Extended Star Formation in Dwarf Spheroidal Galaxies: the Cases of Draco, Sextans, and Ursa Minor

Star formation and chemical enrichment histories of the dwarf spheroidal galaxies (dSphs) Draco, Sextans, and Ursa Minor are investigated by means of chemical evolution models and a simulation code for colour-magnitude diagrams (CMDs). The CMD simulation code is designed to fully consider effects of the chemical evolution on stellar evolution and photometric properties. For this aim, star formation and chemical enrichment histories are calculated consistently in the code. Comparisons between the chemical evolution models and the observed abundance patterns reveal that the star formation rates were very low (1 − 5% of that of the solar neighbourhood disc) and that the initial star formation continued for a long duration (> 3.9 − 6.5 Gyr) in these dSphs. This star formation history can reproduce morphologies of the observed CMDs, such as narrow red giant branches and red horizontal branches and succeeds in solving the second parameter problem of the dSph Draco. Hence, both of the abundance patterns and the morphologies of the CMDs can be explained by the star formation histories characterised by the low star formation rate and the long duration of the star formation period. Because of the low star formation rates, plenty of gas remains at the final epoch of star formation. We suggest that gas stripping by the Galaxy results in termination of star formation in the dSphs.