Stellar populations in the dwarf spheroidal galaxy Leo I

We present a detailed study of the color-magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, also allow us to obtain an accurate sampling of the stellar populations at the faint magnitudes corresponding to the main sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium-burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimates for both the distance modulus and the age(s) of the main stellar components of Leo I are derived. In more detail, from the upper luminosity of the red giant branch and the lower luminosity of the subgiant branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m - M)(V) = 22.00 +/- 0.15 mag and an age of 10-15 Gyr or 9-13 Gyr, adopting a metallicity of Z = 0.0001 or 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip as given by convective pulsating models. A detailed investigation of the age(s) of the Leo I stellar populations is then performed by comparing the CMD with a suitable set of theoretical isochrones and central helium-burning models. By taking into account all the various features, including the lack of RR Lyrae variables, we conclude that the star formation process in Leo I started similar to 10 Gyr (with Z = 0.0001) or similar to 13 Gyr (with Z = 0.0004) ago, and stopped about 1 Gyr ago. Some evidence is reported supporting the mild metal deficiency (Z = 0.0004), whereas no clear indication has been found supporting a star formation history characterized by episodic bursts. The adoption of updated physics, including the inward diffusion of elements as recently presented for globular cluster stars, would yield a slightly larger distance modulus (similar to 0.10 mag) and a slightly lower age for the most ancient stellar component (similar to 1 Gyr).