Hubble Space Telescope Observations of M Subdwarfs

Abstract

We present the results of a Hubble Space Telescope snapshot program to search for very low mass stellar companions to nearby M subdwarfs. None of our nine targetted metal-poor primaries has companions more massive than the hydrogen burning limit, implying that the halo binary fraction is equal to or less than the Galactic disk binary fraction below 0.3 M_solar. In addition, the more distant tertiary VB 12, an sdM3.0 companion to an F subdwarf double, is also unresolved. We show that the relation between WFPC2 F555W and F850LP photometry and ground-based V,I photometry is consistent with theoretical expectations. We also report that two recently observed Hyades M dwarfs appear single.