The cepheid period-luminosity relation and the maser distance to NGC 4258

In a recent paper describing Hubble Space Telescope observations of Cepheids in the spiral galaxy NGC 4258, Newman et al. report that the revised calibrations and methods for the Key Project on the Extragalactic Distance Scale yield that the true distance modulus of this galaxy is mu(0, 4258) = 29.40 +/- 0.09 mag, corresponding to a metric distance of 7.6 +/- 0.3 Mpc. This Cepheid distance, which holds for 18.50 mag as the true distance modulus of the Large Magellanic Cloud (LMC), is not significantly larger than 7.2 +/- 0.5 Mpc, the value determined by Herrnstein et al. from purely geometric considerations on the orbital motions of water maser sources. However, if the metallicity difference Delta[O/H] similar to 0.35 between NGC 4258 and the LMC is taken into account, then the Key Project methods lead to a metallicity-corrected value of mu(0, 4258) = 29.47 +/- 0.09 mag, with mu(0, LMC) = 18.50 mag, namely, to a Cepheid distance of 7.8 +/- 0.3 Mpc, which is 1.2 sigma from the maser determination. One might argue that these results are suggesting that a smaller distance to the LMC, mu(0, LMC) similar to 18.3 mag, should be used to calibrate the Cepheid brightness. However, even though the well-known controversy between short and long distance scale is far from being settled and current determinations of the LMC distance modulus span similar to0.4 mag, it seems that the various empirical methods converge on the value adopted by the Key Project (see Carretta et al.). In this paper we show that the metallicity correction on Cepheid distance determinations, as suggested by pulsation models with three different chemical abundances (Y = 0.25, Z = 0.004; Y = 0.25, Z = 0.008; Y = 0.28, Z = 0.02), might provide a natural way of reaching a close agreement between Cepheid and maser distances to NGC 4258 for a wide variety of LMC distance determinations. In fact, by adopting mu(0, LMC) = 18.50 +/- 0.10 mag we derive a Cepheid distance of 7.3 +/- 0.6 Mpc, which is only 0.2 sigma from the maser determination, while using mu(0, LMC) = 18.60 +/- 0.10 or 18.40 +/- 0.10 mag, the Cepheid and maser distances are 0.8 sigma and 0.4 sigma apart, respectively.