We investigate the color-magnitude diagram ( CMD ) of the Carina dwarf spheroidal galaxy using data of Stetson et al .
( 65 ) and synthetic CMDs based on isochrones of Dotter et al .
( 19 ) , 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 \sim –1.15 and ages \sim 13 , 7 , \sim 3.5 , and \sim 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 \sim 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 .