Effects of episodic gas infall on the chemical abundances in galaxies

The chemical evolution of galaxies that undergo an episode of massive and rapid accretion of metal- poor gas is investigated with models using both simplified and detailed nucleosynthesis recipes. The rapid decrease of the oxygen abundance during infall is followed by a slower evolution which leads back to the closed- box relation, thus forming a loop in the N/ O - O/ H diagram. For large excursions from the closed- box relation, the mass of the infalling material needs to be substantially larger than the gas remaining in the galaxy, and the accretion rate should be larger than the star formation rate. We apply this concept to the encounter of high velocity clouds with galaxies of various masses, finding that the observed properties of these clouds are indeed able to cause substantial effects not only in low mass galaxies, but also in the partial volumes in large massive galaxies that would be affected by the collision. Numerical models with detailed nucleosynthesis prescriptions are constructed. We assume star formation timescales and scaled yields that depend on the galactic mass, and which are adjusted to reproduce the average relations of gas fraction, oxygen abundance, and effective oxygen yield observed in irregular and spiral galaxies. The resulting excursions in the N/ O - O/ H diagram due to a single accretion event involving a high velocity cloud are found to be appreciable, which could thus provide a contribution to the large scatter in the N/ O ratio found among irregular galaxies. Nonetheless, the N/ O - O/ H diagram remains an important indicator for stellar nucleosynthesis.