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

Abstract

The ALMA interferometer, with its unprecedented combination of high-sensitivity and highangular resolution, allows for (sub-)mm wavelength mapping of protostellar systems at Solar System scales. Astrochemistry has benefited from imaging interstellar complex organic molecules in these jet-disk systems. Here we report the first detection of methanol (CH 3 OH) and methyl formate (HCOOCH 3 ) emission towards the triple protostellar system VLA1623-2417 A1+A2+B, obtained in the context of the ALMA Large Program FAUST. Compact methanol emission is detected in lines from u = 45 K up to 61 K and 537 K towards components A1 and B, respectively. LVG analysis of the CH 3 OH lines towards VLA1623-2417 B indicates a size of 0. ′′ 11-0. ′′ 34 (14-45 au), a column density (CH 3 OH) = 10 16 -10 17 cm −2 , kinetic temperature ≥ 170 K, and volume density ≥ 10 8 cm −3 . An LTE approach is used for VLA1623-2417 A1, given the limited u range, and yields rot ≤ 135 K. The methanol emission around both VLA1623-2417 A1 and B shows velocity gradients along the main axis of each disk. Although the axial geometry of the two disks is similar, the observed velocity gradients are reversed. The CH 3 OH spectra from B shows two broad (4-5 km s −1 ) peaks, which are red-and blue-shifted by ∼ 6-7 km s −1 from the systemic velocity. Assuming a chemically enriched ring within the accretion disk, 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.