Contributions of rotation, expansion and line broadening to the morpho-kinematics of the inner CSE of oxygen-rich AGB star R Hya

TL;DR

This study uses ALMA data to analyze how rotation, expansion, and line broadening shape the inner circumstellar environment of the oxygen-rich AGB star R Hya, revealing dominant rotation and shock-induced broadening within 70 au.

Contribution

It provides new insights into the relative roles of rotation, expansion, and line broadening in the inner CSE of R Hya, highlighting the importance of accounting for line broadening effects.

Findings

Rotation is detected with a velocity of a few km/s.

Line broadening is significant within 14 au, likely caused by shocks.

Properly accounting for line broadening affects rotation analysis.

Abstract

We use archival ALMA observations of the CO(2-1) and SiO(5-4) molecular line emissions of AGB star R Hya to illustrate the relative contributions of rotation, expansion and line broadening to the morphology and kinematics of the circumstellar envelope (CSE) within some ~70 au (~0.5 arcsec) from the centre of the star. We give evidence for rotation and important line broadening to dominate the inner region, within ~14 au (~100 mas) from the centre of the star. The former is about an axis that projects a few degrees west of north and has a projected rotation velocity of a few km/s. The latter occurs within some 7-14 au (50-100 mas) from the centre of the star, the line width reaching two to three times its value outside this region. We suggest that it is caused by shocks induced by stellar pulsations and convective cell granulation. We show the importance of properly accounting for the observed line broadening when discussing rotation and evaluating the radial dependence of the rotation velocity.