Rapid He ii→He i recombination and radiation arising from this process

ABSTRACT Recombination of the primordial helium plasma (HeII→HeI, z ≃ 1500 − 3000) isconsidered. This process has an effect on the CMBR anisotropy and CMBR spectrumdistortion. In this work an influence of neutral hydrogen on kinetics of HeII→HeIrecombination is investigated in the frame of the standard cosmological model. It isshownthat small amount ofneutral hydrogen(10 −5 −10 2 oftotal number ofhydrogenions and atoms) leads to acceleration of HeII→HeI recombination at z . 2000 and atz . 1600 quasi-equilibrium HeII→HeI recombination (according to the Saha formula)becomes valid.Key words: cosmology, primordial plasma, recombination, cmbr, anisotropy 1 INTRODUCTIONThe HeII→HeI primordial recombination has an ef-fect on formation of the cosmic microwave background ra-diation (CMBR) anisotropy (e.g. Hu et al., 1995; Seager etal., 1999) and CMBR spectrum distortion (Lyubarsky andSunyaev, 1983; Fahr and Loch, 1991; Dubrovich and Stol-yarov, 1997; Wong et al., 2006). The relative difference ofCMBR anisotropy power spectrum corresponding to SahaHeII→HeI recombination and corresponding to HeII→HeIrecombination by Seager et al. (1999) has a level of up to5% for multipoles l ≃ 1500−3000 (Seager et al., 2000). Suchchanges can be measured in future experiments (e.g. Planckand others).Kinetics of HeII→HeI recombination has been consid-ered in a number of papers (Boschan and Biltzinger, 1998;Novosyadlyj, 2006; Wong and Scott, 2006; and referencestherein). The most used model of helium recombination isthe model suggested by Matsuda et al. (1969) and devel-oped by Seager et al. (1999). A significant modification ofthis model was suggested by Dubrovich and Grachev (2005)who took into account recombination through HeI ortho-states.In the model by Seager et al. (1999) the effect of neu-tral hydrogen on HeII→HeI recombination was neglectedalthough in a number of previous papers (e.g. Hu et al.,1995; Boschan and Biltzinger, 1998; and others) authorspointed out that this effect may be essential. This contra-diction made us investigate the effect of neutral hydrogenon HeII→HeI recombination again and derive HeII→HeI re-combination kinetic equation which takes this effect into ac-count.2 ADDITION TO THE RECOMBINATIONMODELThe main addition to the Seager’s et al. (1999) model istaking into account an interaction of resonant HeI quantawith neutral hydrogen. This interaction is described by thefollowing elementary processes:1. HeI resonant photon ionizes hydrogen atom with emis-sion of electron which has a kinetic energy more than 6 eV:H + γ → H