Numerical Simulations of Convective 3-Dimensional Red Supergiant Envelopes

We explore the three-dimensional properties of convective, luminous ($L\approx10^{4.5}-10^{5}L_\odot$), Hydrogen-rich envelopes of Red Supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using $\texttt{Athena++}$. These computations comprise $\approx30\%$ 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 $M\approx15M_\odot$ 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 $\approx1/3$ of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of $\alpha=3-4$, 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 ($\approx300$ days), with amplitudes consistent with those observed photometrically.