Hubble Space Telescope fine guidance sensor parallaxes of galactic cepheid variable stars:: Period-luminosity relations

We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: l Car, zeta Gem, beta Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on the Hubble Space Telescope. We find absolute parallaxes in milliarcseconds: l Car, 2: 01 +/- 0: 20 mas; zeta Gem, 2: 78 +/- 0: 18 mas; beta Dor, 3: 14 +/- 0: 16 mas; W Sgr, 2: 28 +/- 0: 20 mas; X Sgr, 3: 00 +/- 0: 18 mas; Y Sgr, 2: 13 +/- 0: 29 mas; FF Aql, 2: 81 +/- 0: 18 mas; T Vul, 1: 90 +/- 0: 23 mas; and RT Aur, 2: 40 +/- 0: 19 mas; average sigma(pi)/pi = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit W-VI bandpasses, corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for delta Cep, we construct period-luminosity relations (PLRs) for 10 Galactic Cepheids. We compare our new PLRs with those adopted by several recent investigations, including the Freedman and Sandage H-0 projects. Adopting our PLR would tend to increase the Sandage H-0 value, but leave the Freedman H-0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and W-VI to be identical in the two galaxies within their respective errors. Our data lead to a W-VI distance modulus for the LMC m - M = 18: 50 +/- 0: 03, uncorrected for any metallicity effects. Applying recently derived metallicity corrections yields a corrected LMC distance modulus of (m - M)(0) 18: 40 +/- 0.05. Comparing our PLR to solar-metallicity Cepheids in NGC 4258 results in a distance modulus 29: 28 +/- 0: 08 that agrees with one derived from maser studies.