An Ultraviolet-Optical Color-Metallicity relation for Red Clump Stars using GALEX and Gaia

TL;DR

This study establishes a new ultraviolet-optical color-metallicity relation for red clump stars using GALEX and Gaia data, enabling easier metallicity estimation from photometry.

Contribution

The paper introduces a novel photometric UV-optical color-metallicity relation for red clump stars, validated with a large sample, improving metallicity estimates without spectroscopy.

Findings

Strong correlation between UV-optical color and metallicity in RC stars.

The relation has a scatter of 0.28 dex, comparable to extinction effects.

Photometric metallicity estimates are feasible for large surveys.

Abstract

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.