Speaker: Don M Dixon (Vanderbilt)
Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Binarity and Sub-subgiants
In this study we build on our previous publication that detailed the use of 133 Red Giant Branch and Red Clump stars to characterize rotationally driven chromospheric activity for giants. This was formulated as an empirical relation between excess emission in the near-ultraviolet (NUV excess) and projected rotational velocities (vsini). With a substantially larger selection of 6,332 giants from APOGEE DR17 we perform a deeper analysis that grants insight into the connection between rotation, magnetic activity and binary evolution for giants. Our analysis starts with SED fitting Castelli-Kurucz model atmospheres for 1338 systems with quality Gaia/2MASS photometry, in order to disentangle the effects of metallicity on NUV excess due to line blanketing. From this procedure we derive linear empirical activity relations (ζ-relations) that approximate expected NUV excess and thus stellar activity, due to rotation and metallicity. To investigate the impact of close (< 100 days) binarity on magnetically active giants we use the maximum radial velocity separation (ΔRVmax) of sparsely sampled RV curves to generate nominal single (ΔRVmax < 1 km/s) and nominal close binary (ΔRVmax > 3 km/s) subsamples. We find 98.3% of single stars are rotating less than 10 km/s vs 78.6% for binaries demonstrating the connection between binarity and rapid rotation (vsini > 10 km/s) for evolved stars. We also use APOGEE and Gaia orbit solutions to show that the spin of rapidly rotating giants appear to be synchronized to their orbits, which supports tidal spin up as the source of their enhanced angular momentum. Lastly, we use a 14 Gyr, [M/H] = 0.5 isochrone to identify the reddest/dimmest Sub-subgiant (SSG) stars and find via comparison to our ζ-relations and the Gaia activity index that they represent the most active class of giant binaries. A quarter (5/20) of these systems have also been crossmatched to the APOGEE-GALEX-Gaia white dwarf binary catalog, which may be tentative evidence that mass transfer and/or common envelope scenarios are non-negligible formation channels of SSGs, but different forms of analysis and followup observations are likely needed to confirm the nature of the gravitational companion.
