2022MNRAS.515..543C

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 E_u = 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 $N_{\rm CH_3OH}$ = 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 E_u range, and yields T_rot ≤ 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.