Minsun Kim

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, 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.

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

We have determined the distance to M33 using the tip of the red giant branch (TRGB) and the red clump (RC), from the VI photometry of stars in 10 regions of M33 based on HST/WFPC2 images. The regions used in this study are located at R = 26–178 from the center of M33. The distance modulus to M33 obtained in this study, for an adopted foreground reddening of E(B-V) = 0.04, is (m - M)0,TRGB = 24.81 ± 0.04(random)(systematic) from the TRGB, and (m - M)0,RC = 24.80 ± 0.04(random) ± 0.05(systematic) from the RC, showing an excellent agreement between the two [corresponding to a distance of 916 ± 17(random) kpc and 912 ± 17(random) kpc, respectively]. These results are ≈0.3 mag larger than the Cepheid distances based on the same HST/WFPC2 data and ground-based data. This difference is considered partially due to the uncertainty in the estimates of the total reddening for Cepheids in M33.

REDDENING AND DISTANCE OF THE LOCAL GROUP STARBURST GALAXY IC 10

We estimate the reddening and distance of the nearest starburst galaxy IC 10 using deep near-infrared JHK S photometry obtained with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru telescope. We estimate the foreground reddening toward IC 10 using the UBV photometry of IC 10 from the Local Group Survey, obtaining E(B – V) = 0.52 ± 0.04 mag. We derive the total reddening including the internal reddening, E(B – V) = 0.98 ± 0.06 mag, using the UBV photometry of early-type stars in IC 10 and comparing the JHK S photometry of red giant branch stars in IC 10 and the Small Magellanic Cloud. Using the Two Micron All Sky Survey point-source catalog of 20 Galactic globular clusters, we derive a relation between the metallicity [Fe/H]CG97 and the slope of the red giant branch in the K S – (J – K S) color-magnitude diagram. The mean metallicity of the red giant branch stars in IC 10 is estimated to be [Fe/H]CG97 = –1.08 ± 0.28. The magnitude of the tip of the red giant branch (TRGB) of IC 10 in the K S band is measured to be K S,TRGB = 18.28 ± 0.01. Based on the TRGB method, we estimate the distance modulus of IC 10 to be (m – M)0 = 24.27 ± 0.03(random) ± 0.18(systematic), corresponding to the distance of d = 715 ± 10 ± 60 kpc. This confirms that IC 10 is a member of the Local Group.