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]=4x 10(-5) by factors of up to ~ 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 -> D + H(+) . The reverse reaction is endothermic by 43 K. HeD(+) is reduced since it is formed through He + D(+) -> 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-> 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 ( ~ 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.
American Astronomical Society Meeting Abstracts #188
- Pub Date:
- May 1996