Distribution and kinematics of the HCN(3-2) emission down to the innermost region in the envelope of the O-rich star W Hya

TL;DR

This study uses high-resolution submillimeter observations to map HCN (3-2) emission in W Hya's inner envelope, revealing its formation close to the star and suggesting non-standard mass-loss mechanisms.

Contribution

It provides the first high-resolution imaging of HCN (3-2) in W Hya's inner envelope, confirming in-situ formation and challenging traditional mass-loss models.

Findings

HCN (3-2) emission is centrally peaked and spherically symmetric.

HCN forms within ~10 stellar radii, consistent with shock chemistry.

Detected a ~5 km/s velocity gradient possibly due to rotation or bipolar outflow.

Abstract

We report high angular resolution observations of the HCN (3-2) line emission in the circumstellar envelope of the O-rich star W Hya with the Submillimeter Array. The proximity of this star allows us to image its molecular envelope with a spatial resolution of just ~40 AU, corresponding to about 10 times the stellar diameter. We resolve the HCN (3-2) emission and find that it is centrally peaked and has a roughly spherically symmetrical distribution. This shows that HCN is formed in the innermost region of the envelope (within ~10 stellar radii), which is consistent with predictions from pulsation-driven shock chemistry models, and rules out the scenario in which HCN forms through photochemical reactions in the outer envelope. Our model suggests that the envelope decreases steeply in temperature and increases smoothly in velocity with radius, inconsistent with the standard model for mass-loss driven by radiative pressure on dust grains. We detect a velocity gradient of ~5 km/s in the NW--SE direction over the central 40 AU. This velocity gradient is reminescent of that seen in OH maser lines, and could be caused by the rotation of the envelope or by a weak bipolar outflow.