Don Dixon (Vanderbilt U.)


Title: Rotationally Driven UV Excess of Evolved Stars. Insights from Metallicity and Binarity Abstract: As stars evolve on the main-sequence their rotation is gradually slowed as a result of angular momentum being carried away by their magnetized winds. In addition to this magnetic breaking effect, ascending the red giant branch slows rotation even further due to conservation of angular momentum during envelope expansion. These spin down mechanisms predict that virtually all evolved stars should have undetectable rotation, but a significant number of rapidly rotating (vsini > 10 km/s) giants stars have been discovered. The likely origin of rapid rotation for such giants is they are spun up via some gravitational interaction with a close companion, making them interesting systems in the context of binary evolution. In previous work we investigated the properties of a set of evolved stars and reported strong correlations between spin and excess near-ultraviolet (NUV) emission, showcasing rotationally driven activity for spun up giants. We now expand upon that research using the 17th data release of APOGEE to closely investigate dependencies on metallicity and binarity. With a ~60x increase in sample size we now find a strong metallicity dependence on NUV excess, demonstrating the importance of line blanketing. We also find that close binaries with an evolved primary have both markedly higher NUV excess and vsini compared to more distant binaries or single evolved stars. This suggests that tidal synchronization spins up the primary which generates the bulk of the systems NUV excess via chromospheric activity, rather than the photosphere of the potentially hot companion. Lastly, we discuss the atmospheric effects of saturation and starspots and how respectively they may result in supersaturated stars and the evolutionarily peculiar sub-subgiants.