A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular
  Cluster Crater
  We analyze the resolved stellar populations of the faint stellar system,
Crater, based on deep optical imaging taken with the Hubble Space Telescope.
The HST/ACS-based color-magnitude diagram (CMD) of Crater extends $\sim$4
magnitudes below the oldest main sequence turnoff, providing excellent leverage
on Crater's physical properties. Structurally, Crater has a half-light radius
of $\sim$20 pc and shows no evidence for tidal distortions. Crater is
well-described by a simple stellar population with an age of $\sim$7.5 Gyr,
[M/H]$\sim-1.65$, a M$_{\star}\sim10^4$ M$_{\odot}$, M$_{\rm V}\sim -5.3$,
located at a distance of d$\sim$ 145 kpc, with modest uncertainties in these
properties due to differences in the underlying stellar evolution models. The
sparse sampling of stars above the turnoff and sub-giant branch are likely to
be 1.0-1.4 M$_{\odot}$ binary star systems (blue stragglers) and their evolved
descendants, as opposed to intermediate age main sequence stars. Confusion of
these populations highlights a substantial challenge in accurately
characterizing sparsely populated stellar systems. Our analysis shows that
Crater is not a dwarf galaxy, but instead is an unusually young cluster given
its location in the Milky Way's very outer stellar halo. Crater is similar to
SMC cluster Lindsay 38, and its position and velocity are in good agreement
with observations and models of the Magellanic stream debris, suggesting it may
have accreted from the Magellanic Clouds. However, its age and metallicity are
also in agreement with the age-metallicity relationships of lower mass dwarf
galaxies such as Leo I or Carina. Despite uncertainty over its progenitor
system, Crater appears to have been incorporated into the Galaxy more recently
than $z\sim1$ (8 Gyr ago), providing an important new constraint on the
accretion history of the Milky Way. [abridged]
