EDP Sciences Journals List
Advanced Search
Subscriber Authentication Point
Home All issues Volume 381/2 Article
Free access article

Issue A&A
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 )

Abstract
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


What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.