ΔY/ΔZ from fine structure in the main sequence based on Hipparcos parallaxes

The slope Delta Y/Delta Z 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 Delta Y/Delta Z from fine structure in the main sequence of nearby stars from Hipparcos data for stars with Z less than or equal to Z.. We find a value of about 3, which is consistent with what has been found in extragalactic Ii rr regions and with stellar models for suitable upper limits to the initial masses of supernovae according to the initial mass function slope adopted.