WFPC2 Observations of Massive and Compact Young Star Clusters in M31

Abstract

We present color-magnitude diagrams of four blue massive and compact star clusters in M31: G38, G44, G94, and G293. The diagrams of the four clusters reveal a well-populated upper main sequence and various numbers of supergiants. The U-B and B-V colors of the upper main sequence stars are used to determine reddening estimates of the different lines of sight in the M31 disk. Reddening values range from E_B-V=0.20+/-0.10 to 0.31+/-0.11. We statistically remove field stars on the basis of completeness, magnitude, and color. Isochrone fits to the field-subtracted, reddening-corrected diagrams yield age estimates ranging from 63+/-15 to 160+/-60 Myr. Implications for the recent evolution of the disk near NGC 206 are discussed. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.