Δ Y/Δ Z from fine structure in the main sequence based on Hipparcos parallaxes

The slope ΔY/ΔZ is a quantity of interest in relation to stellar evolution, the initial mass function and the determination of the primordial helium abundance. In this paper we estimate ΔY/ΔZ from fine structure in the main sequence of nearby stars from Hipparcos data for stars with Z ≤ Z⊙. We find a value of about 3, which is consistent with what has been found in extragalactic H ii regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the initial mass function slope adopted.