HST FGS-1r parallaxes for 8 metal-poor stars

The parallaxes of these stars determined by the new Hipparcos reduction average 17% accuracy, in contrast to our new HST parallaxes, which average 1% accuracy and have errors on the individual parallaxes ranging from 85 to 144{mu}as. These parallax data were combined with HST Advanced Camera for Surveys photometry in the F606W and F814W filters to obtain the absolute magnitudes of the stars with an accuracy of 0.02-0.03 mag.

Six of these stars are on the main sequence (MS) (with -2.7<[Fe/H]< -1.8) and are suitable for testing metal-poor stellar evolution models and determining the distances to metal-poor globular clusters (GCs). Using the abundances obtained by O'Malley+ (2017ApJ...838...90O), we find that standard stellar models using the VandenBerg & Clem (2003AJ....126..778V) color transformation do a reasonable job of matching five of the MS stars, with HD 54639 ([Fe/H]=-2.5) being anomalous in its location in the color-magnitude diagram.