SIMBAD references

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