# Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry

## TL;DR

This study uses optical interferometry to measure the surface asymmetries of RGB stars, suggesting convection or magnetic activity as potential causes, and highlights the need for further multi-modal observations.

## Contribution

First interferometric measurements of surface asymmetries on RGB stars, linking asymmetries to stellar surface activity and emphasizing the importance of combined observational approaches.

## Key findings

- Detected surface asymmetries in all three RGB stars.
- Asymmetries tend to be larger in stars with lower surface gravity.
- Possible link between asymmetries and convection or magnetic activity.

## Abstract

Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their atmospheres. Aims. We aim at precisely characterising the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interfer- ometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas, HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause.