2023AJ....165...70C

The main goal of this paper is to use ultraviolet photometry from the Galaxy Evolution Explorer (GALEX) satellite to study chromospheric activity among red giant and core-helium-burning (CHeB) stars. Correlations are sought between several chromospheric and coronal activity indicators for giants that were detected in soft X-rays by the ROentgen SATellite. There is an evident correlation between $\mathrm{log}({L}_{{\rm{X}}}/{L}_{\mathrm{bol}})$ and $\mathrm{log}({F}_{\mathrm{Mg}\,{\rm\small{II}}}/{F}_{\mathrm{bol}})$, where L_X is the X-ray luminosity and F_Mg II_ is the flux from Mg IIh and k emission lines, although there is substantial scatter. Using GALEX far-ultraviolet (FUV) magnitudes, the relationship between an FUV-excess parameter (a proxy for chromospheric and transition-region emission lines) and X-ray luminosity is documented. Correlations found herein are not strong, and may be linked to binarity. There is a varied range of FUV emission among CHeB stars, indicating that giants in this phase may be exhibiting differences or cycles in outer atmospheric activity. Additionally, efforts were made to constrain a relationship between excess FUV and near-ultraviolet emission and the projected surface rotation velocity, $v\sin i$, of giants. No clear relationship was found. Obscurities in this relationship may result from a spread in $\sin i$ inclinations, and/or too few stars with $v\sin i\gt 7$ km s^–1.