Stellar Populations in the Dwarf Spheroidal Galaxy Leo I

We present a detailed study of the color magnitude diagram (CMD) of the dwarfspheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Ourphotometric analysis, confirming previous results on the brighter portion ofthe CMD, allow us to obtain an accurate sampling of the stellar populationsalso at the faint magnitudes corresponding to the Main Sequence. By adopting ahomogeneous and consistent theoretical scenario for both hydrogen and centralhelium-burning evolutionary phases, the various features observed in the CMDare interpreted and reliable estimations for both the distance modulus and theage(s) for the main stellar components of Leo I are derived. More in details,from the upper luminosity of the Red Giant Branch and the lower luminosity ofthe Subgiant Branch we simultaneously constrain the galaxy distance and the ageof the oldest stellar population in Leo I. In this way we obtain a distancemodulus (m-M)_V=22.00$\pm$0.15 mag and an age of 10--15 Gyr or 9--13 Gyr,adopting a metallicity Z=0.0001 and 0.0004, respectively. The reliability ofthis distance modulus has been tested by comparing the observed distribution ofthe Leo I anomalous Cepheids in the period-magnitude diagram with the predictedboundaries of the instability strip, as given by convective pulsating models.Comment: 19 pages, 3 tables, 14 figures To be published in A