Abstract
Abstract Detailed modeling of stellar evolution requires a better understanding of the (magneto)hydrodynamic processes that mix chemical elements and transport angular momentum. Understanding these processes is crucial if we are to accurately interpret observations of chemical abundance anomalies, surface rotation measurements, and asteroseismic data. Here, we use two-dimensional hydrodynamic simulations of the generation and propagation of internal gravity waves in an intermediate-mass star to measure the chemical mixing induced by these waves. We show that such mixing can generally be treated as a diffusive process. We then show that the local diffusion coefficient does not depend on the local fluid velocity, but rather on the wave amplitude. We then use these findings to provide a simple parameterization for this diffusion, which can be incorporated into stellar evolution codes and tested against observations.
Highlights
Accounting for hydrodynamic processes in stellar interiors over stellar evolution times has been the biggest source of uncertainty when comparing theoretical results with observations
The transport of angular momentum by these waves cannot be treated as a diffusive process; Internal gravity waves (IGWs) have an anti-diffusive nature
While it is clear that angular momentum transport by IGWs cannot be parameterized with a diffusion coefficient (Rogers 2015), it is unclear whether the chemical mixing induced by waves could be treated diffusively as previously suggested (Press & Rybicki 1981; Garcia-Lopez & Spruit 1991)
Summary
Accounting for hydrodynamic processes in stellar interiors over stellar evolution times has been the biggest source of uncertainty when comparing theoretical results with observations. This diffusion coefficient is included locally (in space and time) in the stellar evolution calculation While this procedure is rather rudimentary, it is clear from observations that additional mixing within stellar radiative regions is required. The transport of angular momentum by these waves cannot be treated as a diffusive process; IGWs have an anti-diffusive nature That is, they drive, rather than dissipate, shear flows (Buhler 2009). While it is clear that angular momentum transport by IGWs cannot be parameterized with a diffusion coefficient (Rogers 2015), it is unclear whether the chemical mixing induced by waves could be treated diffusively as previously suggested (Press & Rybicki 1981; Garcia-Lopez & Spruit 1991). The purpose of this Letter is to first determine whether wave mixing can be treated diffusively and, if so, to determine how efficient that mixing is and whether it could be reasonably parameterized in 1D stellar evolution models
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