Numerical Simulations of Convective 3-Dimensional Red Supergiant Envelopes

Abstract

We explore the three-dimensional properties of convective, luminous ($L\approx 10^{4.5}-10^5{L}_{\odot }$), Hydrogen-rich envelopes of Red Supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using $\text{Athena++}$. These computations comprise $\approx 30\%$ of the stellar volume, include gas and radiation pressure, and self-consistently track the gravitational potential for the outer $\approx 3M_{\odot }$ of the simulated $M\approx 15M_{\odot }$ stars. This work reveals a radius, $R_{\mathrm{corr}}$, around which the nature of the convection changes. For $r>R_{\mathrm{corr}}$, though still optically thick, diffusion of photons dominates the energy transport. Such a regime is well-studied in less luminous stars, but in RSGs, the near- (or above-) Eddington luminosity (due to opacity enhancements at ionization transitions) leads to the unusual outcome of denser regions moving outwards rather than inward. This region of the star also has a large amount of turbulent pressure, yielding a density structure much more extended than 1D stellar evolution predicts. This "halo" of material will impact predictions for both shock breakout and early lightcurves of Type II-P supernovae. Inside of $R_{\mathrm{corr}}$, we find a nearly flat entropy profile as expected in the efficient regime of mixing-length-theory (MLT). Radiation pressure provides $\approx 1/3$ of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of $\hat{I}\pm =3-4$, consistent with the sizes of convective plumes seen in the simulations. The temporal variability of these 3D models is mostly on the timescale of the convective plume lifetimes ($\approx 300$ days), with amplitudes consistent with those observed photometrically.

24 pages+refs, 22 figures, accepted for publication in ApJ