Using HST Globular Clusters to Derive an Empirical Photometric Metallicity Relation for Low-mass Stars

We present an analysis of low-mass stars in the ACS Survey of Galactic Globular Clusters (AGC). The AGC obtained exquisite V and I-band photometry of 65 globular clusters, measuring their main sequences down to ~0.2 solar masses. We have used these data to derive an empirical color-[Fe/H]-M^v relation for low-mass dwarfs, employing photometry and photometric [Fe/H] as measured by the AGC team. The resulting relation has two major applications to stellar and Galactic astronomy. First, photometric metallicities can be calculated for any M dwarf with a trigonometric parallax and optical photometry. Second, this relation can be used to improve current photometric parallax relations, resulting in more precise distances for millions of low-mass stars. With large surveys such as LSST and GAIA looming, this relation will facilitate a deeper understanding of the structure and chemical history of the Milky Way. We gratefully acknowledge the financial support of NSF AST-1151462 in this work.