Determination of the distance to M33 based on single-epoch I-band Hubble Space Telescope observations of Cepheids

We have determined the distance to M33 using single-epoch I-band observations of Cepheids based on Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 (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 log P>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.; 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, and Madore 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, delta(m-M)(z) = 0.01-0.06, which leads to (m-M)(0) = 24.53-24.58 after this metallicity effect correction. Using the Wesenheit W-I, 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 multiepoch BVRI observations of brighter Cepheids in M33 and are approximate to0.3 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.