature of the source seen system focused on the most simple iCOM, methanol. over multiple velocity components. From these data, we derive the hot gas properties (column density and temperature) for each velocity component. 2. The [BHB2007] 11 protobinary system The [BHB2007] 11 source was originally identified as a single object in the Barnard 59 (B59) molecular cloud, which is located 3. Observations at a distance of 163 ± 5 pc (Gaia second data release: Dzib et al. 2018). B59 hosts a proto-cluster of low-mass young stellar [BHB2007] 11 was observed with ALMA (FAUST Large Pro- objects (YSOs) at different evolutionary stages as first shown by gram 2018.1.01205.L) with two frequency setups in Band 6 Onishi et al. (1999) using the 1–0 transition of 12 CO, 13 CO, and (setups 1 and 2) and one frequency setup in Band 3 (setup 3). C18 O with the NANTEN telescope with a 2.70 beam. Follow- The data exploited here were acquired between 2018 and 2020 ing this, the proto-cluster was extensively observed in infrared using the 12-m and 7-m arrays for setups 1 and 2, and the 12-m bands (Brooke et al. 2007; Forbrich et al. 2009; Covey et al. array for setup 3. The baseline length in the configurations of the 2010; Román-Zúñiga et al. 2010; Sandell et al. 2021), ammo- 12-m array for setups 1 and 2 (C43-4 and C43-1) ranges from nia (NH3 ) emission (Rathborne et al. 2008; Redaelli et al. 2017), 15 m to 1.3 km, and the baseline length in the configuration of and X-rays (Forbrich et al. 2010). The protostellar nature of the the 7-m array ranges from 8.9 m to 48.9 m. For setup 3, the base- youngest and most embedded member of the cluster, [BHB2007] line length in the configurations of the 12-m array (C43-6 and 11 was confirmed by several studies reporting bipolar outflows C43-3) ranges from 15 m to 3 km. The observations were centred (Riaz et al. 2009; Duarte-Cabral et al. 2012), hints of dynam- at α (2000) = 17h11m23.125s, δ (2000) = –27◦ 240 32.8700 . J2056- ical infall (Hara et al. 2013; Alves et al. 2017), and a spectral 4714 and J1427-4206 were used as bandpass calibrators for setup energy distribution (SED) consistent with a Class 0/I protostar 2 (12-m array), and J1700-2610 was used as phase calibrator and with bright far-infrared emission and Lbol ∼ 4.4 L (Sandell et al. flux calibrator. J1427-4206 and J2056-4714 were used as band- 2021). pass calibrator and flux calibrator for setup 1 (12-m array), and Alves et al. (2017) used ALMA in its extended configura- J1700-2610 was used as phase calibrator. tion to observe the dust continuum emission at 1.3 mm and The data were reduced in the Common Astronomy Soft- molecular transitions of CO, C18 O and H2 CO lines (angular res- ware Applications package (CASA) version 5.6.1 (McMullin olution of 0.2200 ). The dust distribution reveals an ∼180 au disc et al. 2007) using a modified version of the ALMA calibra- tion pipeline and an additional in-house calibration routine 1 http://faust-alma.riken.jp to correct for the system temperature and spectral line data A171, page 2 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin Table 1. Observed spectral windows in the observed FAUST data. 0.6 TM 1 0.5 TM 2 7M Spectral window Setup 1 Setup 2 Setup 3 Amplitude (Jy) 0.4 (GHz) (GHz) (GHz) 0.3 spw 25 216.113 243.916 93.181 0.2 spw 27 216.563 244.049 94.405 0.1 spw 29 217.105 244.936 95.000 0.0 spw 31 217.822 245.606 107.014 0 50000 100000 150000 200000 250000 300000 spw 33 218.222 246.700 108.040 uv distance (k ) spw 35 218.440 260.189 104.239 spw 37 219.560 260.255 105.799 0.6 spw 39 219.949 261.687 TM 1 0.5 TM 2 spw 41 231.061 262.004 7M Amplitude (Jy) 0.4 spw 43 213.221 257.896 spw 45 231.322 258.256 0.3 spw 47 231.410 258.549 0.2 spw 49 (a) 233.796 259.035 0.1 0.0 (a) Notes. Continuum window. 0 50000 100000 150000 200000 250000 300000 uv distance (k ) Fig. 1. Amplitude versus uv-distance plots before (top) and after (below) centred on the frequencies listed in Table 1. The bandwidth is self-calibration. 59 MHz for 12 m (62 MHz for 7-m), except for the continuum window with a 1.875 GHz (2 GHz for 7-m) bandwidth. Spectral line imaging was performed with the CASA3 package, while the normalisation2 . Self-calibration was carried out using carefully- data analysis was performed using the IRAM/GILDAS4 package selected, line-free continuum channels for each configuration. as well as the CASSIS5 package. The complex gain corrections derived from the self-calibration were then applied to all channels in the data, and the contin- uum model derived from the self-calibration was then subtracted 4. Line identification from the data to produce continuum-subtracted line data. A self- The line identification was performed using the CASSIS soft- calibration technique was also used to align both amplitudes ware that connects to the JPL and CDMS databases. This is and phases across the multiple configurations to correct for any a standalone software, written in Java, freely delivered to the remaining offsets in the flux density scales and to correct for community to help with visualising, analysing and modelling systematic position offsets that may be caused by either source observations from ground or space-based observatories. CAS- proper motion or other system issues. SIS has been developed at IRAP since 2005 and is part of the For each setup, iterative-phase self-calibration with a very OVGSO6 data centre that is aimed at promoting the Virtual long solution interval was then used for the position align- Observatory (VO) technology. CASSIS displays any spectrum ment across multiple configurations. In addition, amplitude (ASCII, FITS, or GILDAS/CLASS format or the result from offsets between datasets needed to be corrected. To do this, a the query to any Simple Spectral Access Protocol (SSAP) VO model derived from a very deeply cleaned and masked image or EPN-TAP7 VO service from IVOA8 registries) and identifies was used to obtain amplitude corrections per execution block. atomic and molecular species through its link to the databases The dynamic range of the resulting self-calibrated image was such as CDMS (Müller et al. 2005) or JPL (Pickett et al. 1998) improved by a factor of three for setup 1, without reduction in via an SQLite database, or via a direct access to VAMDC9 for the peak flux density of the source. Figure 1 shows an amplitude any available spectroscopic database. versus uv-distance plot of the data from the different configura- The systemic velocity of [BHB2007] 11 is VLSR = 3.6 km s−1 tions before and after this amplitude alignment for setup 1 in the with respect to the local standard of rest (Onishi et al. 1999). continuum spectral window (least noisy). The data were aver- We identified and detected twelve methanol transitions in the aged per baseline across the entire dataset. The improvement in observed spectral windows of the three spectral setups (dust peak the dynamic range was less significant for setup 2 (around 10%), emission) and only one 13 CH3 OH transition (234 GHz but not at because for setup 2, the amplitude scale of the 12-m array (TM 1 94.4 GHz where the rms reached 0.9 K). No deuterated counter- and TM 2) was much better aligned coming out of the calibration parts such as CH2 DOH at 261.7 GHz have been detected. We list, pipeline and per-configuration self-calibration process than they in Table 2, the spectroscopic parameters of the detected transi- were for setup 1. tions using the CDMS database. The spectral resolutions are also The resulting continuum-subtracted line cube was made quoted as δV (km s−1 ) for each detected transition. The methanol using a Briggs robust parameter of 0.5, and we obtained a transitions cover a broad range in energy from Eup ∼ 45 to 537 K. synthesised beam with a 0.4400 × 0.3700 position angle (PA) = −82◦ at 261.687 GHz, and a 0.4100 × 0.3900 PA = 47◦ at 218 GHz. 3 https://casa.nrao.edu/ We estimate an overall systematic uncertainty of 10% for each 4 http://www.iram.fr/IRAMFR/GILDAS 5 dataset. The observed spectral windows (also called spw) are http://cassis.irap.omp.eu 6 https://ov-gso.irap.omp.eu/ 2 https://help.almascience.org/kb/articles/what-errors- 7 Europlanet-Table Access Protocol. could-originate-from-the-correlator-spectral- 8 International Virtual Observatory Alliance. normalization-and-tsys-calibration 9 http://www.vamdc.org/ A171, page 3 of 17 A&A 664, A171 (2022) Table 2. Spectroscopic parameters of the methanol transitions (using the CDMS database) detected towards [BHB2007] 11 and results from the Gaussian fits. CH3 OH Frequency QN Eup Ai j Beam, PA rms δV T mb FWHM VLSR (MHz) (K) (s−1 ) 00 00 ◦ × , (mK) (km s−1 ) (K) (km s−1 ) (km s−1 ) 218440.063(0.013) 4−2,3 –3−1,2 E 45.46 4.69 × 10−5 0.41 × 0.39, 47 158 0.17 1.57(0.06) 3.2(0.1) –2.0 1.46(0.04) 6.1(0.3) 2.8 1.16(0.04) 5.4(0.2) 9.9 243915.788(0.004) 51,4 –41.3 A 49.66 5.97 × 10−5 0.47 × 0.40, –84 59 0.15 2.01(0.04) 3.2(0.1) –2.0 1.67(0.02) 5.9(0.1) 2.8 1.41(0.02) 4.9(0.1) 9.9 247228.587(0.013) 42,2 –51,5 A 60.92 2.12 × 10−5 0.46 × 0.39, –80 34 1.18 1.49(0.06) 2.9(0.2) –2.0 1.01(0.04) 5.6(0.0) 2.8 0.99(0.04) 5.0(0.0) 9.9 234683.37(0.012) 42.3 –51,4 A 60.92 1.87 × 10−5 0.38 × 0.34, 70 74 0.62 1.32(0.07) 2.9(0.1) –2.0 0.92(0.04) 6.2(0.5) 2.8 0.92(0.04) 5.3(0.3) 9.9 234698.519(0.015) 54.2 –63,3 E 122.72 6.34 × 10−6 0.38 × 0.34, 70 73 0.63 0.85(0.05) 3.1(0.2) –2.0 0.53(0.03) 5.1(0.5) 2.8 0.56(0.03) 5.7(0.5) 9.9 232945.797(0.012) 103,7 –112,9 E 190.37 2.13 × 10−5 0.38 × 0.34, 70 79 0.62 1.12(0.07) 3.1(0.2) –2.0 0.84(0.04) 5.9(0.6) 2.8 0.91(0.04) 5.6(0.4) 9.9 247161.95(0.015) 16−2,15 –153,13 E 338.14 2.57 × 10−5 0.46 × 0.39, –80 29 1.18 1.00(0.05) 3.0(0.2) –2.0 0.62(0.03) 5.6(0.0) 2.8 0.72(0.03) 5.0(0.0) 9.9 261704.409(0.015) 12−6,7 –13−5,8 E 359.77 1.78 × 10−5 0.44 × 0.37, –82 104 0.14 1.41(0.05) 3.9(0.2) –2.0 0.53(0.05) 3.6(0.5) 2.8 0.81(0.04) 5.2(0.3) 9.9 233795.666(0.012) 183,15 –174,14 A 446.58 2.20 × 10−5 0.38 × 0.34, 70 89 0.63 0.67(0.09) 2.6(0.4) –2.0 0.48(0.05) 6.3(1.2) 2.8 0.51(0.06) 4.4(0.7) 9.9 247610.918(0.014) 183,15 –182,16 A 446.58 8.29 × 10−5 0.46 × 0.39, –80 49 1.18 1.16(0.06) 2.9(0.2) –2.0 0.67(0.04) 5.6(0.0) 2.8 0.82(0.04) 5.0(0.0) 9.9 246873.301(0.015) 193,16 –192,17 A 490.65 8.27 × 10−5 0.46 × 0.39, –80 19 1.18 1.01(0.06) 2.7(0.2) –2.0 0.62(0.04) 5.6(0.0) 2.8 0.73(0.04) 5.0(0.0) 9.9 246074.605(0.016) 203,17 –202,18 A 537.04 8.25 × 10−5 0.46 × 0.39, –80 29 1.19 0.93(0.08) 3.2(0.3) –2.0 0.65(0.05) 5.6(0.0) 2.8 0.66(0.05) 5.0(0.0) 9.9 13 CH3 OH 234011.58(0.05) 51,5 –41,4 A+ 48.25 5.27 × 10−5 0.38 × 0.34, 70 76 0.62 0.41(0.05) 2.9(0.4) –2.0 0.47(0.03) 5.6(0.0) 2.8 0.30(0.03) 5.0(0.0) 9.9 Notes. The spectral resolution is indicated as δV. The 1σ errors are quoted in parentheses for the main beam temperature, the full width at half maximum, and velocity in the standard of rest. The spectra were extracted over the region above 4σ in the integrated intensity map for each transition. 5. Results H2 CO lines, where low upper energy level (Eup ) transitions trace envelope rotation and infall, misaligned with the disc rotation 5.1. Dust emission seen in high-Eup transitions (Alves et al. 2017, 2018). At the Figure 2 shows the dust continuum maps obtained in the three present resolution, the proto-binary system reported by Alves setups (ACA+12 m combined). The dust distribution is consis- et al. (2019) is not resolved; however, faint extended emission tent with the previous maps of Alves et al. (2017), showing oriented north-west-south-east is detected (at a 3σ level) in the bright emission from the circumbinary disc (inclined ∼56◦ with ∼1-mm bands (setups 1 and 2) beyond the envelope. The radial respect to the line of sight) at the centre of the elongated enve- profile of the continuum emission, taken from increasing ellipti- lope emission. The apparent misalignment between the envelope cal annuli oriented at a position angle of 167◦ east of north (the major axis and the disc major axis suggests a warped disc. This orientation of the inner envelope), is displayed in Fig. 3. The is consistent with the velocity structure previously revealed by contribution from the disc is distinguishable from the fainter A171, page 4 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin Intensity (Jy beam 1) 0.0001 0.0010 0.0100 0.0500 = 100.6 GHz = 225.4 GHz = 252.9 GHz -27°24'30" Declination (J2000) 32" 34" 36" 500 au 17h11m23.3s 23.0s 22.8s 17h11m23.3s 23.0s 22.8s 17h11m23.3s 23.0s 22.8s Right Ascension (J2000) Fig. 2. Dust continuum maps obtained in each setup. The rms noise level is 50 µJy beam−1 for Band 3 observations (setup 3, left panel), 90 µJy beam−1 for setup 1 in Band 6 (middle panel), and 70 µJy beam−1 for setup 2 in Band 6 (right panel). The intensity contour levels in the left panel are –5, 5, 10, 30, 50, 70, and 90 × the noise level, while the intensity contours in the middle and right panels are –5, 5, and 50 and from 100 to 1000 (in steps of 100) × 70 µJy beam−1 the noise level in setup 2. The colour scale at the top is the same for all panels. The red dots indicate the positions of 11A (north) and 11B (south) as reported by Alves et al. (2019). The sources are not resolved with the present resolution. The left panel also shows the integrated intensity contours from CO (2 → 1) emission from the bipolar outflow powered by [BHB2007] 11 (Alves et al. 2017). The synthesised beam is displayed in the bottom left corner of each panel (0.3600 × 0.3400 , 0.3900 × 0.3300 and 0.4200 × 0.2900 for setups 3, 1, and 2, respectively). disk emission surrounding fainter emission Fig. 4. Spectral energy distribution determined for the dust emission within the circumbinary disc (α = 3.0, black line) and in the envelope Fig. 3. Radial profile of dust continuum emission at 252 GHz (setup (α = 3.4, dotted dashed line). The flux densities for the disc and enve- 2) towards [BHB2007] 11. The profile is centred on the continuum lope are indicated as filled circles and open triangles, respectively. The peak. The red line shows a Gaussian fit to the disc’s brightness dis- errors are dominated by the uncertainty in the flux calibration, which is tribution. The dashed line indicates the synthesised beam size. The grey assumed to be ∼10%. The dashed line shows the SED determined from line shows the rms noise level of the continuum emission. the free-free (centimetric) emission detected with the Very Large Array (Alves et al. 2019). structure that surrounds it, as the disc brightness is well repre- sented by a Gaussian distribution whose wings do not encompass a linear fit (in log–log scale) over the fluxes observed in the the extended emission. three setups (Fig. 4). We note that the flux density (and error) With the continuum data aligned in position across the three used to construct the SED are derived from 2D Gaussian fits FAUST setups, we reconstructed the continuum maps with a over the disc emission, where the emission is centrally peaked, common clean beam of 0.3500 . We then computed the SED from while the envelope flux is derived from a contour excluding the A171, page 5 of 17 A&A 664, A171 (2022) Table 3. Peak flux and flux density for the circumbinary disc. Setup Frequency Peak flux Flux density (GHz) (mJ beam−1 ) (Jy) 3 100.6 5.4 ± 0.2 20.9 ± 0.9 1 225.4 49 ± 1 233 ± 7 2 253.0 76 ± 2 331 ± 12 Notes. Uncertainties refer to the 2D Gaussian fit error. disc.10 Table 3 reports the peak flux and flux density for the cir- cumbinary disc, for which we find a spectral index of 3.0 ± 0.1. The same index is reported by Alves et al. (2019), whose obser- vations cover ALMA Bands 3, 6, and 7. In contrast, the dust emission from the inner envelope has a spectral index of 3.4 ± 0.2, which is slightly lower than the canonical value of the inter- stellar medium (ISM; αISM = 3.7, Testi et al. 2014), implying that the grains in the envelope are potentially mildly larger than in the ISM (.10 µm, Agurto-Gangas et al. 2019). 5.2. Molecular emission Figure 5 shows the moment 0 maps (in Jy beam km s−1 ) of the CH3 OH high spectral resolution lines at 218, 244, and 262 GHz using the 12-m array. These three transitions are the only ones observed at high spectral resolution (0.1–0.2 km s−1 ). The con- tours start at 4σ at every 10σ for 218.440 and 243.916 GHz and every 3σ at 261.704 GHz. The ellipse in the bottom left corner represents the ALMA synthesised beam. The emission is com- pact (less than 100 ) and centred at the same location, which means that, in this region, the gas is co-existent, from the low energy transition to the higher energy transition. The beam encompasses both 11A and 11B, separated by 0.200 , which therefore cannot be resolved. The other transitions where the spectral resolution is lower (0.6–1.2 km s−1 ) also show a centred compact emission. It is obvious from Fig. 5 that all methanol transitions do not necessarily trace the same extended structure. The 218 GHz and 244 GHz transitions, which have an upper energy of 45 K and 50 K, respectively, are tracing a more extended region (around 100 ) compared to the highest energy levels at 540 K for the 246.074 GHz transition tracing a seemingly more compact region (around 0.500 ). 5.2.1. CH3 OH line identification We extracted the average spectrum over the region above 4σ in the integrated intensity map for each molecular transition (either in Jy beam−1 or in Kelvins) into the CASSIS software and the high spectral resolution transitions are presented in Fig. 6, Fig. 5. Moment 0 map of the high spectral resolution methanol transi- while the lower spectral resolution transitions of methanol are tions at 218.440 GHz (Eu = 45.46 K), 243.916 GHz (Eu = 49.66 K), and presented in Figs. 7 and 8. From these figures we can clearly 261.704 GHz (Eu = 359.77 K). Contours start at 4σ at every 10σ for distinguish three components, which are visible at –2 km s−1 , 218.440 and 243.916 GHz, and every 3σ at 261.704 GHz. The ellipse in 2.8 km s−1 and 9.9 km s−1 . Table 2 shows the results of the the bottom left corner represents the ALMA synthesised beam. Both sources A and B identified by Alves et al. (2019) are indicated by white best-fit adjustment using the Levenberg–Marquardt fitting within filled circles. 10 The circumbinary disc emission over which the SED was computed CASSIS using a large number of iterations (Tmb in Kelvins, is determined as the first closed contour around it (i.e. not encompassing the extended N-S emission): 15, 120, and 230 times the rms noise of the FWHM and VLSR in km s−1 ). beam-matched maps in setups 3, 1, and 2, respectively. The SED for the The maximum recoverable scale of the 12-m-array data is envelope uses the integrated flux higher than five times the rms noise but 900 –1100 . Since the CH3 OH emission is compact and its size is lower than the contours listed above (i.e. it excludes the circumbinary less than 300 , the missing flux in the 12-m-array data is negli- disc). gible. In fact, we confirmed that the flux of the data combined A171, page 6 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin Fig. 6. T b (K) spectra for the high spectral resolution Methanol tran- Fig. 7. Spectra (in mJy beam−1 ) for the low spectral resolution Methanol sitions at 218.440, 243.916, and 261.704 GHz. The dashed lines corre- transitions. The dashed lines correspond to the results of the best- spond to the results of the best-fit Gaussian at –2 km s−1 , 2.8 km s−1 , and fit Gaussian of the high spectral resolution transitions at –2 km s−1 , 9.9 km s−1 . The –2 km s−1 component of the 261.704 GHz transition is 2.8 km s−1 , and 9.9 km s−1 . blended with a transition of methyl formate (see text). resolution transitions, taking into account the three velocity with the 7-m- and 12-m-array data (maximum recoverable scale: components. At 243.916 GHz, the image component size (decon- 1600 –1900 ) is comparable with that of the 12-m-array data (see volved from beam) for (i) the 9.9 km s−1 velocity component Fig. 9 for the 243.916 GHz transition). We therefore only used the (channel 195) is 0.2700 × 0.2300 (major axis FWHM × minor axis 12-m-array data in the following. FWHM), (ii) the –2 km s−1 (channel 274) is 0.2700 × 0.1600 (major Figure 10 shows the velocity field (moment 1) of the axis FWHM × minor axis FWHM), and (iii) the 2.8 km s−1 two extreme (in upper energy values), high spectral resolu- (channel 242) is 0.5300 × 0.3600 (major axis FWHM × minor tion methanol transitions at ∼50 K (243.916 GHz) and ∼350 K axis FWHM). The 218.440 GHz transition gives similar results, (261.704 GHz) upper energy levels (Eup ). The velocity structure and the 261.704 GHz non-blended components at +9.9 and ranges from –5 to 15 km s−1 in both low and high transitions, and 2.8 km s−1 tend towards a point-source deconvolved source size. it has a northwest-southeast gradient centred on the 11B source at Therefore, the two extreme velocity components seem slightly the systemic velocity of the source. Although we cannot resolve more compact than the 2.8 km s−1 . the protostars in these maps, this could indicate that the methanol 5.2.2. 13 CH3 OH line identification maps are more sensitive to the kinematics of the southern source. Only one transition of the 13 C methanol isotopologue has been Due to the molecular complexity of this source, we explored detected at 234 GHz with Eup = 48 K, and it presents the the line blending from other species. The CH3 OH transi- same characteristics as methanol and methyl formate with three tion at 261.704 GHz is indeed clearly blended, as shown in components, as shown in Fig. 12. In Table 2, we present the Fig. 6, with the 21(7,14)–20(7,13) E transition of methyl formate results from the best-fit Gaussian adjustments for this transition. (CH3 OCHO) at 261715.5180 MHz. Figure 11 shows the methyl Figure 13 shows that the integrated line intensity is much noisier formate transition with the three components at –2 km s−1 , than the other methanol moment 0 maps but is still compact and 2.8 km s−1 , and 9.9 km s−1 indicated as thin dashed lines. It is centred on the same position. clear that the –2 km s−1 methanol transition is blended with the 9.9 km s−1 transition of methyl formate. A subsequent analysis of 6. Modelling of the methanol and dust emission the iCOMs detected in our [BHB2007] 11 FAUST data will be published at a later time. We first performed a local thermodynamic equilibrium (LTE) Within CASA (imfit command), we fitted a 2D Gaussian analysis to estimate the excitation temperature and column den- to the integrated intensity maps of the three high spectral sity for our methanol transitions. To achieve that, we defined A171, page 7 of 17 A&A 664, A171 (2022) the average spectrum for each methanol transition. We then performed a best-fit of the three components and obtained a rotational-diagram (RD) analysis followed by a non-LTE anal- ysis to estimate the H2 volume density, the column density, kinetic temperature, and size of the source. The integrated line intensities used in the following are presented in Table 4. 6.1. LTE rotational-diagram analysis For a molecule in LTE, all excitation temperatures are the same, and the population of each level is given by Ntot Nu = gu e−Eu /kTrot , (1) Q(T rot ) where Q is the partition function, k is the Boltzmann constant, T rot is the rotational temperature (in Kelvins), gu is the statistical weight in the upper level, Eu /k is the energy of the upper level (in Kelvins), Nu is the column density in the upper level, and Ntot is the column density of the species. We can rewrite this equation to obtain Nu Ntot Eu ln = ln − . (2) gu Q(T rot ) kT rot A rotational diagram can be useful to determine whether the emission is optically thick or thin, whether the level popula- tions are described by LTE, and to determine what temperature describes the population distribution in the event that LTE applies (Goldsmith & Langer 1999). Equation (2) can be writ- ten in terms of the observed integrated area W when the lines are optically thin: Fig. 8. Spectra (in mJy beam−1 ) for the low spectral resolution methanol transitions in the 234 GHz spectral windows. The dashed lines corre- 8πkν2 W Ntot Eu spond to the results of the Gaussian components obtained from each ln = ln − , (3) of the high spectral resolution transitions at –2 km s−1 , 2.8 km s−1 , and hc Aul gu 3 Q(T rot ) kT rot 9.9 km s−1 . where h is the Planck constant, Aul is the Einstein coefficient 0.08 (s−1 ) between level up and level low, ν is the frequency for each observed transition, and c is the celerity of light. The error bars 12 m-only should be taken into account for the order 1 polynomial fit in joint (12 m + 7 m) order to obtain a reliable value for the uncertainty on the rota- 0.06 (joint) - (12m) tional temperature as well as the total column density. From the CASSIS Line Analysis module, the user can fit the lines and pro- Intensity [Jy beam-1] duce a detailed .rotd file with the integrated area and rms values for each transition. The user is required to give a value for the 0.04 instrumental calibration uncertainty. The uncertainty (∆W) of the integrated area is computed though the following formula: q √ 0.02 ∆W = (cal/100 × W)2 + (rms 2 × FWHM × δV)2 , (4) where cal is the calibration value (%), W is the integrated area (in K km s−1 ), rms is the noise around the selected species (in 0 K), FWHM is the full width at half maximum (km s−1 ), and δV is the bin size (in km s−1 ). Therefore, the plotted uncertainties are simply -10 0 10 20 VLSR [km s-1] Nu ! ∆W ∆ ln = . (5) gu W Fig. 9. Difference between the 12-m-only data and the joint data for the A-CH3 OH transition at 243.916 GHz. From the fitted straight line (y = ax + b), the slope a is related to the rotational excitation temperature as T rot = −1/a. Then ∆T rot = ∆a/a2 . The intercept b is related to the total column den- a polygon extracted from the average spectrum over a region sity as Ntot = Q(T rot ) × eb . Therefore, ∆Ntot = Q(T rot ) × ∆b × eb . above 4σ in the integrated intensity map of the 218.440 GHz The RD analysis is presented in Fig. 14 along with the methanol transition. This polygon was then used to extract resulting column densities and excitation (also called rotational) A171, page 8 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin CH3OH Eup = 50 K CH3OH Eup = 360 K Fig. 10. Velocity field determined from methanol transitions at low and high energy levels. The contours indicate the continuum intensity levels from the high spatial resolution observations of Alves et al. (2019). The compact emission contours arise from the circumstellar discs around each protostar of the binary system: 11A (northern component) and 11B (southern component). The dashed line in the left panel shows the direction of the cut used to produce the position-velocity diagram displayed in Fig. 17. The cut position angle is 140◦ and its width is 0.500 , encompassing both protostars. Fig. 11. Spectra at VLSR = 3.6 km s−1 of the [BHB2007] 11 source showing the E-CH3 OCHO 21(7,14)–20(7,13) transition at 261.715 GHz blended with the CH3 OH transition at 261.704 GHz. The dashed lines Fig. 12. Spectrum (in mJ beam−1 ) of only detected transition of 13 correspond to the results of the best-fit Gaussian components obtained CH3 OH at 234 GHz. The dashed lines correspond to the results of the from the high spectral resolution methanol transitions at –2 km s−1 , best-fit Gaussian components obtained from the high spectral resolution 2.8 km s−1 , and 9.9 km s−1 for the methyl formate transition. transitions at –2 km s−1 , 2.8 km s−1 , and 9.9 km s−1 . temperature. All the transitions are shown, although the blended and colder envelope compared to the higher energy transitions. transition of the 261.7 GHz methanol transition at –2 km s−1 is Removing the transitions with an upper energy under 150 K does not used in the fit. A large dispersion can be seen in the figures. not change the results because of the wide dispersion in these The low energy transitions are also tracing the more extended points. A171, page 9 of 17 A&A 664, A171 (2022) 28.5 -2 km s−1 component 28.0 N = (1.0 ± 0.4) 1016 cm−2 27.5 T = (167 ± 31) K 27.0 26.5 ln(Nu /gu ) 26.0 25.5 25.0 24.5 24.0 23.5 0 50 100 150 200 250 300 350 400 450 500 550 Eu /k (K) 28.5 2.8 km s−1 component 13 Fig. 13. Moment 0 map of CH3 OH transition at 234 GHz. Contours N = (1.1 ± 0.3) 1016 cm−2 28.0 start at 4σ at every 2σ. The ellipse in the bottom left corner represents the ALMA synthesised beam. Both sources 11A and 11B identified by 27.5 T = (152 ± 16) K Alves et al. (2019) are indicated as white filled circles. 27.0 Table 4. Integrated intensities used for the LTE and non-LTE analysis, ln(Nu /gu ) 26.5 extracted over a region above 4σ in the integrated intensity map of the 26.0 218.440 GHz methanol transition. 25.5 25.0 W(–2 km s−1 ) W(2.8 km s−1 ) W(9.9 km s−1 ) 24.5 GHz K km s−1 K km s−1 K km s−1 24.0 CH3 OH 23.5 0 50 100 150 200 250 300 350 400 450 500 550 218.440∗ 5.4 ± 0.2 9.5 ± 0.2 6.7 ± 0.2 Eu /k (K) 243.916∗ 5.2 ± 0.1 7.9 ± 0.1 5.6 ± 0.1 247.229∗ 3.4 ± 0.1 4.3 ± 0.1 3.9 ± 0.1 28.5 9.9 km s−1 component 234.683∗ 3.1 ± 0.1 4.0 ± 0.1 3.8 ± 0.1 234.698∗ 1.8 ± 0.1 2.5 ± 0.1 2.5 ± 0.1 28.0 N = (1.2 ± 0.4) 1016 cm−2 232.946∗ 2.7 ± 0.1 3.6 ± 0.1 4.2 ± 0.1 27.5 T = (165 ± 27) K 247.162 3.2 ± 0.2 3.7 ± 0.2 3.8 ± 0.2 261.704∗ 1.1 ± 0.1 1.5 ± 0.1 1.9 ± 0.1 27.0 233.796 1.9 ± 0.4 3.2 ± 0.4 2.4 ± 0.4 26.5 ln(Nu /gu ) 247.611 3.6 ± 0.1 4.0 ± 0.1 4.4 ± 0.1 26.0 246.873 2.9 ± 0.1 3.7 ± 0.1 3.9 ± 0.1 25.5 246.075 3.2 ± 0.1 3.9 ± 0.1 3.5 ± 0.1 13 CH3 OH 25.0 24.5 234.011∗ 0.9 ± 0.1 2.3 ± 0.2 1.4 ± 0.2 24.0 Notes. Transitions used in the LVG are indicated by the star symbol. 23.5 0 50 100 150 200 250 300 350 400 450 500 550 Eu /k (K) The estimated parameters for the three components are sim- ilar. At –2 km s−1 : N = (1.0 ± 0.4)1016 cm−2 , T rot = (167 ± Fig. 14. Rotational diagrams of the detected methanol lines for the 31) K. At 2.8 km s−1 : N = (1.1 ± 0.3)1016 cm−2 , T rot = (152 ± −2 km s−1 (top panel), 2.8 km s−1 (middle panel), and 9.9 km s−1 (bottom 16) K. At 9.9 km s−1 : N = (1.2 ± 0.4)1016 cm−2 , T rot = (165 ± panel) components, respectively. 27) K. The column densities are the beam-averaged and the lines are predicted to be optically thin. transitions up to J = 15 for the non-LTE analysis. We assumed an A-/E-CH3 OH ratio equal to 1 and a 12 C/13 C ratio equal to 6.2. Non-LTE LVG analysis 68 (e.g. Milam et al. 2005). To compute the line escape prob- ability as a function of the line optical depth, we adopted a In order to better take into account the possible non-LTE effects, spherical geometry and a line width equal to 3.2 km s−1 for we modelled the methanol line emission via the large veloc- the –2 km s−1 component, 6 km s−1 for the 2.8 km s−1 compo- ity gradient (LVG) code, grelvg, developed by Ceccarelli et al. nent, and 5 km s−1 for the 9.9 km s−1 component, following the (2003). We used the collisional coefficients with para-H2 com- observations. puted by Rabli & Flower (2010) between 10 and 200 K for We ran a large grid of models (∼10 000) to cover the χ2 sur- the first 256 levels and provided by the BASECOL database face in the parameter’s space: the total (A- plus E- ) methanol (Dubernet et al. 2012, 2013). We therefore used the methanol column density N(CH3 OH) from 1 × 1013 to 2 × 1018 cm−2 , the A171, page 10 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin H2 density nH2 from 1 × 105 to 1 × 109 cm−3 , and the temperature T from 15 to 200 K. We then simultaneously fitted the measured 12 C and 13 C -A- and 12 C E- line intensities extracted from the 4σ polygon of the 218.440 GHz methanol transition by comparing them with those predicted by the model, leaving N(CH3 OH), nH2 and T . Since the line emission is unresolved with our spatial res- 𝛘2 olution, we also left the emitting size θs as a free parameter. We performed this same procedure for the three velocity components separately. With regard to the component at –2 km s−1 , we obtained a good fit (reduced χ2 = 1.2 for 5 degrees of freedom) for all lines with the following values: N(A+E CH3 OH) = (1.4 ± 0.6) ×1018 cm−2 , T = (110 ± 10) K, nH2 = (1.0+3 7 −0.5 ) ×10 cm −3 and θs = (0.15 ± 0.10 ). Figure 15 reports the minimum χ2 , 00 00 N(A-CH3OH) = N(E-CH3OH) (cm-2) with respect to the temperature T and density nH2 , as a func- tion of the methanol column density N(CH3 OH-A). The same figure also shows the temperature-density χ2 contour plot at the best-fit N(CH3 OH) (note that the x-axis reports N(CH3 OH)/2). Finally, the third panel of Fig. 15 shows the observed/predicted line intensity ratio as a function of the line upper level energy density (cm-3) Eup . The 12 C lines are predicted to be from moderately (the 234.69 GHz line has the smallest value, τ = 0.64) to heavily (the 243.91 GHz line has the largest value, τ ∼ 12) optically thick, while the 13 C line is optically thin (τ = 0.25). Concerning the component at +9.9 km s−1 , the best fit (reduced χ2 = 1.9) gives values very similar to those obtained for the –2 km s−1 component, namely N(CH3 OH) = (3.0 ± 0.8) ×1018 cm−2 , T = (130 ± 10) K, nH2 = (2.5 ± 2.2) ×107 cm−3 and θs = (0.1200 ± 0.1000 ). Finally, with regard to the component at +2.8 km s−1 , Temperature (K) the best fit (reduced χ2 = 2.7) gives the following values: N(CH3 OH) = (4.0 ± 1.0) ×1018 cm−2 , T = (130 ± 10) K, nH2 = (2.0 ± 0.5) × 106 cm−3 and θs = (0.1300 ± 0.1000 ). Observations/Model We now give a summary of the non-LTE analysis The three velocity components trace gas with similar characteris- tics: hot (with a temperature around 110–130 K), dense (with a H2 density of 2–20 × 106 cm−3 ), enriched in methanol (with a methanol column density [1–5] × 1018 cm−2 ), and relatively compact (0.1200 –0.1500 ). Table 5 lists the results of the analy- sis for the three components. Assuming that the emission in each velocity component originates from a spherically symmet- ric region, one can estimate the approximate H2 column density (∼2 × 1023 cm−2 ) and then the methanol abundance, whose value is very high, ∼10−5 . Eup-E0 (K) Compared to the results of the LTE RD analysis (see Sect. 6.1), the non-LTE analysis predicts slightly lower gas tem- Fig. 15. Results from the non-LTE analysis. Upper panel: results from peratures and much higher methanol column densities, by about the χ2 minimisation for the –2 km s−1 component simultaneously fitting two orders of magnitude. These differences are due to two major the A and E forms with a ratio equal to unity and the 13 C E methanol effects. First, the column densities calculated with the RD are transition with a ratio of 68. The minimum therefore has to be multiplied beam-averaged, whilst the LVG analysis, where the emitting by two to obtain the A+E forms of methanol. Middle panel: density and size is derived, shows that the methanol emission is relatively temperature range obtained for the best-fit methanol column density of compact. This explains the large difference between the two esti- 2 × (7 × 1017 ) cm−2 and source size of 0.1500 . Bottom panel: observed mates. Second, the RD in Fig. 14 show a clear scatter of the integrated intensities versus modelled ones as a function of the upper points around the best-fit straight line, which indicates optically energy level value with respect to the lowest value. These are 7.9 K for thick lines and/or non-LTE effects. They both lead to a mislead- E methanol (red filled circles) and 0 K for A methanol (red filled stars). The 13 C E methanol is represented as a red filled triangle. ing evaluation of the gas temperature (and, to a lesser extent, methanol column density). Finally, even though the source size and the optical depths of the lines can be corrected a posteri- 6.3. Dust opacity towards the binary system objects, 11A ori (using the rotational diagram analysis), non-LTE effects (e.g. and 11B sub-thermally populated lines), if present, cannot be corrected unless a non-LTE analysis is performed. In summary, in the RD As described in Sect. 2, [BHB2007] 11 is a binary system analysis, all of the above mentioned effects lead to an under- where the two objects (11A and 11B) are embedded in their estimation of the methanol column density and incorrect gas respective circumstellar discs (Alves et al. 2019). The discs temperature. are both detected in the continuum at 0.94 cm with the A171, page 11 of 17 A&A 664, A171 (2022) Table 5. Results from the non-LTE analysis. –2 km s−1 2.8 km s−1 9.9 km s−1 N(CH3 OH) (cm−2 ) (1.4 ± 0.6) × 1018 (4.0 ± 1.0) × 1018 (3.0 ± 0.8) × 1018 T (K) 110 ± 10 130 ± 10 130 ± 10 nH2 (cm−3 ) (1.0+3 −0.5 ) × 10 7 (2.0 ± 0.5) × 106 (2.5 ± 2.2) × 107 θs (00 ) 00 0.15 ± 0.10 00 0.1300 ± 0.1000 0.1200 ± 0.1000 Karl G. Jansky Very Large Array (JVLA) and at 1.3 mm with 7. Origin of the observed methanol components ALMA. Following the findings of De Simone et al. (2020) that the dust in front of the hot corino of NGC1333 IRAS4A1 com- In the following, we discuss the possible origin of the three pletely absorbs its methanol lines in the millimeter wavelengths, velocity components detected in our methanol spectra. In order here we aim to estimate the possible absorption of the methanol to better understand this point, we created a position-velocity emission specifically related to the compact disc scales around (PV) diagram for the methanol transition at 243.916 GHz (Eup ∼ each of the sources 11A and 11B. Within this scope, we used the 50 K) displayed in Fig. 17. The PV plot is produced from a cut dust continuum observations by Alves et al. (2019) at 225 GHz, oriented parallel to the velocity gradient observed in the moment who found a flux density F225 equal to 5.2 ± 0.53 mJy towards 1 map of the methanol emission (Fig. 10), whose PA is 140◦ east 11A and 5.62 ± 0.45 mJy towards 11B. The deconvolved sizes of north. The cut width is 0.500 , the value of which is comparable are 7.3(±1.3) × 5.4(±1.1) au2 and 4.9(±1.1) × 3.7(±1.0) au2 for to the angular resolution of the FAUST data and encompasses 11A and 11B, respectively. Thus, the 225 GHz continuum emis- both core 11A and core 11B, separated by only 0.200 . Unlike the sion is more compact in 11B than 11A, despite having a similar H2 CO PV plot, which shows a smooth Keplerian profile over flux density. the velocity range (Alves et al. 2019), a fragmented profile is We can constrain the dust optical depth τν and temperature clearly seen in the methanol PV plot. The three substructures towards each of the two sources using the following equation: correspond to the three velocity components observed in the methanol spectra (a similar distribution is observed at the higher Fν = Bν (T d ) (1 − e−τν ) Ωs , (6) Eup transition). Although we cannot locate the methanol emission with where Bν (T d ) is the Planck function, which depends on the dust respect to the binary system due to our limited angular res- temperature, T d , and Ωs is the source solid angle, derived from olution, we notice that blueshifted and redshifted emission in the continuum observations. The source solid angle is defined the high velocity channels where the S/N is larger than 5 as ∼A/d2 , where A is the source’s emitting area (derived from (–4.89 km s−1 in blue and 13.3 km s−1 in red) tends to be aligned the source’s size) and d2 is the distance to the sources. The dust with source 11B (Fig. 18, left panel), alike the high-velocity optical depth can be estimated by inverting Eq. (6) as follows: components of the CO emission shown by Alves et al. (2019). The methanol emission at velocity channels near the systemic ! Fν τν = −ln 1 − . (7) velocity of the source (∼3.6 km s−1 ) also tends to peak near 11A. Bν (T d ) Ωs The three methanol components appear in a compact configura- Finally, using Eq. (7), we can estimate the possible dust absorp- tion that is compatible with the compact methanol seen in the tion coefficient using the following equation: moment 0 maps (Fig. 5). pred It is clear from Figs. 6, 7, and 8 that the three velocity compo- Iν nents are not completely spectrally independent and that they are eτν = . (8) Iνobs affected by the fact that they blend into one another. In Fig. 19, we present the integrated intensity and velocity field maps from As we do not know the dust temperature, T d , of the sources, the 243 GHz methanol transition for the three velocity compo- we derived the dust optical depth for temperatures ranging nents in the [–7–0] km s−1 range for the –2 km s−1 component, between 100 and 500 K with steps of 10 K from 100 K to 200 K, the [0–6] km s−1 range for the 2.8 km s−1 component, and the [6– and by steps of 20 K from 200 to 500 K, following the method 16] km s−1 range for the 9.9 km s−1 component. The three posi- described in Bouvier et al. (2021). The observed 225 GHz flux tions are clearly identified in the moment 0 maps: the 2.8 km s−1 provides a lower limit to T d . Specifically, the condition Fν /Ωs ≤ component in between [BHB2007] 11A and [BHB2007] 11B, Bν (T d ) implies T d ≥ 130 K in 11A and T d ≥ 300 K in 11B (as while the –2 and 9.9 km s−1 are located north-west and south- it is more compact). Then, for each dust temperature, we can east of [BHB2007] 11B. There is no clear evidence of a velocity derive the dust opacity at 225 GHz, reported in Fig. 16. The fig- structure in these two velocity ranges, which points to the conclu- ure shows that τν is well constrained in the two sources, except sion of the results from a compact shock emission (see Sect. 7.3). around the minimum Td for each of them, where the error bars The 2.8 km s−1 seems to be more sensitive to the disc rota- become very large. Specifically, in 11A, τν varies from 5+48 −5 at tion (as in the H2 CO centroid map in Alves et al. 2019), and +0.2 T d ∼ 130 K to 0.3−0.1 at T d = 500 K; in 11B, τν varies from 3+9 −3 that could explain the gradient that is seen more clearly than at T d ∼ 300 K to 0.8+0.7 −0.6 at T d = 500 K. The dust opacities tend in the other two components. Also, Fig. 18 (right panel) must to be optically thick, and taking the largest dust optical depths be analysed with caution as the overlap of velocity ranges from gives τν ≤ 53 and τν ≤ 12 in 11A and 11B, respectively. There- one component to another can mislead us into concluding that fore, the possible methanol lines from the hot corinos associated we have an elongated structure for the methanol emission. We with 11A and 11B, if they exist, can be completely attenuated by added coloured borders to identify the three velocity components the foreground dust. observed in our methanol observations in this figure. However, A171, page 12 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin 102 102 [BHB2007] 11 A [BHB2007] 11 B =[5+48 +0.2 5 - 0.3 0.1 ] =[3+93 - 0.8+0.7 0.6 ] 101 101 100 100 1 1 10 10 100 150 200 250 300 350 400 450 500 100 150 200 250 300 350 400 450 500 Td [K] Td [K] Fig. 16. Dust optical depth, τν , at 225 GHz as function of the dust temperature, T d , derived from the observations of Alves et al. (2019) and following the method described in Bouvier et al. (2021). The lines show τν as derived from Eq. (7). The errors are calculated using the error propagation method described in Appendix C.2. of Bouvier et al. (2021) and include the calibration uncertainty of 15% (Alves et al. 2019) and the source size uncertainty. The left and right panels refer to 11A and 11B, respectively. 1.0 a S/N larger than 10.0 (which corresponds to a position accu- racy better than 0.0200 ). The right panel of Fig. 18 shows the 2D 0.8 9.9 km s-1 Gaussian centroid map with the location of each methanol emis- 2.8 km s-1 sion peak (black circle) as a function of velocity. The velocity 0.6 −2.0 km s-1 components satisfying the conditions above span from −4.1 to 12.7 km s−1 . 0.4 Offset (arcsec) It is noticeable that the methanol emission with velocities 0.2 between 0 and 8 km s−1 tends to lie in the region in between sources 11A and 11B, while the gas with velocities <0 km s−1 and 0 >8 km s−1 seems to approach source B, as is also seen in the CO (2–1) observations presented in Alves et al. (2017, 2020). Their −0.2 analyses show an increasing velocity towards 11B that seems to indicate acceleration of infalling gas from the circumbinary disc −0.4 onto 11B. This can be shown on the right side of Fig. 18, where −0.6 we also mark the CO emission as circles with green borders and PA = 140° the methanol emission as circles with black borders. The CO −0.8 emission observed by Alves et al. (2017) and shown in the left Width = 0.5ʺ panel of our Fig. 2 primarily traces large-scale outflowing gas −1.0 emission, launched from a distance of ∼100 au from the disc centre and propagating perpendicularly to the envelope major 20 15 10 5 0 −5 −10 axis. However, velocity components faster than the outflows are LSRK radio velocity (km s-1) detected within the circumbinary disc at only a few au. These high-velocity CO components reach up to 15 km s−1 , and the Fig. 17. PV diagram obtained from the CH3 OH emission at blueshifted and redshifted velocities (VLSR < −1.5 km s−1 and 243.916 GHz. The PA and width of the PV cut is indicated in the VLSR > 9 km s−1 , respectively) are centred on and aligned with figure. We note that the direction of the cut used to produce the 11B, the higher velocities being closest to the position of the pro- position-velocity diagram is shown in Fig. 10 as a grey dashed line. tostar. We note that no high velocity components are seen near The three velocity components observed in the methanol spectra are [BHB2007] 11A. The increasing velocity towards [BHB2007] also indicated. 11B indicates acceleration of infalling gas from the circumbinary disc onto [BHB2007] 11B. higher spatial resolution observations of methanol are necessary From our methanol analysis, an increasing velocity is not to determine the velocity structure of methanol accurately. as clear as in CO, possibly due to our lower spatial resolu- Since the emission in each velocity channel is compact and tion, but also due to the fact that the three velocity components centrally peaked, we performed 2D Gaussian fits on the emis- identified in our methanol spectra are partially overlapping. The sion in each channel to obtain the position of their intensity peak. information provided by the PV diagram and centroid maps give The accuracy on the relative position is given by Θ/(2 × S/N), constraints on the origin of the three velocity components, which where Θ is the synthesised beam size and S/N is the signal-to- is described in the following sub-section. noise ratio of the peak with respect to the map noise (Condon 1997; Condon et al. 1998). Thus, the uncertainty in the position is 7.1. The –2.0 and +9.9 km s−1 components inversely proportional to the S/N in each velocity channel. Cen- troid fitting to the compact molecular emission yields astrometry Based on the arguments above, these two velocity components with precisions better than those of the spatial resolution in the are likely associated with the gas streaming towards 11B. The case of a high S/N. For this calculation, we only considered most extreme components, −5 km s−1 and 13 km s−1 , are con- intensity peaks brighter than five times its Gaussian fit error and sistent with the high-velocity components observed in CO lines A171, page 13 of 17 A&A 664, A171 (2022) Fig. 18. Analysis of the origin of the observed methanol components. Left panel: methanol intensity contours at 243.916 GHz plotted over the dust continuum map of Alves et al. (2019). The contours show the methanol emission at extreme velocity channels detected at a 3σ level (1σ = 1.8 mJy beam−1 ). The blue and red contours correspond to emission at the −4.89 km s−1 and 13.3 km s−1 velocity channels, respectively. White contours show emission (from a 20σ level) from velocity channels near the systemic velocity of the source (2.91 km s−1 ). The synthesised beam of the methanol (FAUST) data is shown in the lower left corner. Right panel: position of intensity peaks (circles) from the 2D Gaussian fit in each velocity channels separated by ∼0.15 km s−1 . Colour code for the edges of the circle: red for the 9.9 km s−1 methanol component, green for the 2.8 km s−1 methanol component, and blue for the –2 km s−1 methanol component, while the CO (2–1) high-velocity components are in cyan. The circle size is defined by the uncertainty on the absolute position of the emission peak in each velocity component (largest circle has ∼0.0200 ). vlsr (km s 1) vlsr (km s 1) vlsr (km s 1) 4 2 0 2 4 8 10 -27°24'32.4" -27°24'32.4" -27°24'32.4" 32.6" 32.6" 32.6" Declination (J2000) Declination (J2000) Declination (J2000) 32.8" 32.8" 11A 32.8" 11A 11A 33.0" 11B 33.0" 11B 33.0" 11B 33.2" 33.2" 33.2" 33.4" 33.4" 33.4" 50 au 50 au 50 au 17h11m23.14s 23.09s 17h11m23.14s 23.09s 17h11m23.14s 23.09s Right Ascension (J2000) Right Ascension (J2000) Right Ascension (J2000) Fig. 19. Moment 0 and moment 1 maps for the 51,4 –41,3 transition of A-CH3 OH . Top: integrated intensity maps of the high spectral resolution methanol transition at 243.916 GHz (Eu = 49.66 K) for the three components ([–7–0] km s−1 for the –2 km s−1 component, [0–6] km s−1 for the 2.8 km s−1 component, and [6–16] km s−1 for the 9.9 km s−1 component). Contours start at 4σ at every 4σ. The ellipse in the bottom left corner represents the ALMA synthesised beam. Both sources A and B identified by Alves et al. (2019) are indicated as white filled circles. Bottom: velocity field maps of the high spectral resolution methanol transition at 243.916 GHz (Eu = 49.66 K) for the three components. A171, page 14 of 17 C. Vastel et al.: FAUST. V. Hot methanol in the [BHB2007] 11 protobinary system; hot corino versus shock origin distributed along the filaments and confined within the circumbi- co-desorption of methanol with water. Again based on Ferrero nary disc (see also higher spatial resolution observations by et al. (2020), this would happen at about 110 K, namely in Alves et al. 2019, and their Fig. 2 and S2). These components between the dust sublimation temperature of a pure methanol exceed the Keplerian velocities observed in the H2 CO maps ice. of Alves et al. (2019; −3 km s−1 . vKep . 12 km s−1 ), and are The first question to answer is whether the measured temper- distributed around 11B. Our non-LTE analysis of the methanol ature, 110–130 K (Sect. 6.2), is compatible with the warming of lines indicates that this gas is very warm (100–140 K), dense the dust from the two sources or if it necessitates a non-thermal (∼107 cm−3 ) and very enriched in methanol (abundance w.r.t mechanism. To this end, we can estimate the approximate dust H2 ∼ 10−5 ). We discuss the physical origin of gaseous methanol temperature of a dust grain heated by a L? source at a distance later in this section. of r. When the dust is optically thin, the dust temperature profile heated by a central source can be approximated by the following equation (Ceccarelli et al. 2000a): 7.2. The +2.8 km s−1 component This component seems to be associated with 11A when consid- " L? #0.25  r −0.5 ering the arguments based on the velocity field, PV diagram, and T d [K] = 75 . (9) centroid maps above. This gas is also warm (120–140 K), dense 27L 150 au (∼2 × 106 cm−3 ), and very enriched in methanol (abundance with respect to H2 ∼ 10−5 ), which would point to a similar origin Considering that the two sources 11A and 11B have a total bolo- of the methanol emission of the other two components. More- metric luminosity of 4.4 L (Sandell et al. 2021) and that the over, the H2 CO (Eup ∼ 70 K) observations of Alves et al. (2019), distance between them is 28 au (0.200 ), the dust temperature taken at comparable spectral resolution but a factor ∼2 better would be ∼93 K. As already stated in Sect. 6.3, the dust is angular resolution, show relatively warm gas associated with optically thick, therefore the dust temperature should be above 11A. In fact, the emission observed in the velocity range of 2.5– ∼93 K. This means that all the dust in between 11A and 11B 5.0 km s−1 is spatially coincident with 11A, which is consistent could be warm enough for the methanol to sublimate, within with our findings (see Fig. S3 in Alves et al. 2019). This com- the uncertainties discussed above on the frozen methanol sub- ponent is close to the systemic velocity of 3.6 km s−1 and could limation temperature. A fortiori, if 11A and 11B host one or be (partially) tracing the Keplerian motion of the disc (similar two hot corinos, the observed methanol could just originate to H2 CO), and so a velocity gradient is expected and identified from them. However, the above analysis on the location of the in Fig. 19 (middle figure, bottom panel). Nevertheless, due to the gas (Sects. 7.1 and 7.2) and the dust optical depth towards 11A limited angular resolution of our maps, we should not completely and 11B (Sect. 6.3) is not in favour of the hypothesis that discard that this velocity component has some contribution from the observed hot methanol originates in the 11A or 11B cores, 11B or from gas located in between sources. therefore invalidating their hot corino nature. Alternatively, the observed methanol could be the result of the sputtering of the grain mantles in shocks (e.g. Caselli et al. 7.3. Origin of the gaseous methanol: Shock versus thermal 1997; Jiménez-Serra et al. 2008; De Simone et al. 2020; Codella desorption et al. 2020) created by the streaming of the gas towards 11A Methanol is known to have a very low abundance ≤10−8 in cold and 11B and impacting the quiescent gas of the circumbinary objects (e.g. Friberg et al. 1988; Nagy et al. 2019), while its envelope/disc or, possibly, the two circumstellar discs. The mass abundance is high (10−7 –105 ) in hot cores/corinos (e.g. Gibb contained in the shocked gas traced by methanol would then be et al. 2000; Cazaux et al. 2003) and molecular outflow shocks about 10−6 M , namely a very minor fraction of the dynamical (e.g. Bachiller et al. 1995; Codella et al. 2020). It is commonly mass (∼2.2 M ) or even the mass of the larger circumbinary disc accepted that methanol is indeed synthesised on the grain sur- (∼0.08 M , Alves et al. 2019). faces (e.g. Watanabe & Kouchi 2002; Rimola et al. 2014) during The gas kinematics in a binary system is expected to be the cold prestellar phase and released from the grain mantles more complex compared to a simple protostar formation, with into the gas phase by thermal and non-thermal processes. Hot accretion shocks and structures that are related to the transi- cores/corinos are therefore the results of thermal processes (e.g. tion from the circumbinary material to the circumstellar discs Maret et al. 2005; Taquet et al. 2016), while molecular outflow (Matsumoto et al. 2019). Alves et al. (2019) computed a mass triggers non-thermal desorption via shocks (e.g. Caselli et al. accretion rate from the circumbinary disc into the circumstellar 1997; Jiménez-Serra et al. 2008). disc of ∼1.1 × 10−5 M yr−1 , which is also consistent with other The temperature derived from the non-LTE analysis of the Class 0/I protostars. Also, the infall motion, traced by the high- methanol lines tells us that the gas is hot and dense. The latter velocity CO, is perfectly consistent with the dynamical mass of would also imply that the dust and gas are thermally cou- the system. A similar situation to that of [BHB2007] 11 is seen pled (e.g. Goldsmith 2001) and, hence, the dust is also warm in the young binary system IRAS16293 A, where some molec- enough for methanol to be thermally desorbed, based on the ular tracers such as HNCO, NH2 CHO, and t-HCOOH coincide recent estimates of the methanol binding energy by Ferrero with the location of the dust sub-structures detected in the con- et al. (2020). These authors found the CO binding energy ranges tinuum emission, away from the the two binaries (Maureira et al. between ∼4400 and ∼6500 K, which would correspond to a 2020). Also, the CH3 OH emission and the HCOOCH3 molecu- dust sublimation temperature of about 80 to 120 K, assuming lar emission detected by Oya et al. (2016) likely originate in the that the grain mantles have been warmed up for ∼104 yr11 . Of ring-like region with a radius of 50 au surrounding the protostar, course, it is possible that methanol is trapped in the water-rich based on their kinematic structures (Oya & Yamamoto 2020), ices, so the sublimation temperature could be dominated by the with temperatures rising to more than 300 K. The evidence pre- sented here (dust opacity, compactness of the three components, 11 We note that these sublimation temperatures could be lower by less centroid analysis) point to the hypothesis of a shocked region due than 10 K if the heating of the ices is 1 Myr instead. to the large-scale streaming material feeding the proto-binary A171, page 15 of 17 A&A 664, A171 (2022) system (see Fig. 18). 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I., Caselli, P., et al. 2016, ApJ, 830, L6 for the project Astro-Chemical Origins (ACO), grant agreement no. 811312. Jiménez-Serra, I., Vasyunin, A. I., Spezzano, S., et al. 2021, ApJ, 917, 44 I.J.-S. has received partial support from the Spanish State Research Agency Jin, M., & Garrod, R. T. 2020, ApJS, 249, 26 (project number PID2019-105552RB-C41). D.J. is supported by NRC Canada Jørgensen, J. K., van der Wiel, M. H. D., Coutens, A., et al. 2016, A&A, 595, and by an NSERC Discovery Grant. S.Y. thanks the support by Grant-in- A117 Aids from Ministry of Education, Culture, Sports, Science, and Technologies Lee, C.-F., Codella, C., Li, Z.-Y., & Liu, S.-Y. 2019, ApJ, 876, 63 of Japan (18H05222). This paper makes use of the following ALMA data: Maret, S., Ceccarelli, C., Tielens, A. G. G. M., et al. 2005, A&A, 442, 527 ADS/JAO.ALMA#2018.1.01205.L. 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Fermi 5, Leiden, The Netherlands 50125, Florence, Italy 31 Department of Physics and Astronomy, University College London, 7 National Radio Astronomy Observatory, PO Box O, Socorro, Gower Street, London WC1E 6BT, UK NM 87801, USA 32 Materials Science and Engineering, College of Engineering, 8 European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo Garching bei München, Germany 135-8548, Japan 9 RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, 33 Departament de Física Quàntica i Astrofísica, Institut de Ciències Saitama 351-0198, Japan del Cosmos, Universitat de Barcelona (IEEC-UB), Martí Franquès, 10 Department of Physics, The University of Tokyo, 7-3-1, Hongo, 1, 08028 Barcelona, Catalunya, Spain Bunkyo-ku, Tokyo 113-0033, Japan 34 Astronomical Institute, Graduate School of Science, Tohoku Uni- 11 Research Center for the Early Universe, The University of Tokyo, versity, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Japan A171, page 17 of 17