2.3. Helium-3 - Complicated Evolution

As may be seen in Fig. 1, the predicted primordial abundance of ³He behaves similarly to that of D, decreasing monotonically with . Again, the reason is the same: ³He is being burned to the more tightly bound nucleus ⁴He. The higher the nucleon abundance, the faster the burning and the less ³He survives. In contrast to the behavior of D/H versus , the decrease of ³He/H with increasing is much slower. This is simply a reflection of the much tighter binding of ³He compared to D. Although the BBN predictions for the abundance of ³He have similarly small uncertainties (8 - 10%) to those of D, it is much more difficult to exploit the observations of ³He to test and constrain BBN. The complication is the evolutionary history of the ³He abundance since BBN.

Although any deuterium cycled through stars is burned to he during the stars' pre-main sequence evolution, ³He will survive in the cooler stellar exteriors while being destroyed in the hotter interiors. For the more abundant lower mass stars which are cooler, a larger fraction of prestellar ³He (along with the ³He produced from prestellar D) survives. Indeed, for sufficiently low-mass stars (less than a few solar masses) incomplete burning actually leads to a buildup of newly synthesized ³He (to be contrasted with the prestellar D and ³He) which may - or may not - be returned to the interstellar medium. In fact, some planetary nebulae are observed to be highly enriched in ³He. So, the evolution of ³He is complex with stellar destruction competing with primordial and stellar production. Indeed, if all low mass stars were as prolific producers of ³He as indicated by some planetary nebulae, the solar system and local interstellar medium abundances of ³He should far exceed those inferred from observations. Thus, at least some low mass stars must be net destroyers of ³He. Given this necessarily complex and uncertain picture of production, destruction and survival, it is difficult to use current observational data to infer the primordial abundance of ³He. Unless and until ³He is observed in high redshift (i.e., early Universe), low metallicity (i.e., nearly unevolved) systems, it will provide only a weak check on the consistency of BBN.