Stellar Convection Zone Evolution as the Origin of Anomalous Rotational Evolution Epochs

Low-mass stars are born spinning. Their spin slows down over billions of years, because magnetic fields created inside these stars act like brakes. A star’s age can be estimated from how quickly it spins. This is one of the few techniques available to estimate ages of individual stars. However, the predictable slowing of a star’s spin is interrupted twice in a star’s life, so-called "stalling epochs." One epoch happens early in a star’s life, the other happens toward the end of its life. The investigator and students from the University of North Georgia will explore a hypothesis that stalling epochs begin when surface convection zones in low-mass stars grow larger. They will test if this growth weakens magnetic field braking or creates a strong connection between a star’s surface and its faster spinning core. This work will help improve the accuracy of spin-based age estimates. This project also supports educational outreach efforts of the investigator through the creation of Astronomy on Wheels. The investigator and a team of students will promote astronomy through events at local schools and festivals. The researchers will investigate that convection zone growth triggers stalling epochs by creating a two-zone stellar rotational evolution model. Their approach will modify existing formulations to capture convection zone dynamics more accurately, which will be supplied by one-dimensional stellar evolution models. Physics included in the stellar evolution models will be explored to constrain the relevant timescales for convection zone growth, and the total convection zone growth expected for stars of different masses. This will help researchers to distinguish between physical mechanisms linking convection zone growth to stalling: weakened magnetic braking or internal angular momentum transport. The results will be used to connect early-age and late-age stalling - currently thought to be independent phenomena - and make testable predictions about which stars should exhibit stalling behavior. Furthermore, the two-zone model will be extended by explicitly including a third zone that accounts for the evolution of convective cores, and including terms that quantify latent angular momentum stored in convective flows. In doing so, the proposed program will advance an explanation for stalling based on stellar structure and evolution and improve age estimates from stellar rotation by clarifying how stars evolve through stalling epochs. 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.