MODEL COLOR-MAGNITUDE DIAGRAMS FOR HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF LOCAL GROUP DWARF GALAXIES

In this Letter, we discuss a method to conduct a quantitative study of the star formation history (SFH) of Local Group (LG) galaxies using H ubble Space Telescope (HST) data. This method has proved to be success ful in the analysis of the SFH of the same kind of galaxies using grou nd-based observations. It is based on the comparison of observed CMDs with a set of model CMDs. The latter are computed assuming different e volutionary scenarios and include a detailed simulation of observation al effects. CMDs obtained with HST are similar to 3 mag deeper than ty pical CMDs obtained from ground-based telescopes, allowing the observa tion, for all LG galaxies, of a part of the CMD that up till now had r emained accessible only for the very nearest galaxies. A very importan t feature that will become accessible with HST is the horizontal branc h plus the red clump. The distribution of stars along this structure i s quite sensitive to age and metallicity and should provide a very imp ortant improvement in the time resolution of the SFH for stars older t han similar or equal to 2-3 Gyr. We show and discuss four model CMDs t hat would be comparable with CMDs from deep HST observations. These mo del CMDs represent the following evolutionary scenarios corresponding to a wide range of dwarf galaxy subtypes from dI to dE: case A, a cons tant SFR from 15 Gyr ago to the present time; case 8, same as case A, but with the SFR stopped 0.5 Gyr ago; case C, a constant SFR in the ag e range 10-9 Gyr; and case D, same as case C but in the age range 15-1 2 Gyr. In all four cases a range of metallicity from Z = 0.0001 to Z = 0.004 has been assumed. The present analysis is just a first qualitat ive approach to what one may expect to find in the CMDs of LG galaxies . However, a complete set of model CMDs must be computed to analyze th e data for each galaxy, using the crowding effects derived for that pa rticular galaxy.