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 , allow us to obtain an accurate sampling of the stellar populations also 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 estimations for both the distance modulus and the age ( 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 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 \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 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 .
The 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 has started at \sim 10 Gyr ( with Z =0.0001 ) or \sim 13 Gyr ( with Z =0.0004 ) ago and it 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 which includes the inward diffusion of elements , as recently presented for globular cluster stars , would yield a slightly larger distance modulus ( \sim 0.10 mag ) and a slightly lower age for the most ancient stellar component ( \sim 1 Gyr ) .