Systematic uncertainties in the determination of the primordial He abundance

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany e-mail: dsauer@mpa-garching.mpg.de, jedamzik@mpa-garching.mpg.de

Corresponding author: D. Sauer, dsauer@mpa-garching.mpg.de

Received: 25 April 2001
Accepted: 25 October 2001

Abstract

The primordial helium abundance Y_p is commonly inferred from abundance determinations in low-metallicity extragalactic -regions. Such determinations may be subject to systematic uncertainties that are investigated here. Particular attention is paid to two effects: icf-corrections for “imperfect” ionization structure leading to significant amounts of (unobservable) neutral helium or hydrogen and “tcf”-corrections due to non-uniform temperature. Model Hii-regions with a large number of parameters are constructed and it is shown that required corrections are almost exclusively functions of two physical parameters: the number of helium- to hydrogen-ionizing photons in the illuminating continuum , and the ratio of width to radius of the Strömgren sphere. For clouds of sufficient helium-ionizing photons and non-negligible width of the Strömgren sphere, a significant overestimate of helium abundances may result. Such clouds show radiation softness parameters in the range coincident with the range of η in observed -regions. Existing data of -regions indeed seem to display a correlation which is consistent with a typical ~ overestimate of helium abundances due to these effects. In case such an interpretation prevails, and in the absence of other compensating effects, a significant downward revision of Y_p may result. It is argued that caution should be exercised regarding the validity of commonly quoted error bars on Y_p.