The Populations of Carina. I. Decoding the Color--Magnitude Diagram
  We investigate the color-magnitude diagram (CMD) of the Carina dwarf
spheroidal galaxy using data of Stetson et al. (2011) and synthetic CMDs based
on isochrones of Dotter et al. (2008), in terms of the parameters [Fe/H], age,
and [alpha/Fe], for the cases when (i) [alpha/Fe] is held constant and (ii)
[alpha/Fe] is varied. The data are well described by four basic epochs of star
formation, having [Fe/H] = -1.85, -1.5, -1.2, and ~-1.15 and ages ~13, 7, ~3.5,
and ~1.5 Gyr, respectively (for [alpha/Fe] = 0.1 (constant [alpha/Fe]) and
[alpha/Fe] = 0.2, 0.1, -0.2, -0.2 (variable [alpha/Fe])), with small spreads in
[Fe/H] and age of order 0.1 dex and 1 - 3 Gyr. Within an elliptical radius 13.1
arcmin, the mass fractions of the populations, at their times of formation,
were (in decreasing age order) 0.34, 0.39, 0.23, and 0.04. This formalism
reproduces five observed CMD features (two distinct subgiant branches of old
and intermediate-age populations, two younger, main-sequence components, and
the small color dispersion on the red giant branch (RGB)). The parameters of
the youngest population are less certain than those of the others, and given it
is less centrally concentrated it may not be directly related to them.
High-resolution spectroscopically analyzed RGB samples appear statistically
incomplete compared with those selected using radial velocity, which contain
bluer stars comprising ~5 - 10% of the samples. We conjecture these objects
may, at least in part, be members of the youngest population. We use the CMD
simulations to obtain insight into the population structure of Carina's upper
RGB.
