Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-09-17
Astronomy and Astrophysics
Astrophysics
10 pages, 6 Postscript figures, accepted for publication in the Astronomy and Astrophysics
Scientific paper
10.1051/0004-6361:20011303
This paper investigates the effect of collisional enhancement of the hydrogen lines on the derivation of the helium abundances in low metallicity H II regions. For this, we have constructed a grid of photoionization models relevant for the analysis of giant \hii regions in blue compact galaxies. We show that the effect of collisional excitation on the Halpha/Hbeta ratio can be quite important (up to 8% or more). The impact of this effect on the determination of the helium mass fraction has been tracked on four low-metallicity blue compact galaxies for which Keck spectra are available and which are among the best objects for the quest of the pregalactic helium abundance. We find that taking into account the effects of collisional excitation of hydrogen results in an upward correction of the helium mass fraction Y by up to 5%. However, combining with other systematic effects usually not considered in the determination of the helium abundance in low-metallicity galaxies, the resulting uncertainty should be much less.
Izotov Yu. I.
Stasinska Grazina
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