UC Santa Barbara

Modeling of massive star (M > 10 solar masses) outer envelopes has remained a challenge for decades. Due to the complex physics involved, 1D models of stellar envelopes can only be evolved under many approximations that attempt to incorporate (or alleviate) the intricate interactions between matter and radiation. To reveal the multi-dimensional nature of massive star envelopes, we performed 3D radiation hydrodynamic simulations of the main sequence and post-main sequence evolution of massive stars using Athena++. These 3D models capture the detailed structures and interactions of the gas and radiation fields, in particular the time-dependent, vigorous turbulence excited by iron and helium opacity peaks in the near-surface convection zones. This turbulence becomes trans-sonic and creates large density fluctuations that propagate to the surface, eliminating the common notion of a spatially confined convection zone and a constant-radius photosphere. Strong anti-correlations between radiation flux and density decrease the radiation pressure force by up to 80%, rendering the dynamical pressure of the turbulence essential in maintaining force balance. As predicted by Henyey et. al. (1965), we show that this turbulent pressure support impacts the adiabatic temperature gradient and significantly reduces the superadiabaticity of these convection zones. Turbulent motions propagating to the surface from the Fe convection zone have significant observational impacts. The dynamic surface topography generates stochastic low-frequency brightness variability that is consistent with that observed in similar stars by recent photometric surveys (e.g. TESS). Additionally, we used the frequency-dependent Monte Carlo radiation transport code Sedona to self-consistently synthesize the spectral features of these turbulent stellar envelopes, revealing that the time-dependent surface velocities generate spectral line broadening and variability. Our work proposes future improvements to 1D stellar evolution models and suggests the need for a novel understanding of how turbulent surface velocities affect spectral line profiles.