THE FIRST STEPS OF THE EXTRAGALACTIC DISTANCE LADDER

Recent distance determinations by means of the Cepheid period-magnitud e relations are based on a true distance modulus of m-M=18.5 to the La rge Magellanic Cloud. It has further been assumed that for the LMC Cep heids the average E(B-V)=0.10 and that the ratio of interstellar extin ction A(V) in the visual to E(B - V), i.e., R(V), is 3.1. In order to determine the distance to the LMC with its lower than solar metallicit y, it is also assumed that the period-magnitude relation is independen t of metallicity. Here we reexamine the different assumptions, i.e., t he reddening and distance to the LMC and to the Galactic Cepheids. We also study the dependence of the period-magnitude relations for differ ent heavy element abundances. In that process we rediscuss the distanc es to the LMC, to M31 and the Small Magellanic Cloud. Adopting the the oretical result, that the period-luminosity relation is independent of metallicity, we derive for the V and B bands the dependence of the pe riod-magnitude relations on Z. The use of these relations changes the determination of the interstellar extinction for the three M31 fields, studied by Freedman and Madore, and yields very good agreement for th e distance moduli of the three fields; namely m-M=24.59+/-0.02, or (m- M)(M31)-(m-M)(LMC)=6.09+/-0.02, where the error limits refer only to t he internal agreement for the three fields. We adopt a foreground redd ening of E(B-V)=0.08 for M31 and estimate an average E(B-V)=0.18 for t he LMC Cepheids. With a distance modulus to the Pleiades, m-M=5.57, we derive a distance modulus to the LMC of m-M=18.42+/-0.11, and m-M=18. 84+/-0.1 for the SMC. These error limits are only estimates. We also p oint out a way to get a rough estimate about large metallicity differe nces of Cepheids observed in external galaxies. (C) 1997 American Astr onomical Society.