Revisiting ΔY/ΔZ from multiple main sequences in globular clusters: insight from nearby stars

For nearby K dwarfs, the broadening of the observed main sequence (MS) at low metallicities is much narrower than expected from isochrones with the standard helium-to-metal enrichment ratio ΔY/ΔZ ∼ 2. Though the latter value fits well the MS around solar metallicity, and agrees with independent measurements from H ii regions as well as with theoretical stellar yields and chemical evolution models, a much higher ΔY/ΔZ ∼ 10 is necessary to reproduce the broadening observed for nearby subdwarfs. This result resembles, on a milder scale, the very high ΔY/ΔZ estimated from the multiple MSs in ω Cen and NGC 2808. Although not 'inverted' as in ω Cen, where the metal-rich MS 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 MS 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 ΔY/ΔZ in multiple MSs. 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 asY ≃ 0.3.