An Ultraviolet–Optical Color–Metallicity Relation for Red Clump Stars Using GALEX and Gaia

Abstract

Abstract Although core helium-burning red clump (RC) stars are faint at ultraviolet wavelengths, their ultraviolet (UV)–optical color is a unique and accessible probe of their physical properties. Using data from the Galaxy Evolution Explorer All Sky Imaging Survey, Gaia Data Release 2, and the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR14 survey, we find that spectroscopic metallicity is strongly correlated with the location of an RC star in the UV–optical color–magnitude diagram. The RC has a wide spread in (NUV–G)0 color of over 4 mag compared to a 0.7 mag range in (G BP–G RP)0. We propose a photometric, dust-corrected, UV–optical (NUV–G)0 color–metallicity [Fe/H] relation using a sample of 5,175 RC stars from APOGEE. We show that this relation has a scatter of 0.16 dex and is easier to obtain for large, wide-field samples than for spectroscopic metallicities. Importantly, the effect may be comparable to the spread in RC color attributed to extinction in other studies.