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