FAUST XV. A disk wind mapped by CH$_3$OH and SiO in the inner 300 au of the NGC 1333 IRAS 4A2 protostar

TL;DR

This study uses high-resolution ALMA observations to map the inner 300 au of the IRAS 4A2 protostar, revealing a disk wind traced by SiO and CH$_3$OH, providing new insights into star formation processes.

Contribution

First detection of a disk wind candidate in SiO and second in CH$_3$OH, linking molecular emission to the base of a wide-angle disk wind in a protostar.

Findings

Identified three emission zones with distinct kinematics and compositions.

Estimated CH$_3$OH/SiO abundance ratio of 120-400 in the intermediate region.

Proposed that CH$_3$OH and SiO trace the base of a wide-angle disk wind.

Abstract

Context. Understanding the connection between outflows, winds, accretion and disks in the inner protostellar regions is crucial for comprehending star and planet formation process. Aims. We aim to we explore the inner 300 au of the protostar IRAS 4A2 as part of the ALMA FAUST Large Program. Methods. We analysed the kinematical structures of SiO and CH$_3$OH emission with 50 au resolution. Results. The emission arises from three zones: i) a very compact and unresolved region ($<$50 au) dominated by the ice sublimation zone, at $\pm$1.5 km s$^{-1}$ with respect to vsys, traced by methanol; ii) an intermediate region (between 50 au and 150 au) traced by both SiO and CH$_3$OH, between 2 and 6 km s$^{-1}$ with respect to vsys, with an inverted velocity gradient (with respect to the large scale emission), whose origin is not clear; iii) an extended region ($>$150 au) traced by SiO, above 7 km s$^{-1}$ with respect to vsys, and dominated by the outflow. In the intermediate region we estimated a CH$_3$OH/SiO abundance ratio of about 120-400 and a SiO/H$_2$ abundance of 10$^{-8}$. We explored various possibilities to explain the origin of this region such as, rotating disk/inner envelope, jet on the plane of the sky/precessing, wide angle disk wind. Conclusions. We propose that CH$_3$OH and SiO in the inner 100 au probe the base of a wide-angle disk wind. The material accelerated in the wind crosses the plane of the sky, giving rise to the observed inverted velocity gradient, and sputtering the grain mantles and cores releasing CH$_3$OH and SiO. This is the first detection of a disk wind candidate in SiO, and the second ever in CH$_3$OH.