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Model Color-Magnitude Diagrams for HST Observations of Local Group Dwarf Galaxies

In this paper, we discuss a method to conduct a quantitative study of the star formation history (SFH) of Local Group (LG) galaxies using Hubble Space Telescope (HST) data. This method has proven to be successful in the analysis of the SFH of the same kind of galaxies using ground-based observations. It is based on the comparison of observed CMDs with a set of model CMDs. The latter are computed assuming different evolutionary scenarios, and include a detailed simulation of observational effects. CMDs obtained with HST are ∼ 3 mags deeper than typical CMDs obtained from ground-based telescopes, allowing the observation, for all LG galaxies, of a part of the CMD that up till now had remained accessible only for the very nearest galaxies. A very important feature that will become accessible with HST is the horizontal-branch plus the red-clump. The distribution of stars along this structure is quite sensitive to age and metallicity and should provide a very important improvement in the time resolution of the SFH for stars older than ≃ 2 − 3 Gyr. We show and discuss four model CMDs which would be comparable with CMDs from deep HST observations. These model CMDs represent the following evolutionary scenarios corresponding to a wide range of dwarf galaxy sub-types from dI to dE: A) a constant SFR from 15 Gyr – 2 – ago to the present time; B) as A), but with the SFR stopped 0.5 Gyr ago; C) a constant SFR in the age range 10-9 Gyr and D) as C) but in the age range 15-12 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 qualitative 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 analize the data for each galaxy, using the crowding effects derived for that particular galaxy.