HOTDISK. Finding Massive Protostellar Disks with Water and Refractory Molecular Species

TL;DR

This study uses high-resolution ALMA observations to identify vibrationally excited water, NaCl, and SiS as effective tracers of compact, rotating disks around massive protostars, revealing common hot-disk chemical patterns.

Contribution

It demonstrates that vibrationally excited water, NaCl, and SiS are powerful and common tracers of massive protostellar disks at ~100 au scales, improving understanding of disk chemistry.

Findings

Vibrationally excited water detected in 7 of 10 sources.

Detected NaCl and SiS in 5 sources, supporting disk rotation.

Hot-disk tracers are more compact than traditional hot-core molecules.

Abstract

We present high-angular-resolution ($\sim0.05^{\prime\prime}$) ALMA Band~6 observations from the HOTDISK project (Hot-Origin Tracer survey of DISKs of massive protostars) aimed at investigating the "hot-disk" chemical pattern traced by vibrationally excited water, NaCl, SiS, and SiO in the innermost regions around massive protostars. Ten targets were selected based on strong CH$_3$CN emission exhibiting clear rotational signatures and centrally concentrated SiO emission from lower-resolution observations. We detect vibrationally excited water emission toward 7 of the 10 sources. In all detections, the blueshifted and redshifted components are compact and located on opposite sides of the 1.3 mm continuum peak, with velocity gradients approximately perpendicular to the outflow axes, consistent with rotation on disk scales. Emission from NaCl and SiS is detected toward 5 of these 7 sources and exhibits similar kinematics, further supporting the presence of compact rotating structures. In contrast, commonly used hot-core tracers (e.g., CH$_3$CN and SO$_2$) primarily probe larger-scale envelope gas. These results demonstrate that vibrationally excited water, NaCl, and SiS are powerful tracers of disk structures on $\sim$100 au scales, when observed at sufficient angular resolution and sensitivity. The high detection rate suggests that hot-disk chemical patterns - and thus compact rotating disks - are common in massive star-forming regions, at least among sources with well-developed rotating envelopes.