# Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths

## TL;DR

This study uses high-resolution ALMA observations to resolve the extended atmospheres of four AGB stars, revealing hotspots, asymmetries, and dynamic motions, providing new insights into their atmospheric structure and variability.

## Contribution

First direct high-resolution imaging of multiple AGB star atmospheres at (sub)millimetre wavelengths, revealing detailed structures and variability not previously observed.

## Key findings

- Resolved all four AGB stars' atmospheres and mapped brightness temperature structures.
- Confirmed hotspots with temperatures >3000 to 10000 K on W Hya and R Dor.
- Detected dynamic surface motions and deviations from spherical symmetry.

## Abstract

The initial conditions for the mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. We used the Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor and R Leo. We interpreted the observations using a parameterised atmosphere model. We resolve all four AGB stars and determine the brightness temperature structure between $1$ and $2$ stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures $>3000$ to $10000$~K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least $10.6\pm1.4$~km~s$^{-1}$. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential.