3He: Does the problem persists?

TL;DR

This paper investigates the persistent discrepancy in 3He abundance measurements in planetary nebulae, suggesting that most low-mass stars deplete 3He more than standard models predict, but more data is needed for definitive conclusions.

Contribution

It provides new upper limits on 3He in several PNe and proposes revised chemical evolution models accounting for widespread 3He depletion in low-mass stars.

Findings

Most low-mass stars likely undergo enhanced 3He depletion.

Current observations are insufficient to fully resolve the 3He problem.

Revised models can reconcile observed 3He levels with stellar evolution theories.

Abstract

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.