Determination of the Distance to M33 Based on Single Epoch I-band Hst Observations of Cepheids

We have determined the distance to M33 using single epoch I-band observations of Cepheids based on HST/WFPC2 images of five fields in M33. Combining the HST I-band photometry and the periods determined from the ground-based observations (DIRECT) for 21 Cepheids with logP > 0.8 in the sample of 32 Cepheids, we derive a distance modulus of (m−M)0 = 24.52±0.14(random)±0.13(systematic) for an adopted total reddening of M33, E(B−V ) = 0.20±0.04 (E(V −I) = 0.27±0.05) given by Freedman et al. (2001), the reddening to the LMC, E(B−V ) = 0.10, and the distance to the LMC, (m−M)0 = 18.50. If the total reddening to M33 of E(B−V ) = 0.10±0.09 given by Freedman, Wilson, & Madore (1991) is used, the Cepheid distance modulus based on the I-band photometry will be increased by 0.20. Metallicity effect on the Cepheid distance to M33 is estimated to be small, δ(m−M)Z = 0.01 to 0.06, which leads to (m−M)0 = 24.53 to 24.58 after this metallicity effect correction. Using the Wesenheit WI , an extinction-free parameter, we derive a similar value, (m−M)0 = 24.52±0.15(random)±0.11(systematic). These results are in reasonable agreement with those based on the ground-based multi-epoch BV RI observations of brighter Cepheids in M33, and are ≈ 0.3 smaller than those based on the tip of the red giant branch and the red clump. It is needed to estimate better the reddening to Cepheids in M33. Subject headings: galaxies: distances and redshifts — galaxies: individual (M33 (NGC 598)) — stars: Cepheids