Ion chemistry in the early universe

Abstract

Aims. We reassess the role of HeH + with the aid of newly calculated rates that use entirely ab initio methods, which thereby allow us to compute with higher accuracy the relevant abundances within the global chemical network of the early universe. A comparison with the similar role of the ionic molecule LiH + is also presented. Methods. Quantum calculations were carried out for the gas-phase reaction of HeH + with H atoms using our new in-house code, based on the negative imaginary potential method. Integral cross-sections and reactive rate coefficients obtained under the general conditions of early universe chemistry are presented and discussed. Results. Using the new reaction rate, the abundance of HeH + in the early universe is found to be more than one order of magnitude higher than in previous studies. Our more accurate findings increase our confidence in detecting cosmological signatures of HeH + .

PHOTON-INDUCED EVOLUTIONARY RATES OF LiHe+(1Σ+) IN EARLY UNIVERSE FROM ACCURATE QUANTUM COMPUTATIONS

“…The new, corrected rate values are thus affecting also the computed production/destruction rates of LiHe + as a function of the redshift values reported earlier by the data of Figure 5 in Bovino et al (2011a). The behavior of all production and destruction processes as a function of z is now exactly the same as that reported in Figure 5 of Bovino et al (2011a), only lowered uniformly by a factor of about 22.…”

“…The new calculations change the computed cross-sections downward by about an order of magnitude with respect to those reported by Bovino et al (2011a).…”

“…Several groups have proposed chemical models and evaluated the abundances of those primordial molecules (H 2 , H + 2 , LiH, LiH + , HeH + ), which are deemed to be operative under these special conditions. As a consequence, a great deal of work has been invested to obtain accurate rate coefficients for formation/destruction reactions involving such species Bulut et al 2008;Čurìk & Greene 2007Čurìk & Greene , 2008Bennett et al 2008;Bovino et al 2009Bovino et al , 2010aBovino et al , 2010bBovino et al , 2011a. The importance of these primordial molecules has been introduced earlier by Dubrovich (1993) and discussed in several papers since that initial proposal (e.g., see the previous references).…”

Search for hydrogen-helium molecular species in space

“…Then, Engel et al (2005), by calculating the spectra of HeH + and opacity data, proposed that HeH + could be detected in cold helium stars. Recent chemical models, such as that developed by Bovino et al (2011Bovino et al ( , 2012 have also predicted that HeH + could be the most abundant molecule of the post-recombination era and could survive so as to be observed at low redshifts.…”

The He + ${\rm H}_2^+$H2+ → HeH+ + H reaction: Ab initio studies of the potential energy surface, benchmark time-independent quantum dynamics in an extended energy range and comparison with experiments

“…II. Although several classical trajectories, 32 stereodynamical, 33 and approximated quantum dynamical studies 34,35 have already been performed with the PESs of Ref. 31, rigorous quantum dynamical calculations were still lacking, so it was not clear whether these PESs really improve the agreement with the experimental data.…”