College of Science & Engineering
The evolution of stars and their ultimate fate as white dwarfs, white dwarf supernovae (SN Type Ia), neutron stars (core-collapse SN Type II), or black holes remains at the center of research in astronomy, and relates to diverse research questions in time domain astronomy, supernova research, nuclear astrophysics, galactic chemical evolution, asteroseismology, stellar populations, and the formation and evolution of stars and structure in the early universe. Despite tremendous progress, the fundamental macroscopic stellar physics process of convection remains poorly understood in some important aspects. Mixing-length theory, the standard treatment of convection in stellar evolution, is valid, and in fact surprisingly accurate, in regimes where the nuclear and thermal time scales are much longer than the convective turn-over time scales, such as main-sequence evolution. However, mergers of convective shells, such as the C- and O-burning shells of massive stars, as well as convective boundary mixing can only be realistically captured in complex and computationally intensive 3D hydrodynamics simulations.
In previous work, this group has validated a 1D model of convective boundary mixing involving a set of coefficients that can be determined from brief 3D stellar hydrodynamics simulations in conditions that apply to massive stars near the ends of their lives. This project involves an assessment of conditions under which two convective shells that are located right on top of each other can in fact merge, so that substantial amounts of material are exchanged. The researchers will simulate this process via series of alternating 1D simulations with the 1D model, and 3D simulations. 3D stellar hydrodynamical simulations of mergers of H- and He-burning shells and of C- and O-burning shells in massive stars will help the group to understand what the consequences are when nearby shells exchange their chemical abundance mixtures, leading to potentially eruptive and violent outbursts.
Additional previous work studied the ingestion of fuel from above the oxygen burning shell in a massive star, fitting results of the 3D simulations to the parameters of a model of convective boundary mixing suitable for use in 1D stellar evolution codes. Studies in 1D indicate that with these ingestion rates the convection zone above the oxygen burning shell could incorporate enough material from above it to reach the carbon burning shell. The 1D studies indicate that such burning shell mergers could cause odd-Z elements like potassium, scandium, and chlorine to be produced in greater abundance. The researchers have established an ability to produce from 3D simulations over brief time periods 1D model descriptions that can carry the star forward in time in 1D over longer periods. They will use a sequence of such brief 3D simulations alternating with longer 1D simulations to describe the approach to and ultimate shell merger process.
In the process of carrying out these simulations, the researchers are generating a large database of results from 3D simulations of convective boundary mixing in stellar interiors. They are organizing this database so that they can mine it, comparing it to a variety of potentially useful 1D models of the mixing process. This database, together with the tools that mine it, analyze it, and display results, is unique. It represents a significant investment, not only in the group's time-developing simulation codes and data analysis tools, but also in computer time on one of the most powerful computing systems in the world. A broad impact of the project will therefore be this database and tools that make it useful in stellar evolution research. The researchers have started to make analytic tools combined with data and processing capabilities available to the community on-line in a useful format, so that it will enable research by many others. The simulations will clarify the nucleosynthetic yields for stars that experience convective-reactive flash events. The researchers are working with the NuGrid collaboration to see that their results are incorporated into datasets that are available to the community for chemical evolution simulations of galaxies and structures in the early universe. The results for nucleosynthesis will depend upon nuclear reaction rates of unstable species that are becoming feasible to investigate through experiments. The researchers are therefore collaborating with the NSF's Joint Institute for Nuclear Astrophysics.