The Primordial Helium Abundance as Determined from Chemical Evolution of Irregular Galaxies

Astronomy and Astrophysics – Astronomy

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Galaxies: Irregular, Nuclear Reactions, Nucleosynthesis, Abundances

Scientific paper

In a previous study we explored the effects of Galactic chemical evolution on the determination of the primordial 4He abundance, Yp, from the zero metallicity intercept of the observed Y versus Z correlations. In the present paper we derive formulae in which one of the approximations made in that study is removed in the fit to the Y versus Z correlation data. We also investigate the effects on the Yp value of various approaches to fitting the data and find that a simultaneous fitting of all the data is the best approach to determining Yp. We examine the recently suggested method of using N/O versus 0/H as a means to determine the primary nitrogen abundance at low metallicity. As in the previous work it is found that the Y versus N/H data require a combination of primary and secondary nitrogen in all fits. We determine that Yp = 0.227±0.006 (1 σ). This value is ˜1.5 σ below the minimum value allowed by the standard homogeneous big bang model with three light neutrino species.

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