A HUBBLE SPACE TELESCOPE STUDY OF THE ENIGMATIC MILKY WAY HALO GLOBULAR CLUSTER CRATER* * Based on observations made 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 NAS 5-26555. These observations are associated with program #13746.

We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Crater’s color–magnitude diagram (CMD) extends ∼4 mag below the oldest main-sequence (MS) turnoff. Structurally, we find that Crater has a half-light radius of ∼20 pc and no evidence for tidal distortions. We model Crater’s CMD as a simple stellar population (SSP) and alternatively by solving for its full star formation history. In both cases, Crater is well described by an SSP with an age of ∼7.5 Gyr, a metallicity of [M/H] ∼ −1.65, a total stellar mass of , and a luminosity of , located at a distance of d ∼ 145 kpc, with modest uncertainties due to differences in the underlying stellar evolution models. We argue that the sparse sampling of stars above the turnoff and subgiant branch are likely to be 1.0–1.4 blue stragglers and their evolved descendants, as opposed to intermediate-age MS stars. We find that Crater is an unusually young cluster given its location in the Galaxy’s outer halo. We discuss scenarios for Crater’s origin, including the possibility of being stripped from the SMC or the accretion from lower-mass dwarfs such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than z ∼ 1 (8 Gyr ago), providing an important new constraint on the accretion history of the Galaxy.