The dY/dZ in the Chemical Evolution of Galaxies with the Multiphase Model

Abstract

The relation between helium and oxygen abundances resulting from chemical evolution models applied to spiral galaxies is analysed. The models are of the kind referred to as “multiphase models” (Ferrini et al. 1992) and have been successfully applied to the Solar Neighbourhood — without and with thick disc — (Pardi & Ferrini 1994; Ferrini, Matteucci &Pardi 1994); the Galactic Disc (Ferrini et al. 1994); the Galactic Bulge (Mollá & Ferrini 1994) and outer spiral galaxies (Mollá, Ferrini & Díaz 1994). From the comparison between models and observations, helium production yields higher than presently calculated by factors between 5 and 10 are implied, suggesting that a revision of the nucleosynthesis prescriptions may be needed.