HE-3 IN PLANETARY-NEBULAE - A CHALLENGE TO STELLAR EVOLUTION MODELS

The discrepancy between the observed abundances of He-3 in the interst ellar medium and those predicted by stellar and Galactic chemical evol ution remains largely unexplained. In this paper, we attempt to shed s ome light on this unsolved problem by presenting a quantitative compar ison of the He-3 abundances recently measured in six planetary nebulae (IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, NGC 7662) with the c orresponding predictions of stellar evolution theory. The determinatio n of the mass of the planetary nebulae progenitors allows us to dismis s, with a good degree of confidence, the hypothesis that the abundance of He-3 in the envelopes of all low-mass stars (M less than or simila r to 2.5 M.) is strongly reduced with respect to the standard theoreti cal values by some mixing mechanism acting in the latest phases of ste llar evolution. The abundance versus mass correlation, with allowance made for the limitation of the sample, is in fact found to be fully co nsistent with the classical prediction of stellar evolution. We examin e the implications of this result on the Galactic evolution of He-3 wi th the help of a series of models with standard and nonstandard (i.e., He-3-depleted) nucleosynthesis prescriptions in varying percentages o f low-mass stars. The results are found to be consistent with the abun dances determined in the presolar material and in the local interstell ar medium only if the vast majority of low-mass stars (more than 70%-8 0%) follow nonstandard prescriptions. This implies that either the sam ple of planetary nebulae under examination is highly biased and theref ore not representative of the whole population of low-mass stars, or t he solution to the He-3 problem lies elsewhere.