Quantifying mixing by shear instabilities in stellar interiors: differential rotation

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Understanding the details of the interiors of stars and of their evolution with time requires a combination of careful observations and sophisticated theoretical models. This research team at the University of California-Santa Cruz will provide improved, numerically-validated quantitative computer models for the transport and mixing of gases and energy within stellar interiors. The research will bring up to date prescriptions derived nearly 30 years ago that have never strictly been tested. The work is of fundamental importance for scientific investigations into the internal rotation of stars. This is particularly relevant today, when stellar interiors are being probed with asteroseismology observations. The principal investigator will continue to advise young researchers at all levels. She is involved at the executive level in two international graduate training programs, the Woods Hole Geophysical Fluid Dynamics (GFD) program, and the Kavli Summer Program in Astrophysics. She helps run the programs and advises students through them. The next GFD program will be focused on topics similar to this research project.The PI and her team will obtain robust quantitative estimates for mixing (of heat, momentum, angular momentum, and chemical species) induced by hydrodynamic shear instabilities in stably stratified regions of rotating stars. Numerical simulations will be run in rotating Cartesian domains located at various latitudes in the star using the shearing-box formalism. The simulations will be used to measure, for given shearing rate, rotation rate, and fluid parameters, global outcomes such as the turbulent Reynolds stresses, turbulent fluxes of heat, and the chemical species. Two regimes will be explored, one in which the effects of thermal diffusion are important (the low Peclet number limit) and one in which they are not. The latter is the standard shear instability problem, while the former has been invoked as an alternative mechanism called secular shear instability. The results will be used to validate or invalidate existing mixing prescriptions, and create new prescriptions as necessary. The team also plans to look at both baroclinically stable and unstable cases.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.