Galactic evolution of D, 3He and 4He

TL;DR

This paper reviews the uncertainties and observational challenges in understanding the Galactic evolution of light nuclides D, 3He, and 4He, highlighting the need for improved models and measurements.

Contribution

It discusses the current state of knowledge, discrepancies, and potential mechanisms affecting the evolution of these light elements in the Galaxy.

Findings

Deuterium abundance varies by a factor of 3 locally.

3He abundance remains nearly constant, suggesting negligible stellar production.

The evolution of 4He in the Galaxy is still unresolved.

Abstract

The uncertainties which still plague our understanding of the evolution of the light nuclides D, 3He and 4He in the Galaxy are described. Measurements of the local abundance of deuterium range over a factor of 3. The observed dispersion can be reconciled with the predictions on deuterium evolution from standard Galactic chemical evolution models, if the true local abundance of deuterium proves to be high, but not too high, and lower observed values are due to depletion onto dust grains. The nearly constancy of the 3He abundance with both time and position within the Galaxy implies a negligible production of this element in stars, at variance with predictions from standard stellar models which, however, do agree with the (few) measurements of 3He in planetary nebulae. Thermohaline mixing, inhibited by magnetic fields in a small fraction of low-mass stars, could in principle explain the complexity of the overall scenario. However, complete grids of stellar yields taking this mechanism into account are not available for use in chemical evolution models yet. Much effort has been devoted to unravel the origin of the extreme helium-rich stars which seem to inhabit the most massive Galactic globular clusters. Yet, the issue of 4He evolution is far from being fully settled even in the disc of the Milky Way.