Hot methanol from the inner region of the HH 212 protostellar system

TL;DR

This study uses ALMA observations to analyze hot methanol emission in the innermost region of the HH 212 protostellar system, revealing insights into jet-disk interactions and potential disk wind origins.

Contribution

It provides the first detailed characterization of hot methanol in the inner 100 AU of HH 212, suggesting methanol traces a disk wind rather than a Keplerian disk.

Findings

Methanol emission originates from a compact, hot region elongated along the jet.

Velocity structure indicates rotation and outflow motions, not Keplerian rotation.

Methanol likely traces a disk wind launched at a few AU from the protostar.

Abstract

The mechanisms leading to the formation of disks around young stellar objects (YSOs) and to the launching of the associated jets are crucial to the understanding of the earliest stages of star and planet formation. HH 212 is a privileged laboratory to study a pristine jet-disk system. Therefore we investigate the innermost region ($<100$ AU) around the HH 212-MM1 protostar through ALMA band\,7 observations of methanol. The 8 GHz bandwidth spectrum towards the peak of the continuum emission of the HH 212 system reveals at least 19 transitions of methanol. Several of these lines (among which several vibrationally excited lines in the v$_{\rm t}=1,2$ states) have upper energies above 500 K. They originate from a compact ($<135$ AU in diameter), hot ($\sim 295$ K) region elongated along the direction of the SiO jet. We performed a fit in the $uv$ plane of various velocity channels of the strongest high-excitation lines. The blue- and red-shifted velocity centroids are shifted roughly symmetrically on either side of the jet axis, indicating that the line-of-sight velocity beyond 0.7 km s$^{-1}$ from systemic is dominated by rotational motions. The velocity increases moving away from the protostar further indicating that the emission of methanol is not associated with a Keplerian disk or rotating-infalling cavity, and it is more likely associated with outflowing gas. We speculate that CH$_3$OH traces a disk wind gas accelerated at the base. The launching region would be at a radius of a few astronomical units from the YSO.