Retrieving stellar parameters and dynamics of AGB stars with Gaia parallax measurements and CO⁵BOLD RHD simulations

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange, CS 34229 Nice, France
² Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
³ Institute of Applied Physics, TU Wien, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria

Received: 4 April 2024
Accepted: 16 July 2024

Abstract

Context. The complex dynamics of asymptotic giant branch (AGB) stars and the resulting stellar winds have a significant impact on the measurements of stellar parameters and amplify their uncertainties. Three-dimensional (3D) radiative hydrodynamic (RHD) simulations of convection suggest that convection-related structures at the surface of AGB star affect the photocentre displacement and the parallax uncertainty measured by Gaia.

Aims. We explore the impact of the convection on the photocentre variability and aim to establish analytical laws between the photo-centre displacement and stellar parameters to retrieve such parameters from the parallax uncertainty.

Methods. We used a selection of 31 RHD simulations with CO⁵BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [320–1050 nm]. From these maps, we calculated the photocentre position and temporal fluctuations. We then compared the synthetic standard deviation to the parallax uncertainty of a sample of 53 Mira stars observed with Gaia.

Results. The simulations show a displacement of the photocentre across the surface ranging from 4 to 13% of the corresponding stellar radius, in agreement with previous studies. We provide an analytical law relating the pulsation period of the simulations and the photocentre displacement as well as the pulsation period and stellar parameters. By combining these laws, we retrieve the surface gravity, the effective temperature, and the radius for the stars in our sample.

Conclusions. Our analysis highlights an original procedure to retrieve stellar parameters by using both state-of-the-art 3D numerical simulations of AGB stellar convection and parallax observations of AGB stars. This will help us refine our understanding of these giants.