Three Dimensional Radiation Hydrodynamic Simulations of Massive Star Envelopes

Abstract

(Abridged) Stars more massive than $20-30M_{\odot}$ are so luminous that the radiation force on the cooler, more opaque outer layers can balance or exceed the force of gravity. These near or super-Eddington outer envelopes represent a long standing challenge for calculating the evolution of massive stars in one dimension, a situation that limits our understanding of the stellar progenitors of some of the most exciting and energetic explosions in the universe. In particular, the proximity to the Eddington limit has been the suspected cause for the variability, large mass loss rate and giant eruptions of an enigmatic class of massive stars: the luminous blue variables (LBVs). When in quiescence, LBVs are usually found on the hot ($T_{eff} \approx 2 - 4 \times 10^4$ K) S Dor instability strip. While in outburst, most LBVs stay on the cold S Dor instability strip with a $T_{eff} \approx 9000$ K. Here we show that physically realistic three dimensional global radiation hydrodynamic simulations of radiation dominated massive stars with the largest supercomputers in the world naturally reproduce many observed properties of LBVs, specifically their location in the Hertzsprung-Russell (HR) diagram and their episodic mass loss with rates of $10^{-7}-10^{-5} M_{\odot}/yr$. The helium opacity peak is found to play an important role to determine these properties, which is not realized in the traditional one dimensional models of massive stars. The simulations also predict that convection causes irregular envelope oscillations and 10-30% brightness variations on a typical timescale of a few days. The variability is more prominent in our models that are on the cool part of the S Dor instability. These calculations pave the way to a quantitative understanding of the structure, stability and the dominant mode of mass loss of massive stars.