Revisiting Delta Y/Delta Z from multiple main sequences in Globular Clusters: insight from nearby stars

TL;DR

This paper investigates the broadening of the main sequence in nearby stars and globular clusters, suggesting that current stellar models may overestimate helium content, and proposes empirical revisions to better match observations.

Contribution

It provides empirical evidence and a new formula to revise helium enrichment ratios in stellar models based on observed main sequence broadening.

Findings

Standard helium-to-metal enrichment ratio fits solar metallicity stars.

Higher ratios are needed to explain subdwarfs and multiple main sequences.

Revised models suggest lower helium content in globular cluster subpopulations.

Abstract

For nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the Main Sequence around solar metallicity, and agrees with independent measurements from HII regions as well as with theoretical stellar yields and chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high DY/DZ estimated from the multiple Main Sequences in Omega Cen and NGC 2808. Although not "inverted" as in omega Cen, where the metal-rich Main Sequence is bluer than the metal-poor one, the broadening observed for nearby subdwarfs is much narrower than stellar models predict for a standard helium content. We use this empirical evidence to argue that a revision of lower Main Sequence stellar models, suggested from nearby stars, could significantly reduce the helium content inferred for the subpopulations of those globular clusters. A simple formula based on empirically calibrated homology relations is constructed, for an alternative estimate of DY/DZ in multiple main sequences. We find that, under the most favourable assumptions, the estimated helium content for the enriched populations could decrease from Y~0.4 to as low as Y~0.3.