Context :

Aims : To study the effects of galactic winds on the stellar metallicity distributions and on the evolution of Draco and Ursa Minor dwarf spheroidal galaxies , we compared the predictions of chemical evolution models , adopting different prescriptions for the galactic winds , with the photometrically-derived stellar metallicity distributions ( SMDs ) of both galaxies .

Methods : The chemical evolution models for Draco and Ursa Minor , which are able to reproduce several observational features of these two galaxies , take up-to-date nucleosynthesis into account for intermediate-mass stars and supernovae of both types , as well as the effect of these objects on the energetics of the systems .

Results : For both galaxies , the model that best fits the data contains an intense continuous galactic wind , occurring at a rate proportional to the star formation rate .
Models with a wind rate assumed to be proportional only to the supernova rate also reproduce the observed SMD , but do not match the gas mass , whereas the models with no galactic winds fail to reproduce the observed SMDs .
In the case of Ursa Minor , the same model as in previous works reproduces the observed distribution very well with no need to modify the main parameters of the model ( \nu = 0.1 Gyr ^ { -1 } and w _ { i } = 10 ) .
In the case Draco , on the other hand , the observed SMD requires a model with a lower supernova type Ia thermalization efficiency ( \eta _ { SNeIa } = 0.5 instead of \eta _ { SNeIa } = 1.0 ) in order to delay the galactic wind , whereas all the other parameters are kept the same ( \nu = 0.05 Gyr ^ { -1 } , w _ { i } = 4 ) .

Conclusions : The model results strongly suggest that intense and continuous galactic winds play a very important role in the evolution of local dSphs by removing a large fraction of the gas content of the galaxy and defining the pattern of the abundance ratios and of the SMD .