Collaborative Research: CDS&E: 3-D Stellar Hydrodynamics of Convective Penetration and Convective Boundary Mixing in Massive Stars

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Convection is a ubiquitous feature in Nature and occurs whenever energy transport by radiation and conduction is insufficient. It can be found in the Earth's atmosphere (e.g., cumulus clouds) and mantle (continental drift), and inside stars, where it can impact the chemical enrichment of their outer envelopes as well their Main Sequence lifetime. Despite tremendous progress, convection within stars is still poorly understood in some important respects, and our current best physical theory (mixing length theory; MLT), is inadequate in many situations. The team of researchers will apply a new 1-D convection model to improve the treatment of convection, particularly convective penetration or overshooting due to convective boundary mixing (CBM). This approach promises improved estimates of Main Sequence lifetimes as well as a more correct understanding of how adjacent "burning" shells within the cores of massive stars exchange matter and energy through convection. The award will also support the development of instructional software and modules suitable for undergraduates and college-bound high school students as well as summer research by undergraduate students attending the participating institutions. Simulation data will also be made available to other researchers. The team will work to improve the understanding of stellar evolution by removing key uncertainties that arise in standard MLT. This will be done by applying a new 1-D model of convection developed in their previous award. This model employs a new constraint equation which enables an improved estimate of the extent of convective penetration for a given the kinetic energy of convective motions at the Schwarzschild boundary. This kinetic energy cannot be known from a 1-D stellar evolution computation but can be determined accurately and inexpensively through a short 3-D simulation. The team will advance 1-D stellar evolution simulations using this more accurate representation of convective overshooting, and hence of CBM, by using the kinetic energy estimates together with the new constraint equation implemented in 1-D MESA code, while occasionally recalibrating the kinetic energy estimates with short yet inexpensive 3-D simulations. Using these periodic recalibrations, this procedure eliminates ad hoc free parameters, including the mixing length, from the 1-D model of convection and CBM. 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.