ar X iv : a st ro - p h / 96 09 18 4 v 1 2 6 Se p 19 96 3 He in Planetary Nebulae : a Challenge to Stellar Evolution Models

The discrepancy between the observed abundances of He in the interstellar medium 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 He abundances recently measured in six planetary nebulae (IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, NGC 7662) with the corresponding predictions of stellar evolution theory. The determination of the mass of the planetary nebulae progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of He in the envelope of all low–mass stars (M ∼< 2.5 M⊙) 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 He with the help of a series of models with standard and non–standard (i.e. He depleted) nucleosynthesis prescriptions in varying percentages of low–mass stars. The results are found to be consistent with the abundances determined in the pre–solar material and in the local interstellar medium only if the vast majority of low–mass stars (more than 70–80 %) follows non–standard prescriptions. This implies that either the sample of planetary nebulae under exam is highly biased and therefore not representative of the whole population of low–mass stars, or the solution to the He problem lies elsewhere. Subject headings: galaxies: evolution nucleosynthesis, abundances; planetary nebulae: individual, central stars, abundances