Thomas G. Barnes, III (McDonald Observatory), Fritz Benedict (UT Austin),
Barbara McArthur (UT Austin), Michael W. Feast (Univ. of Cape Town),
Horace A. Smith (Michigan State Univ.), Katrien Kolenberg (CfA),
Denise Taylor (STScI), Thomas Harrison (New Mexico State Univ.),
John Menzies (South African Astr, Obs.), Edmund Nelan (STSCI),
Richard J. Patterson (Univ. of Virginia), and Wendy L. Freedman (Carnegie Observatories)
ABSTRACT
Our program to determine HST parallaxes for the Pop II variable stars XZ Cyg, UV Oct, RZ Cep, SU Dra, Kappa Pav, and VY Py is nearing completion. The approach we have used follows that used success fully for high-precision parallaxes of RR Lyr (Benedict et al. 2002, AJ, 123, 473) and of ten Cepheids (Benedict, G. F. et al. 2007 AJ, 133, 1810). The HST Fine Guidance Sensor is the only current instrument on or off the earth that can consistently deliver high precision (0.2 mas) for critical parallaxes. Combined with ground-based photometric and spectroscopic studies of these stars, we expect the HST program to deliver parallaxes with uncertainties of 3 - 14%.