Determination of the Distance to M33 Based on the Tip of the Red Giant Branch and the Red Clump

Abstract

We have determined the distance to M33 using the tip of the red giantbranch (TRGB) and the red clump (RC), from the VI photometry of stars in10 regions of M33 based on HST/WFPC2 images. The regions used in thisstudy are located at R=2.6{\prime}-17.8{\prime} from the center of M33.The distance modulus to M33 obtained in this study, for an adoptedforeground reddening of E(B-V)=0.04, is(m-M)_{0,TRGB}=24.81+/-0.04(random)^{+0.15}_{-0.11}(systematic) from the TRGB, and(m-M)_{0,RC}=24.80+/-0.04(random)+/-0.05(systematic) from theRC, showing an excellent agreement between the two [corresponding to adistance of 916+/-17(random) kpc and 912+/-17(random) kpc,respectively]. These results are ~0.3 mag larger than the Cepheiddistances based on the same HST/WFPC2 data and ground-based data. Thisdifference is considered partially due to the uncertainty in theestimates of the total reddening for Cepheids in M33. Based onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.