The Cepheid Period‐Luminosity Relation and the Maser Distance to NGC 4258

In a recent paper describing HST observations of Cepheids in the spiralgalaxy NGC 4258, Newman et al. (2001) report that the revised calibrations andmethods for the Key Project on the Extragalactic Distance Scale yield that thetrue distance modulus of this galaxy is 29.40+-0.09 mag, corresponding to ametric distance of 7.6+-0.3 Mpc. This Cepheid distance, which holds for 18.50mag as the true distance modulus of the LMC, is not significantly larger than7.2+-0.5 Mpc, the value determined by Herrnstein et al. (1999) from purelygeometric considerations on the orbital motions of water maser sources.However, if the metallicity difference D[O/H]~0.35 between NGC 4258 and LMC istaken into account, then the Key Project methods lead to ametallicity-corrected value of 29.47+-0.09 mag, with 18.50 mag for the LMC,namely to a Cepheid distance of 7.8+-0.3 Mpc, which is 1.2 sigma from the maserdetermination. In this paper we show that the metallicity correction on Cepheiddistance determinations, as suggested by pulsation models, might provide thenatural way of reaching a close agreement between Cepheid and maser distance toNGC 4258 for a wide variety of LMC distance determinations.Comment: 24 pages, 1 postscript figure accepted for publication on the Astrophysical Journa