Ion chemistry in the early universe

Revisiting the role of HeH⁺ with new quantum calculations

¹ Department of Chemistry, “Sapienza” University of Rome P.le A. Moro 5, 00185 Rome, Italy
e-mail: s.bovino@caspur.it; fa.gianturn@caspur.it
² INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 17 February 2011
Accepted: 9 March 2011

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⁺.