³He: Does the problem persist?

To understand the chemical evolution of the Galaxy, we need to understand the contribution of planetary nebulae (PNe) in the ³He abundance. ³He abundances have been detected only in a couple of PNe; their abundances are consistent with the standard models, in which ³He is produced in significant quantities by stars of 1-2M⊙. However, for all the other PNe observed to date there have been no detections, and therefore only upper limits in their abundance can be calculated. Observations of the ³He⁺ emission line using the Very Large Array (VLA) towards the PNe IC 418, NGC 6572 and NGC 7009 were used to obtain upper limits for their ³He abundance. Because the abundance of ³He in HII regions, the interstellar medium (ISM) and the proto-Solar system is much lower than what is predicted, new chemical models were proposed. The resulting evolution of ³He, based on stellar evolution models, can be consistent with the values determined in pre-solar material and the ISM if 96 per cent of the population of stars with mass below ~2.5MM⊙ has undergone enhanced ³He depletion. This implies that unless the combined sample of PNe that has been observed so far is very atypical, the solution to the 'The ³He Problem' lies elsewhere. However, the results presented here suggest that more observations are needed in order to make a strong conclusion about the stellar evolution models.