2001/04/30 by D. Sauer, Daniel Sauer, Karsten Jedamzik +1 · 4 citations
Physics and Astronomy · #Galaxies: Formation, Evolution, Phenomena #Scientific Research and Discoveries #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1051/0004-6361:20011567
published as Astron.Astrophys. 381 (2002) 361-373 · 14 pages, 18 figures, updated to match version accepted by Astronomy & Astrophysics
arxiv created 2001/11/02 · openalex publication_date 2002/01/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/01
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.