Convectively Driven 3D Turbulence in Massive Star Envelopes. I. A 1D Implementation of Diffusive Radiative Transport

Abstract

Abstract Massive (M > 30 M ⊙) stars exhibit luminosities that are near the Eddington limit for electron scattering, causing the increase in opacity associated with iron at T ≈ 180 , 000 K to trigger supersonic convection in their outer envelopes. Three-dimensional radiative hydrodynamics simulations by Jiang and collaborators with the Athena++ computational tool have found order-of-magnitude density and radiative flux fluctuations in these convective regions, even at optical depths ≫ 100 . We show here that radiation can diffuse out of a parcel during the timescale of convection in these optically thick parts of the star, motivating our use of a “pseudo” Mach number to characterize both the fluctuation amplitudes and their correlations. In this first paper, we derive the impact of these fluctuations on the radiative pressure gradient needed to carry a given radiative luminosity. This implementation leads to a remarkable improvement between 1D and 3D radiative pressure gradients, and builds confidence in our path to an eventual 1D implementation of these intrinsically 3D envelopes. However, simply reducing the radiation pressure gradient is not enough to implement a new 1D model. Rather, we must also account for the impact of two other aspects of turbulent convection: the substantial pressure and the ability to transport an appreciable fraction of the luminosity, which will be addressed in upcoming works. This turbulent convection also arises in other instances where the stellar luminosity approaches the Eddington luminosity. Hence, our effort should apply to other astrophysical situations where an opacity peak arises in a near Eddington-limited, radiation pressure-dominated plasma.