Issue |
A&A
Volume 381, Number 2, JanuaryII 2002
|
|
---|---|---|
Page(s) | 361 - 373 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20011567 | |
Published online | 15 January 2002 |
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
The primordial helium abundance Yp 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
Yp may result. It is argued that caution should be
exercised regarding the validity of commonly quoted error
bars on Yp.
Key words: cosmology: early universe / ISM: HII-regions, abundances
© ESO, 2002
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