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Date and Time of the Query: 2019-03-18 T22:56:23 PDT
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For refcode 1994A&A...288..921C:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1994A&A...288..921C Planetary nebulae and the helium-to-metals enrichment ratio C. Chiappini and W. J. Maciel Instituto Astronomico e Geofisico da USP, Av. Miguel Stefano 4200, CEP 04301-904 Sao Paulo SP, Brazil Received 16 November 1993 / Accepted 25 February 1994 Abstract. A sample of planetary nebulae (PM), 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 PM 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. Key words: planetary nebulae: general - Galaxy: evolution - Galaxy: abundances
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