SIMBAD references

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 ∼ 2.5M☉ 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.