EDP Sciences
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Volume 381, Number 2, January II 2002
Page(s) 361 - 373
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20011567

A&A 381, 361-373 (2002)
DOI: 10.1051/0004-6361:20011567

Systematic uncertainties in the determination of the primordial $^\mathsf{4}$He abundance

D. Sauer and K. Jedamzik

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

(Received 25 April 2001 / Accepted 25 October 2001 )

The primordial helium abundance $Y_{\rm p}$ is commonly inferred from abundance determinations in low-metallicity extragalactic $\ion{H}{ii}$-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 Q(He0)/Q(H0), and the ratio of width to radius $\delta r_{\rm S}/r_{\rm S}$ of the Strömgren sphere. For clouds of sufficient helium-ionizing photons $Q(He^{0})/Q(H^{0}) \ga 0.15$ 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 $-0.4 \la \log\,\eta \la 0.3$ coincident with the range of $\eta$ in observed $\ion{H}{ii}$-regions. Existing data of $\ion{H}{ii}$-regions indeed seem to display a correlation which is consistent with a typical ~ $ 2{-}4\%$ 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 $Y_{\rm p}$ may result. It is argued that caution should be exercised regarding the validity of commonly quoted error bars on $Y_{\rm p}$.

Key words: cosmology: early universe -- ISM: $\ion{H}{ii}$-regions, abundances

Offprint request: D. Sauer, dsauer@mpa-garching.mpg.de

© ESO 2002