Fornax is one of the most massive dwarf spheroidal galaxies in the LocalGroup. The Fornax field star population is dominated by intermediate age starsbut star formation was going on over almost its entire history. It has beenproposed that Fornax experienced a minor merger event. Despite recent progress,only the high metallicity end of Fornax field stars ([Fe/H]>-1.2 dex) has beensampled in larger number via high resolution spectroscopy. We want to betterunderstand the full chemical evolution of this galaxy by better sampling thewhole metallicity range, including more metal poor stars. We use the VLT-FLAMESmulti-fibre spectrograph in high-resolution mode to determine the abundances ofseveral alpha, iron-peak and neutron-capture elements in a sample of 47individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. Wecombine these abundances with accurate age estimates derived from the ageprobability distribution from the colour-magnitude diagram of Fornax. Similarto other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax aretypically alpha-rich while the young metal-rich stars are alpha-poor. In theclassical scenario of the time delay between SNe II and SNe Ia, we confirm thatSNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyrsago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia andAGB stars in the abundance pattern of the younger stellar population of Fornax.Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with aninitial mass function that lacks the most massive stars and with star formationthat kept going on throughout the whole history of Fornax. We find that massivestars kept enriching the interstellar medium in alpha-elements, although theywere not the main contributor to the iron enrichment.
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