The effect of metallicity on the Cepheid Period-Luminosity relation from a Baade-Wesselink analysis of Cepheids in the Galaxy and in the Small Magellanic Cloud

We have applied the near-IR Barnes-Evans realization of the Baade-Wesselink method as calibrated by Fouque & Gieren (1997) to five metal-poor Cepheids with periods between 13 and 17 days in the Small Magellanic Cloud as well as to a sample of 34 Galactic Cepheids to determine the effect of metallicity on the period-luminosity (P-L) relation. For ten of the Galactic Cepheids we present new accurate and well sampled radial-velocity curves. The Baade-Wesselink analysis provides accurate individual distances and luminosities for the Cepheids in the two samples, allowing us to constrain directly, in a purely differential way, the metallicity effect on the Cepheid P-L relation. For the Galactic Cepheids we provide a new set of P-L relations which have zero-points in excellent agreement with astrometric and interferometric determinations. These relations can be used directly for the determination of distances to solar-metallicity samples of Cepheids in distant galaxies, circumventing any corrections for metallicity effects on the zero-point and slope of the P-L relation. We find evidence for both such metallicity effects in Our data. Comparing our two samples of Cepheids at a mean period of about 15 days, we find a weak effect of metallicity on the luminosity similar to that adopted by the HST Key Project on the Extragalactic Distance Scale. The effect is smaller for the V band, where we find DeltaM(V)/Delta[Fe/H]=-0.21+/-0.19, and larger for the Wesenheit index W, where we find DeltaM(W)/Delta[Fe/H]=-0.29+/-0.19. For the I and K bands we find DeltaM(I)/Delta[Fe/H]=-0.23+/-0.19 and DeltaM(K)/Delta[Fe/H]=-0.21+/-0.19, respectively. The error estimates are 1 sigma statistical errors. It seems now well established that metal-poor Cepheids with periods longer than about 10 days are intrinsically fainter in all these bands than their metal-rich counterparts of identical period. Correcting the LMC distance estimate of Fouque et al. (2003) for this metal licit effect leads to a revised LMC distance modulus of (m-M)(0)=18.48+/-0.07, which is also in excellent agreement with the value of (m-M)(0) 18.50+/-0.10 adopted by the Key Project. From our SMC Cepheid distances we determine the SMC distance to be 18.88+/-0.13 mag irrespective of metallicity.