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 \Delta Y / \Delta Z \sim 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 \Delta Y / \Delta Z \sim 10 is necessary to reproduce the broadening observed for nearby subdwarfs .
This result resembles , on a milder scale , the very high \Delta Y / \Delta Z 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 \Delta Y / \Delta Z in multiple main sequences .
We find that , under the most favourable assumptions , the estimated helium content for the enriched populations could decrease from Y \simeq 0.4 to as low as Y \simeq 0.3 .