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

Abstract

The discrepancy between the observed abundances of 3He in the ISM and those predicted by stellar and galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (PNe) with the corresponding predictions of stellar evolution theory. The determination of the mass of the PNe progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of 3He in the envelope of all low-mass stars is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the galactic evolution of 3He with the help of a series of models with standard and non-standard nucleosynthesis prescriptions. The results are found to be consistent with the observed galactic abundances only 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.