Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...310l..67f&link_type=abstract
Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X), vol. 310, Nov. 15, 1986, p. L67-L70.
Astronomy and Astrophysics
Astrophysics
58
Abundance, Atomic Collisions, Big Bang Cosmology, Helium, Planetary Nebulae, H Ii Regions, Hydrogen, Intergalactic Media, Nuclear Fusion
Scientific paper
Attention is drawn to some consequences of recent ab initio quantal calculations of the cross section for collisional excitation of level three of neutral helium from the metastable 2 3 S state. For conditions appropriate for extragalactic H II regions, such as those used to determine the primordial helium abundance, collisions enhance the 5876 A and 6678 A lines used to deduce the He/H ratio by factors of the order of 5-30 percent. Collisional excitation is relatively more important in these H II regions than in galactic H II regions because of the high electron temperatures associated with these metal-deficient objects. The effect of collisions is to cause the helium abundance to be overestimated by as much as 30 percent. The mean of a small sample of O-He deficient objects is a helium mass fraction of roughly 0.207 + or - 0.016. Three objects in the sample have helium mass fractions significantly below current big bang predictions.
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