Primordial Helium and DeltaY/DeltaZ from H II Regions and from Fine Structure in the Main Sequence Based on HIPPARCOS Parallaxes

Abstract

The primordial helium abundance YP is important for cosmology and the ratio DeltaY/DeltaZ of the changes relative to primordial abundances constrains models of stellar evolution. While the most accurate estimates of YP come from emission lines in extragalactic H II regions, they involve an extrapolation to zero metallicity which itself is closely tied up with the slope DeltaY/DeltaZ. Recently certain systematic effects have come to light in this exercise which make it useful to have an independent estimate of DeltaY/DeltaZ from fine structure in the main sequence of nearby stars. We derive such an estimate from Hipparcos data for stars with Z <= Zsolar and find values between 2 and 3, which are consistent with stellar models, but still have a large uncertainty.