Three-Dimensional Hydrodynamic Simulations of Convective Nuclear Burning In Massive Stars Near Iron Core Collapse

Non-spherical structure in massive stars at the point of iron core collapse can have a qualitative impact on the properties of the ensuing core-collapse supernova explosions and the multi-messenger signals they produce. Strong perturbations can aid successful explosions by strengthening turbulence in the post-shock region. Here, we report on a set of $4\pi$ 3D hydrodynamic simulations of O- and Si-shell burning in massive star models of varied initial masses using MESA and the FLASH simulation framework. We evolve four separate 3D models for roughly the final ten minutes prior to, and including, iron core collapse. We consider initial 1D MESA models with masses of 14-, 20-, and 25 $M_{\odot}$ to survey a range of O/Si shell density and compositional configurations. We characterize the convective shells in our 3D models and compare them to the corresponding 1D models. In general, we find that the angle-average convective speeds in our 3D simulations near collapse are three to four times larger than the convective speeds predicted by MESA at the same epoch for our chosen mixing length parameter of $\alpha_{\rm{MLT}}=1.5$. In three of our simulations, we observe significant power in the spherical harmonic decomposition of the radial velocity field at harmonic indices of $\ell=1-3$ near collapse. Our results suggest that large-scale modes are common in massive stars near collapse and should be considered a key aspect of pre-supernova progenitor models.