The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA

TL;DR

This paper presents high-resolution ALMA observations of an AGB star's atmosphere, revealing shock-heated hot gas with chromospheric features, which impacts models of stellar evolution and mass loss.

Contribution

It provides the first resolved observations of shock-heated gas in an AGB star's atmosphere, highlighting the importance of shocks with longer cooling times.

Findings

Detection of hot, chromospheric gas around W Hya

Small filling factor of the hot gas

Shocks with longer cooling times are necessary

Abstract

Our current understanding of the chemistry and mass-loss processes in solar-like stars at the end of their evolution depends critically on the description of convection, pulsations and shocks in the extended stellar atmosphere. Three-dimensional hydrodynamical stellar atmosphere models provide observational predictions, but so far the resolution to constrain the complex temperature and velocity structures seen in the models has been lacking. Here we present submillimeter continuum and line observations that resolve the atmosphere of the asymptotic giant branch star W Hya. We show that hot gas with chromospheric characteristics exists around the star. Its filling factor is shown to be small. The existence of such gas requires shocks with a cooling time larger than commonly assumed. A shocked hot layer will be an important ingredient in the models of stellar convection, pulsation and chemistry that underlie our current understanding of the late stages of stellar evolution.