Abstract details

In the framework of an ongoing HST/ACS project (HST program # 10586, PI: Aloisi) we obtained deep multi-color imaging data of the very metal-poor Blue Compact Dwarf galaxy IZw18. The data were acquired in time-series fashion to allow the identification of Cepheids. The main aim of this project is to constrain both the distance and the Star Formation History of the galaxy. However, as a byproduct the data also provide new insight into the properties of Classical Cepheids at very low metallicities. We identified Classical Cepheids in IZw18 by appling the image subtraction technique (ISIS 2.2, Alard 2000) to the HST/ACS time series data of the galaxy. The light curves can be calibrated to an absolute magnitude scale when combined with independent estimates of the galaxy distance. To interpret the results we computed new theoretical pulsation models of Classical Cepheids specifically for the extremely low metallicity of this primordial galaxy (Z=0.0004, Y=0.24). The new models allow us to constrain the metallicity dependence of the Cepheid Period-Luminosity relation in the low metallicity regime (Z from 0.0004 to 0.008). We also investigated the effect of different assumptions in the mass-luminosity relation and mixing-length parameter on the classical pulsation relations (Period-Luminosity-Color, Wesenheit and Period-Luminosity). The observed light curves and mean magnitudes and colors are compared with the predictions of the new pulsation models.