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, ζ Gem, β 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 Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: l Car, 2.01± 0.20 ; ζ Gem, 2.78±0.18 ; β Dor, 3.14±0.16 ; W Sgr, 2.28±0.20 ; X Sgr, 3.00±0.18 ; Y Sgr, 2.13±0.29 ; FF Aql, 2.81±0.18 ; T Vul, 1.90±0.23 ; and RT Aur, 2.40±0.19 , an average σπ/π = 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 WV I bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct Period-Luminosity relations for ten Galactic Cepheids. We compare our new Period-Luminosity relations with those adopted by several recent investigations, including the Freedman and Sandage H0 projects. Adopting our Period-Luminosity relationship would tend to increase the Sandage H0 value, but leave the Freedman H0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and WV I identical in the two galaxies within their respective errors. Our data lead to a WV I distance modulus for the Large Magellanic Cloud, m-M = 18.50±0.03, uncorrected for any metallicity effects. Applying recently derived metalllcity corrections yields a corrected LMC distance modulus of (mM)0=18.40±0.05. Comparing our Period-Luminosity relationship to solar-metallicity Cepheids in NGC 4258 results in a distance modulus, 29.28 ± 0.08, which agrees with that derived from maser studies. Subject headings: astrometry — interferometry — stars: distances — stars: individual (l Car, ζ Gem, β Dor, W Sgr, X Sgr, Y Sgr, δ Cep, FF Aql, T Vul, RT Aur) — stars: binary — distance scale calibration — stars: Cepheids — stars: variables — galaxies: individual (Large Magellanic Cloud, NGC 4258)