Correlations in Chromospheric and Coronal Activity Indicators of Giant Stars

Abstract

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 ii h 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 .

Keywords

• Late stellar evolution
• Giant stars
• X-ray stars
• Ultraviolet astronomy