VLT/FLAMES spectroscopy of red giant branch stars in the Carina dwarf spheroidal galaxy

Context. The ages of individual red giant branch stars can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the first dredge-up). This means that they trace the interstellar medium in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. Aims. Colour-magnitude diagram analysis has shown the Carina dwarf spheroidal to have had an unusually episodic star formation history and this is expected to be reflected in the abundances of different chemical elements. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode (R approximate to 20 000) to measure the abundances of several chemical elements, including Fe, Mg, Ca and Ba, in a sample of 35 individual Red Giant Branch stars in the Carina dwarf spheroidal galaxy. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation episodes the stars in our sample belong to, and thus to define the relationship between star formation and chemical enrichment during these two episodes. Results. As is expected from the star formation history, Carina contains two distinct populations of Red Giant Branch stars: one old (greater than or similar to 10 Gyr), which we have found to be metal-poor ([Fe/H] 0); the other intermediate age (approximate to 2-6 Gyr), which we have found to have a metallicity range (-1.8 <[Fe/H] <-1.2) with a large spread in [alpha/Fe] abundance, going from extremely low values ([Mg/Fe] <-0.3) to the same mean values as the older population ( similar to 0.3). Conclusions. We show that the chemical enrichment history of the Carina dwarf spheroidal was different for each star formation episode. The earliest was short (similar to 2-3 Gyr) and resulted in the rapid chemical enrichment of the whole galaxy to [Fe/H] similar to -1.5 with both SNe II and SNe Ia contributions. The subsequent episode occured after a gap of similar to 3-4 Gyr, forming similar to 70% of the stars in the Carina dSph, but it appears to have resulted in relatively little evolution in either [Fe/H] or [alpha/Fe]