Multiple Main Sequence of Globular Clusters as a Result of Inhomogeneous Big Bang Nucleosynthesis

TL;DR

This paper proposes that inhomogeneous big bang nucleosynthesis caused helium enhancement in certain globular cluster stars, explaining their multiple main sequences and predicting associated heavy element enrichment.

Contribution

It introduces a novel mechanism linking inhomogeneous big bang nucleosynthesis to multiple main sequences in globular clusters, supported by nucleosynthesis calculations and diffusion analysis.

Findings

Helium enhancement can be achieved with specific baryon-to-photon ratios.

Heavy elements around mass number 100 are predicted to be enriched.

Diffusion does not diminish helium enhancement in stars.

Abstract

A new mechanism for enhancing the helium abundance in the blue main sequence stars of omega Centauri and NGC 2808 is investigated. We suggest that helium enhancement was caused by the inhomogeneous big bang nucleosynthesis. Regions with extremely high baryon-to-photon ratios are assumed to be caused by the baryogenesis. Its mass scale is also assumed to be 10^6 Msun. An example of the mechanisms to realize these two things was already proposed as the Affleck-Dine baryogenesis. As the baryon-to-photon ratio becomes larger, the primordial helium abundance is enhanced. We calculated the big bang nucleosynthesis and found that there exists a parameter region yielding enough helium to account for the split of the main sequence in the aforementioned globular clusters while keeping the abundances of other elements compatible with observations. Our mechanism predicts that heavy elements with the mass number of around 100 is enhanced in the blue main sequence stars. We estimate the timescales of diffusion of the enhanced helium and mass accretion in several stages after the nucleosynthesis to investigate whether these processes diminish the enhancement of helium. We found that the diffusion does not influence the helium content. A cloud with a sufficiently large baryon-to-photon ratio to account for the multiple main sequence collapsed immediately after the recombination. Subsequently, the cloud accreted the ambient matter with the normal helium content. If the star formation occurred both in the collapsed core and the accreted envelope, then the resultant star cluster has a double main sequence.