THE STELLAR POPULATIONS OF THE CARINA DWARF SPHEROIDAL GALAXY .1. A NEW COLOR MAGNITUDE DIAGRAM FOR THE GIANT AND HORIZONTAL BRANCHES

We report on the first in a series of studies of the Carina dwarf spheroidal galaxy, a nearby satellite of our Galaxy. Our two major results are: (1) precise BI photometry (sigma(B-I) less than or similar 0.05 for V less tha or similar 22) for 11 489 stars in the Carina field, and (2) the detection of two, morphologically distinct, horizontal branches in Carina, which confirms that star formation occurred in two well separated episodes. The old horizontal branch and RR Lyrae instability strip belong to a greater than or similar to 10 Gyr stellar population, while the populous red-clump horizontal branch belongs, presumably, to a approximately 6 Gyr stellar population. We derive a distance modulus (m - M)0=20.09+/-0.06 for Carina from the apparent magnitudes of the old horizontal branch and the tip of the red giant branch (RGB), and discuss modifications to the previously estimated distance, total magnitude, and stellar ages of Carina. Using the color of the RGB, we estimate the metallicities of the younger and older populations to be [Fe/H] congruent-to -2.0 and -2.2, respectively.