LOCAL RADIATION HYDRODYNAMIC SIMULATIONS OF MASSIVE STAR ENVELOPES AT THE IRON OPACITY PEAK

Abstract

We perform three dimensional radiation hydrodynamic simulations of the\nstructure and dynamics of radiation dominated envelopes of massive stars at the\nlocation of the iron opacity peak. One dimensional hydrostatic calculations\npredict an unstable density inversion at this location, whereas our simulations\nreveal a complex interplay of convective and radiative transport whose behavior\ndepends on the ratio of the photon diffusion time to the dynamical time. The\nlatter is set by the ratio of the optical depth per pressure scale height,\n$\\tau_0$, to $\\tau_c=c/c_g$, where $c_g \\approx$ 50 km/s is the isothermal\nsound speed in the gas alone. When $\\tau_0 \\gg \\tau_c$, convection reduces the\nradiation acceleration and removes the density inversion. The turbulent energy\ntransport in the simulations agrees with mixing length theory and provides its\nfirst numerical calibration in the radiation dominated regime. When $\\tau_0 \\ll\n\\tau_c$, convection becomes inefficient and the turbulent energy transport is\nnegligible. The turbulent velocities exceed $c_g$, driving shocks and large\ndensity fluctuations that allow photons to preferentially diffuse out through\nlow-density regions. However, the effective radiation acceleration is still\nlarger than the gravitational acceleration so that the time average density\nprofile contains a modest density inversion. In addition, the simulated\nenvelope undergoes large-scale oscillations with periods of a few hours. The\nturbulent velocity field may affect the broadening of spectral lines and\ntherefore stellar rotation measurements in massive stars, while the time\nvariable outer atmosphere could lead to variations in their mass loss and\nstellar radius.\n