A DECam View of the Diffuse Dwarf Galaxy Crater II: The Colour-Magnitude
  Diagram
  We present a deep Blanco/DECam colour-magnitude diagram (CMD) for the large
but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only
old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow
red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB
stars. Comparing the CMD with [Fe/H] = -2.0 and [$\alpha$/Fe] = +0.3
alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main
event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star
formation events Crater II shows similarity to more massive dwarfs that have
intermediate age populations, however for Crater II there was early quenching
of the star formation and no intermediate age or younger stars are present. The
spatial distribution of Crater II stars overall is elliptical in the plane of
the sky, the detailed distribution shows a lack of strong central
concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main
sequence stars show a slightly higher central concentration when compared to
the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of
Crater II: $\mu_{\alpha}\cos \delta$=-0.14 $\pm$ 0.07 , $\mu_{\delta}$=-0.10
$\pm$ 0.04 mas yr$^{-1}$, approximately perpendicular to the semi-major axis of
Crater II. Our results provide constraints on the star formation and chemical
enrichment history of Crater II, but cannot definitively determine whether or
not substantial mass has been lost over its lifetime.
