THE CEPHEID PERIOD-LUMINOSITY RELATION FROM INDEPENDENT DISTANCES OF 100 GALACTIC VARIABLES

We have determined distances to 100 galactic classical Cepheids on a h omogeneous system with an improved version of the surface brightness t echnique, using the extensive northern Cepheid observations of Moffett and Barnes, and the southern Cepheid observations of Gieren. The peri od-luminosity relations in the V bandpass obtained for the northern an d southern Cepheid samples are found consistent, and the absolute magn itudes have been combined into a two-hemisphere PL(V) relation which i s M(V) = - 1.371 - 2.986 log P. +/- 0.095 +/- 0.094 This is the curren tly most precise Cepheid PL(V) relation from the surface brightness te chnique. Typically M(V) can be obtained from this relation for Cepheid s with periods in the range 3 to 45 days to better than +/-0.06 mag, i f all the scatter in the relation is observational, or +/-0.27 mag, if all the scatter is cosmic in origin. We compare our result to other r ecent observational determinations of the PL(V) relation and find cons istency among the results produced by the surface brightness method, m ain-sequence fitting of open clusters and associations containing Ceph eids, Hbeta photometry, and statistical parallaxes. In particular, the re is no significant difference between the surface brightness and ZAM S-fitting Cepheid distance scales. The agreement between the ZAMS-fitt ing distance scale and the fully independent surface brightness distan ce scale provides extremely persuasive evidence that both methods are free of significant systematic errors at the +/- 0.10 mag level. We sh ow that there is no significant correlation between the residuals of t he M(V) from the PL(V) relation and the residuals of the intrinsic (B - V) colors of the Cepheids from the mean period-color relation, makin g it impossible to find a PLC relation from our data. After correcting the Cepheid distances for the small, theoretically predicted metallic ity dependence of the surface brightness technique, we find no signifi cant trend in the M(V) residuals from our PL(V) relation with the gala ctocentric distances of the Cepheids. While the Cepheids become intrin sically bluer as their radial distances from the Galactic center incre ase, their mean absolute magnitudes, at a given period, do not depend on their galactocentric distances at a significant level, which is an important property in view of the use of Cepheid variables as extragal actic distance indicators.