FAUST VI. VLA 1623--2417 B: a new laboratory for astrochemistry around protostars on 50 au scale

C. Codella et al.
The ALMA (Atacama Large Millimeter Array) interferometer, with its unprecedented combination of high sensitivity and high angular resolution, allows for (sub-)mm wavelength mapping of protostellar systems at Solar system scales. Astrochemistry has benefitted from imaging interstellar complex organic molecules in these jet–disc systems. Here, we report the first detection of methanol (CH3OH) and methyl formate (HCOOCH3) emission towards the triple protostellar system VLA1623−2417 A1+A2+B, obtained in the context of the ALMA Large Programme FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar-like protostars). Compact methanol emission is detected in lines from Eu = 45 K up to 61 K and 537 K towards components A1 and B, respectively. Large velocity gradient analysis of the CH3OH lines towards VLA1623−2417 B indicates a size of 0.11–0.34 arcsec (14–45 au), a column density NCH3OH = 1016–1017 cm−2, kinetic temperature ≥ 170 K, and volume density ≥ 108 cm−3. A local thermodynamic equilibrium approach is used for VLA1623−2417 A1, given the limited Eu range, and yields Trot ≤ 135 K. The methanol emission around both VLA1623−2417 A1 and B shows velocity gradients along the main axis of each disc. Although the axial geometry of the two discs is similar, the observed velocity gradients are reversed. The CH3OH spectra from B show two broad (4–5 km s−1) peaks, which are red- and blueshifted by ∼ 6–7 km s−1 from the systemic velocity. Assuming a chemically enriched ring within the accretion disc, close to the centrifugal barrier, its radius is calculated to be 33 au. The methanol spectra towards A1 are somewhat narrower (∼ 4 km s−1), implying a radius of 12–24 au.
This project has received funding from: (1) the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, for the Project ‘The Dawn of Organic Chemistry’ (DOC), grant agreement no. 741002; (2) the PRIN-INAF 2016 The Cradle of Life – GENESIS-SKA (General Conditions in Early Planetary Systems for the rise of life with SKA); (3) a Grant-in-Aid from Japan Society for the Promotion of Sci- ence (KAKENHI: Nos. 18H05222, 20H05844, 20H05845); (4) the Spanish FEDER under project number ESP2017-86582-C4-1-R; (5) DGAPA, UNAM grants IN112417 and IN112820, and CONACyT, Mexico; (6) ANR of France under contract number ANR-16-CE31- 0013; (7) the French National Research Agency in the framework of the Investissements d’Avenir programme (ANR-15-IDEX-02), through the funding of the ‘Origin of Life’ project of the Univ. Grenoble-Alpes, (8) the European Union’s Horizon 2020 research and innovation programmes under projects ‘Astro-Chemistry Ori- gins’ (ACO), Grant No. 811312; (9) the National Research Council Canada and an NSERC Dico v ery Grant to DJ. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.01205.L. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Obser- vatory is operated by ESO, A UI/NRA O, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.