3He: Does the problem persists?
  To understand the chemical evolution of the Galaxy, we need to understand the
contribution of 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_sun. However, for all the other PNe observed to date there have been no
detections, therefore only upper limits in their abundance can be calculated.
Observations of the 3He+ emission line using the 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 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
percent of the population of stars with mass below 2.5M_sun 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.
