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We perform three dimensional radiation hydrodynamic simulations of thestructure and dynamics of radiation dominated envelopes of massive stars at thelocation of the iron opacity peak. One dimensional hydrostatic calculationspredict an unstable density inversion at this location, whereas our simulationsreveal a complex interplay of convective and radiative transport whose behaviordepends on the ratio of the photon diffusion time to the dynamical time. Thelatter is set by the ratio of the optical depth per pressure scale height,$\tau_0$, to $\tau_c=c/c_g$, where $c_g \approx$ 50 km/s is the isothermalsound speed in the gas alone. When $\tau_0 \gg \tau_c$, convection reduces theradiation acceleration and removes the density inversion. The turbulent energytransport in the simulations agrees with mixing length theory and provides itsfirst numerical calibration in the radiation dominated regime. When $\tau_0 \ll\tau_c$, convection becomes inefficient and the turbulent energy transport isnegligible. The turbulent velocities exceed $c_g$, driving shocks and largedensity fluctuations that allow photons to preferentially diffuse out throughlow-density regions. However, the effective radiation acceleration is stilllarger than the gravitational acceleration so that the time average densityprofile contains a modest density inversion. In addition, the simulatedenvelope undergoes large-scale oscillations with periods of a few hours. Theturbulent velocity field may affect the broadening of spectral lines andtherefore stellar rotation measurements in massive stars, while the timevariable outer atmosphere could lead to variations in their mass loss andstellar radius.

Local Radiation Hydrodynamic Simulations of Massive Star Envelopes at the Iron Opacity Peak