PLANETARY-NEBULAE AND THE HELIUM-TO-METALS ENRICHMENT RATIO

A sample of planetary nebulae (PN), galactic and extragalactic H II re gions and blue compact galaxies (BCG) was used in order to obtain the helium-to-metals enrichment ratio, dY/dZ. Adopting a simple linear var iation for the helium abundance with metals, and taking into account t he contamination of the observed helium abundance in PN by the fresh h elium produced in their central stars, it is shown that dY/dZ > 3, and that the linear model is limited to low metallicities. Finally, we co nclude that the simple model of galactic chemical evolution is unable to explain the observed enrichment ratio unless infall of gaseous mate rial on to the galactic disk is considered.