The Cepheid Period - Luminosity relation and the maser distance to NGC 4258

In a recent paper describing HST observations of Cepheids in the spiral galaxy NGC 4258, Newman et al. (2001) 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 μ0,4258 = 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. (1999) from purely geometric considerations on the orbital motions of water maser sources. However, if the metallicity difference ∆[O/H]∼ 0.35 between NGC 4258 and LMC is taken into account, then the Key Project methods lead to a metallicity-corrected value of μ0,4258 = 29.47±0.09 mag, with μ0,LMC=18.50 mag, namely to a Cepheid distance of 7.8±0.3 Mpc which is 1.2σ from the maser determination. One might argue that these results are suggesting that a smaller distance to the LMC, μ0,LMC ∼ 18.3 mag, should be used to calibrate the Cepheid brightness (see, e.g., Paczynski 1999). 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 are spanning ∼ 0.4 mag, it seems that the various empirical methods converge on the value adopted by the Key Project (see Carretta et al. 2000). 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 the natural way of reaching a close agreement between Cepheid and maser distance to NGC 4258 for a wide variety of LMC distance determinations. In fact, by adopting μ0,LMC = 18.50±0.10 mag we derive a Cepheid distance of 7.3±0.6 Mpc which is only 0.2σ from the maser determination, while using μ0,LMC = 18.60±0.10 mag or 18.40±0.10 the Cepheid and maser distances are 0.8σ and 0.4σ, respectively, apart.