Building three-dimensional giant stellar models for common envelope simulations

Abstract

We build a three-dimensional (3D) red supergiant (RSG) stellar model by transporting a 1D stellar model to a 3D numerical grid, mimicking core nuclear power by depositing energy into an inner shell, and mimicking stellar emission by cooling grid cells with densities below the photospheric density. The goal is to use such models for common envelope evolution (CEE) and grazing envelope evolution simulations. We do not relax the model; rather, we let it perform its natural pulsation. We find that when we mimic photospheric emission by cooling low-density grid cells, the oscillations decay slowly on a timescale much longer than without photospheric cooling. When we mimic both nuclear energy production, by depositing the stellar luminosity in an inner shell above the inert core of the stellar model, and photospheric cooling, the oscillations do not decay; instead, their amplitude slowly increases with time. The main pulsational period is about 1 year, comparable to the stellar dynamical time, suggesting a fundamental radial pulsation mode. The non-spherical structure of the stellar model and rapid low-amplitude temporal variations in the average stellar radius tentatively hint at the presence of non-radial oscillation modes on top of the fundamental radial mode. We also obtain convection, as RSG stars should have. We conclude that energy deposition in the inner envelope at the stellar luminosity and the cooling of the outer low-density numerical cells are a way of preparing a pulsating giant star to simulate CEE and grazing-envelope evolution. This procedure requires no relaxation of the stellar model.