An Ultraviolet-Optical Color-Metallicity relation for Red Clump Stars
  using GALEX and Gaia
  Although core helium-burning red clump (RC) stars are faint at ultraviolet
wavelengths, their ultraviolet-optical color is a unique and accessible probe
of their physical properties. Using data from the GALEX All Sky Imaging Survey,
Gaia Data Release 2 and the SDSS 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, over 4 magnitudes, compared to a 0.7-magnitude range in (G$_{BP}$ -
G$_{RP}$)$_0$. We propose a photometric, dust-corrected, ultraviolet-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 0.28 dex and is
easier to obtain for large, wide-field samples than spectroscopic
metallicities. Importantly, the effect may be comparable to the spread in RC
color attributed to extinction in other studies.
