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Enhanced abundance of primordial HeH+
Though only recently detected in space, the helium hydride ion (HeH+) is thought to be the first molecule ever to have formed in the early Universe. Novotný et al. report state-specific rate coefficients for the dissociative reaction of HeH+ with electrons, obtained using a cryogenic ion storage ring combined with a merged electron beam (see the Perspective by Bovino and Galli). They detect substantial rotational dependence and a decrease of the rates for the lowest states of HeH+, far below the values listed in astrochemistry databases and those previously applied in early-Universe models. These results suggest high abundance of this important primordial molecule at redshifts of first star and galaxy formation.
Abstract
The epoch of first star formation in the early Universe was dominated by simple atomic and molecular species consisting mainly of two elements: hydrogen and helium. Gaining insight into this constitutive era requires a thorough understanding of molecular reactivity under primordial conditions. We used a cryogenic ion storage ring combined with a merged electron beam to measure state-specific rate coefficients of dissociative recombination, a process by which electrons destroy molecular ions. We found a pronounced decrease of the electron recombination rates for the lowest rotational states of the helium hydride ion (HeH+), compared with previous measurements at room temperature. The reduced destruction of cold HeH+ translates into an enhanced abundance of this primordial molecule at redshifts of first star and galaxy formation.
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