Determination of the Distance to M33 Based on Single‐EpochI‐BandHubble Space TelescopeObservations of Cepheids

Abstract

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, δ(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 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 multiepoch BVRI 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.