Convective Boundary Mixing in Massive Stars

The biggest stars can produce bright explosions like supernova or collapse into black holes. One important question is whether fresh fuel from the outer part of the star can get mixed into the center of the star, where it could burn, extending the life of the star. Observations of these stars suggest they live longer than might be expected, so some mixing must be occurring in the star. This work will model mixing in massive stars using computer simulations. The simulations will determine what fraction of the star gets mixed. These results will be used to make new predictions of the lifetimes of such stars, and they will be compared to observations. The results will also be used to predict how many supernova and black holes we expect to see. Each summer, this project will support high school students to complete independent research projects supervised by the project team. This award will support the Research Experience in Astronomy at CIERA for High school students program at Northwestern. This is a highly interactive three-week program that provides high school students experience with astronomy research in an atmosphere of team-style learning, hands-on training, and mentorship from professional scientists. Although rare, massive stars are disproportionately important in astrophysics. They are progenitors of neutron stars and black holes, and they chemically enrich their environments through winds and/or mass loss. Accurate stellar and population synthesis models of intermediate- and high-mass stars are required to robustly predict properties of stellar remnants and nucleosynthetic yields. The lives and deaths of these stars are intrinsically linked to mixing that occurs at the boundary of their convective cores. If fresh fuel can mix into the core, it can extend the star’s main-sequence lifetime and alter its subsequent evolution. Constraining convective boundary mixing is essential for accurate and robust neutron star and black hole population synthesis modeling. This investigator will derive convective boundary mixing parameterizations from multi-dimensional numerical simulations. The team will run a suite of three-dimensional global spherical numerical simulations to measure convective penetration. They will determine how convective penetration varies with stellar mass, age, and rotation rate. The parameterization of convective penetration will be implemented in the MESA code, and we will validate it by comparing to asteroseismic observations. Finally, they will use the new parameterization in the population synthesis code POSYDON to determine how our new parameterization affects compact object binary merger rates. 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.