Rotationally Driven Activity of Red Giant Stars in the UV

Abstract

The magnetic activity of stars is known to be produced by stellar dynamos, which are driven by rotation and convection of the stellar atmosphere. However, before a star evolves into a giant, rotation is greatly diminished due to wind angular momentum loss. Additionally, once a star begins ascent of the red giant branch, its envelope begins to swell, and conservation of angular momentum forces rotation to slow further. As a consequence of this evolutionary spin down, giant stars are generally observed to be inactive because they lack the necessary rotation to stimulate strong dynamo action. Given the expectation that giants spin slowly, most stellar activity research has favored observations of dwarf stars. A considerable number of giants have been discovered to have appreciable rotation rates and magnetic fields, but how their stellar activity relates to dwarf stars remains uncertain. In this dissertation, we sample giants observed by the Sloan Digital Sky Survey APOGEE and investigate their stellar activity through empirically derived relationships between near-UV (NUV) excess and projected rotational velocity (vsini). Our relations are initially fit to 133 red giant stars, where we demonstrate NUV excess is strongly correlated to vsini. Furthermore, these relations are found to share trends with M dwarf rotation-activity relations, including saturation/supersaturation. This suggests similar dynamo origins for giants and cool dwarf stars. After expanding our sample to 7,286 giants, we fit a linear correction function ζ([M/H]) to account for metallicity dependence due to line-blanketing. Analysis using ζ([M/H]) corrected NUV excess reveals rotationally active giants typically have large radial velocity variations, which supports tidal synchronization as the dominant channel for rotationally active giants. We also find convincing evidence that giants dimmer/redder than the red giant branch, i.e. sub-subgiants, are especially active synchronized giant binaries. Our rotation-activity relations serve as a general approach for advancing activity research for post-main-sequence stars. Our results add new insight into giant activity by demonstrating fundamental similarity to cool dwarfs and differences between single and binary giants.