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Session 12 - Cosmology, Large-Scale Structure and Distance Scales.
Display session, Monday, June 10
Great Hall,
We present a comprehensive H/D/He chemistry of the postrecombination epoch of the early universe. Fractional abundances of e^-, H, H^+, H^-, D, D^+, D^-, He, He^+, H_2, H_2^+, HD, HD^+, He_2^+, HeH^+, HeD^+, H_3^+, and H_2D^+ are calculated for redshifts z between 4000 and 10. 90 reactions are included with their rate coefficients taken from the most recent theoretical or experimental evaluations. Enhancements to the formation of HeH^+ and HeD^+ through stimulated radiative association are also included. Rate coefficients for this process will be presented.
We find the isotope abundance ratios [D^-]/[H^-], [HD^+]/[H_2^+], [HD]/[H_2], and [H_2D^+]/[H_3^+] to be enhanced compared to the primordial abundance ratio [D]/[H]=4\times 10^-5 by factors of up to \sim 50. Conversely, the [D^+]/[H^+] and [HeD^+]/[HeH^+] ratios are reduced by a factor of about 10^5 since for z<40, D^+ is significantly depleted due to D^+ + H \to D + H^+. The reverse reaction is endothermic by 43 K. HeD^+ is reduced since it is formed through He + D^+ \to HeD^+ + \nu.
The calculations are in fair agreement with Palla et al. (1995, ApJ, 451, 44) for H, H^+, H^-, H_2, and HD. Palla et al. appear to have overestimated the asymptotic (i.e., for z\to 0) fractional abundances of H_2^+, H_3^+, and H_2D^+ by factors ranging from 10^2 to 10^7 while HD^+ was underestimated by about a factor of 10^4. The ratios [HD]/[H_2] and [H_2D^+]/[H_3^+] are in fair agreement, but the HD^+ discrepancy persists for [HD^+]/[H_2^+].
Dubrovich (1993, Astron. Lett., 19, 53) has suggested that if the abundance of H_2D^+ is large enough, it may be detected by Thomson scattering of cosmic background radiation (CBR) photons. Palla et al. found the H_2D^+ fractional abundance to be to small (\sim 10^-16) to have an effect on the CBR spectrum; we confirm this result by calculating an even smaller H_2D^+ fractional abundance.
This work was supported by NSF grants OSR-9353227 and AST 93-01099.
