Calibration of the Distance Scale from Cepheids

Abstract

We apply the infrared surface brightness method of Fouque and Gieren to a sample of 32 Galactic Cepheids with excellent photometric and radial velocity data. The distance solutions are fully consistent with recent direct interferometric Cepheid distance measurements, and with Hipparcos parallaxes of nearby Cepheid variables, but are more accurate than these determinations. Fitting the slopes observed for large samples of LMC Cepheids to our Galactic data, we derive absolute period-luminosity (PL) relations in the VIWJHK bands which are more accurate than previous work. Comparing the Galactic and LMC PL relations, we derive the LMC distance modulus in all these bands which can be made to agree extremely well under reasonable assumptions for both, the reddening law, and the adopted reddenings of the LMC Cepheids. Our current best LMC distance modulus determination from this technique is 18.55 ± 0.06 mag. The effect of metallicity on the PL relation is discussed. Our Galactic Cepheid distance determinations yield Galactic Cepheid PL relations which are steeper than their LMC counterparts, in all photometric bands, which could be the signature of a metallicity effect. When determining Cepheid distances to solar-metallicity galaxies, it may be advantageous to use the direct Galactic calibration of the PL relation from the infrared surface brightness technique rather than a LMC PL relation, minimizing possible metallicity-related effects on the distance determination.