000002080 001__ 2080
000002080 005__ 20160701171658.0
000002080 0247_ $$2DOI$$a10.1093/mnras/stw893
000002080 037__ $$aASTROimport-237
000002080 100__ $$aGuzmán, F.
000002080 245__ $$aH, He-like recombination spectra - I. l-changing collisions for hydrogen
000002080 260__ $$c2016
000002080 520__ $$aHydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He+/H+ abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole l-changing collisions (nl → nl' = l ± 1) between energy-degenerate states within an n-shell. The work of Pengelly  Seaton has, for half-a-century, been considered the definitive study which `solved' the problem. Recent work by Vrinceanu et al. recommended the use of rate coefficients from a semiclassical approximation which are nearly an order of magnitude smaller than those of Pengelly  Seaton, with the result that significantly higher densities are needed for the nl populations to come into local thermodynamic equilibrium. Here, we compare predicted H I emissivities from the two works and find widespread differences, of up to ˜10 per cent. This far exceeds the 1 per cent precision required to obtain the primordial He/H abundance ratio from observations so as to constrain big bang cosmologies. We recommend using the rate coefficients of Pengelly  Seaton for l-changing collisions, to describe the H recombination spectrum, based-on their quantum mechanical representation of the long-range dipole interaction. 
000002080 700__ $$a Badnell, N. R.
000002080 700__ $$a Williams, R. J. R.
000002080 700__ $$a van Hoof, P. A. M.
000002080 700__ $$a Chatzikos, M.
000002080 700__ $$a Ferland, G. J.
000002080 773__ $$c3498-3504$$i4$$pMonthly Notices of the Royal Astronomical Society$$v459$$y2016
000002080 85642 $$ahttp://esoads.eso.org/abs/2016MNRAS.459.3498G
000002080 905__ $$apublished in
000002080 980__ $$aREFERD