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

TL;DR

This study uses ALMA to detect and analyze complex organic molecules, specifically methanol and methyl formate, in the protostellar system VLA 1623-2417, revealing detailed chemical and kinematic structures at 50 au scales.

Contribution

First detection of methanol and methyl formate emission in the VLA 1623-2417 system, providing high-resolution insights into astrochemistry around protostars at Solar System scales.

Findings

Detected methanol emission with high excitation lines towards components A1 and B.

Reversed velocity gradients observed in the disks, indicating complex kinematic structures.

Identified chemically enriched rings near the centrifugal barrier at ~12-33 au.

Abstract

The ALMA 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 benefited from imaging interstellar complex organic molecules in these jet-disk 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 Program FAUST. Compact methanol emission is detected in lines from Eu = 45 K up to 61 K and 537 K towards components A1 and B, respectively. LVG 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(CH3OH) = 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 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 disk. Although the axial geometry of the two disks is similar, the observed velocity gradients are reversed. The CH3OH spectra from B shows two broad (4-5 km s-1) peaks, which are red- and blue-shifted by about 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 (about 4 km s-1), implying a radius of 12-24 au.