PRIMORDIAL HELIUM AND ΔYZ from H II Regions and from Fine Structure in the Main Sequence Based on Hipparcos Parallaxes*

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The primordial helium abundance YP is important for cosmology and the ratio ΔY/ΔZ 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 ΔY/ΔZ. Recently certain systematic effects have come to light in this exercise which make it useful to have an independent estimate of ΔY/ΔZ from fine structure in the main sequence of nearby stars. We derive such an estimate from Hipparcos data for stars with Z ≤ Z⊙ and find values between 2 and 3, which are consistent with stellar models, but still have a large uncertainty.

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