Figure 1: Example of an intensity map for a snapshot of a simulation of the AGB atmosphere dynamics. Image credit: A. Chiavassa, B. Freytag, M. Schultheis.

Researchers from the Observatoire de la Côte d'Azur obtain an important result for the use of Gaia data to determine physics of the stars in the Asymptotic Giant Branch (AGB) and show that parallax variations on Gaia's measurements of AGBs can be used to extract the fundamental parameters for these stars.

Low to intermediate mass stars, as for example the Sun, evolve towards the asymptotic branch of giants, showing increased mass loss during their evolution. These stars are characterized by:

• large-amplitude variations in radius, brightness, and surface temperature; and
• a high rate of mass loss due to an interaction between pulsation, dust formation, and radiative pressure on the surface.

Their complex dynamics affect measurements and amplify uncertainties on their stellar parameters.

The Gaia space telescope has delivered high-precision astrometric measurements (positions, parallaxes, and proper motions) for more than a billion sources with its second data release in April 2018. Among all these objects, AGB stars are affected by the complexity of their atmospheric dynamics, which can affect the position of the photocentre and, in turn, their parallaxes.

The surface of AGB stars (see Figure 1) is covered by a few large convective cells (with a long life span). These are supported by smaller-scale structures with a shorter duration. The atmosphere above the surface is made up of shock waves, that are produced inside the star and shaped by the top of the convection zone, when these shock waves travel outward. In the presence of light asymmetries, the position of the photocentre does not coincide with the barycentre of the star and changes as the convective pattern changes over time (see Figure 2).

Figure 2: Position of the photocentre calculated for a simulation in the Gaia G band filter. The different snapshots are connected with a line segment. The red dotted lines intersect at the position of the geometric centre. Image credit: A. Chiavassa, B. Freytag, M. Schultheis.

In order to quantify these movements, it is necessary to use a theoretical approach based on multi-dimensional (and in particular three-dimensional, 3D) radiation-hydrodynamics simulations of the movement of gas in the atmospheric layers of stars, coupled with radiation. In these models, the entire envelope of the star is simulated over time.

The displacement of the photocentre was obtained through simulations and compared with the measurement uncertainty on the parallax of a sample of AGB stars in the solar neighbourhood. It was found that there is a good agreement with the observations, suggesting that the variability related to convection largely explains the Gaia parallax uncertainties. In addition, these researchers have shown that, in the simulations, larger displacements of the photocentre correspond to longer pulsation periods. Consequently, parallax variations on Gaia's measurements could be used to extract the fundamental parameters of these stars.

This is Gaia's first result on AGB physics which is described in the A&A letter to the editor "Heading Gaia to measure atmospheric dynamics in AGB stars". AGBs are crucial contributors to the chemical enrichment of galaxies because they lose huge amounts of their mass. The vigorous convection that characterizes them could be the basis of the mass loss mechanism and only radiation-hydrodynamics simulations can help astronomers fully understand all the physical processes that characterize it.

With future releases of Gaia, more precise parallax measurements will become available which will hopefully improve these characterisations even further.