Uncertainty in the metal abundance dependence of the Cepheid variable period-luminosity (PL) relation remains one of the outstanding sources of systematic error in the extragalactic distance scale and the Hubble constant. To test for such a metallicity dependence, we have used the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) to observe Cepheids in two fields in the nearby spiral galaxy M101, which span a range in oxygen abundance of 0.7 ± 0.15 dex. A differential analysis of the PL relations in V and I in the two fields yields a marginally significant change in the inferred distance modulus on metal abundance, with δ(m − M) 0 /δ[O/H] = −0.24 ± 0.16 mag dex −1 . The trend is in the theoretically predicted sense that metal-rich Cepheids appear brighter and closer than metal-poor stars. External comparisions of Cepheid distances with those derived from three other distance indicators, in particular the tip of the red giant branch method, further constrain the magnitude of any Z-dependence of the PL relation at V and I. The overall effects of any metallicity dependence on the distance scale derived with HST will be of the order of a few percent or less for most applications, though distances to individual galaxies at the extremes of the metal abundance range may be affected at the 10% level.
We report on the identiÐcation of 255 candidate variable stars in a Ðeld located some from the 1@ .7 center of the late-type spiral galaxy M101 \ NGC 5457, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space T elescope. Photometric measurements in the F555W and F814W ÐltersÈanalyzed independently with the DAOPHOT/ALLFRAME and DoPHOT software suitesÈhave been transformed to the Johnson V and Kron-Cousins I standard magnitude systems. Periods and intensity-averaged mean magnitudes for 61 carefully selected candidate Cepheid variables with periods in the range 10È48 days indicate a reddening-corrected mean distance modulus (if the true modulus of the Large Magellanic Cloud is 18.50^0.10, and if there (m [ M) 0 \ 29.05^0.14 is no dependence of the period-luminosity relation on metal abundance) ; results consistent with this are obtained whether or not the sample is expanded to include a larger fraction of the candidates. Applying a metallicity-dependent correction of ]0.16^0.10 mag would increase this estimate to (m [ M) 0 \ 29.21^0.17 mag.
We report on the discovery of Cepheids in the Virgo spiral galaxy NGC 4535, based on observations made with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. NGC 4535 is one of 18 galaxies observed as a part of The HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. NGC 4535 was observed over 13 epochs using the F555W filter, and over 9 epochs using the F814W filter.The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent programs, DoPHOT and DAOPHOT II/ALLFRAME. Period-luminosity relations in the V and I bands were constructed using 39 high-quality Cepheids present in our set of 50 variable candidates. We obtain a distance modulus of 31.02 ± 0.26 mag, corresponding to a distance of 16.0±1.9 Mpc. Our distance estimate is based on values of µ 0 = 18.50±0.10 mag and E(V-I ) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.