3He in Planetary Nebulae: A Challenge to Stellar Evolution Models

The discrepancy between the observed abundances of 3He in the ISM and thosepredicted by stellar and galactic chemical evolution remains largelyunexplained. In this paper, we attempt to shed some light on this unsolvedproblem by presenting a quantitative comparison of the 3He abundances recentlymeasured in six planetary nebulae (PNe) with the corresponding predictions ofstellar evolution theory. The determination of the mass of the PNe progenitorsallows us to dismiss, to a good degree of confidence, the hypothesis that theabundance of 3He in the envelope of all low-mass stars is strongly reduced withrespect to the standard theoretical values by some mixing mechanism acting inthe latest phases of stellar evolution. The abundance versus mass correlation,allowance made for the limitation of the sample, is in fact found to be fullyconsistent with the classical prediction of stellar evolution. We examine theimplications of this result on the galactic evolution of 3He with the help of aseries of models with standard and non-standard nucleosynthesis prescriptions.The results are found to be consistent with the observed galactic abundancesonly if the vast majority of low-mass stars follows non-standard prescriptions.This implies that either the sample of PNe nebulae under exam is highly biased,or the solution to the 3He problem lies elsewhere.Comment: 13 pages, LaTeX, 4 Figures. To be published in The Astrophysical Journa