Publication Date:
2016-05-12
Description:
Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He i and H i emission lines can be used to measure the He + /H + abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole l -changing collisions ( nl -〉 nl ' = l ± 1) between energy-degenerate states within an n -shell. The work of Pengelly & Seaton has, for half-a-century, been considered the definitive study which ‘solved’ the problem. Recent work by Vrinceanu et al. recommended the use of rate coefficients from a semiclassical approximation which are nearly an order of magnitude smaller than those of Pengelly & Seaton, with the result that significantly higher densities are needed for the nl populations to come into local thermodynamic equilibrium. Here, we compare predicted H i emissivities from the two works and find widespread differences, of up to 10 per cent. This far exceeds the 1 per cent precision required to obtain the primordial He/H abundance ratio from observations so as to constrain big bang cosmologies. We recommend using the rate coefficients of Pengelly & Seaton for l -changing collisions, to describe the H recombination spectrum, based-on their quantum mechanical representation of the long-range dipole interaction.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
