Mary Geer Dethero
Using a realistic stratification in density, temperature, and luminosity obtained from the stellar evolution code MESA (Lester and Gies, 2018), we produce global multi-dimensional hydrodynamic simulations with the MUSIC code. These MESA models were produced as a best match to the observational data for BW Aqr, but the models fail to produce the observed properties of the stars and predict the two stars to be the same age; improvement to stellar evolution modeling is necessary to understand this particular binary pair, as well as the wealth of new stellar observations available from recent space missions. In this contribution we study individually the properties of non-local stellar convection and convective overshooting in both the primary and secondary star in the eclipsing binary, near the beginning of the red giant branch and the first dredge-up. Using our recent enhanced diffusion model for convective overshooting and penetration (Pratt et al 2017) proposed for one-dimensional stellar evolution calculations, we compare the amount of mixing due to convective overshooting between these stars.
BW Aquarii is a bright eclipsing binary system often used to test stellar models, particularly of convective overshooting (Maxted, Research Notes of the AAS, 2018. Claret and Torres, ApJ 2018. Clausen A&A 1991.) We use the stellar structures produced by: Lester & Gies (2018). Structure, equation of state, and opacities are produced with the stellar structure and evolution code MESA (Modules for Experiments in Stellar Astrophysics). These models incorporate an overshooting diffusion coefficient (Freytag et al 1996, Claret and Torres 2018) where the B star has a larger amount overshooting than the A star by a factor of four.
For each star, we identify a point near the first dredge-up on the red giant branch. This is defined by the extent of the convection zone. Using MUSIC, we perform simulations with identical resolution with respect to the pressure scale height at the bottom of the convection zone. The global simulations of each star includes 95% of the stellar radius.
Preliminary results show that for the binary stars in BW Aquarii at the first dredge-up there is a difference in overshooting of 0.2 Hp (A star) vs 0.7 Hp (B star), measured with the extreme value theory statistical method of Pratt, Baraffe, et al. A&A 2017. Because both stars have the same (1) evolutionary state, (2) metallicity, and (3) similar masses, we emphasize the importance of (4) stellar structure for defining the depth of convective overshooting. We show that the evolution of convective overshooting cannot be described as a fixed constant times the pressure scale height.
Both realistic physical models (compressibility, boundary conditions, global simulations), and realistic Astronomy input (EOS, opacity, chemistry, stratification) are necessary to derive new stellar evolution models based on realistic fluid simulations. These models can then be implemented in stellar evolution codes like MESA and broadly used for stellar and galactic Astrophysics.