CHEMICAL EVOLUTION WITH BURSTS OF STAR FORMATION - ELEMENT RATIOS IN DWARF GALAXIES

We demonstrate that galaxies in which star formation proceeds in a small number of bursts evolve their chemical elements in ratios which are very different from those in galaxies with continuing star formation. Systematic changes of element ratios with overall chemical abundance are determined to a large extent by the onset and duration of the star formation bursts and can be very different from those seen in the solar neighborhood. In particular, we show that an underabundance of oxygen relative to iron, such as is observed in the Large Magellanic Cloud, occurs naturally when star formation proceeds in a small number of widely separated bursts, as is inferred from the age distribution of LMC field stars and clusters. There is no need to invoke either variations in the stellar initial mass function or metal-enhanced winds.