Search for hydrogen-helium molecular species in space

¹ Sorbonne Universités, UPMC Univ. Paris 06, UMR – CNRS 7616, Laboratoire de Chimie Théorique, 75005 Paris, France
e-mail: yves.ellinger@upmc.fr
² Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, 69622 Villeurbanne, France
e-mail: marie-christine.bacchus@univ-lyon1.fr
³ Sorbonne Universités, UPMC Univ. Paris 06 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
e-mail: jean-pierre.maillard@iap.fr

Received: 25 June 2017
Accepted: 27 July 2017

Abstract

Context. Helium, the second most abundant element in the Universe, with a relative abundance of He/H ~ 1/10, has never been observed in any other form than that of a neutral atom (He) or an ion (He⁺) in the interstellar medium. Since He is a noble gas its non-observation as part of neutral molecular systems is understandable, but it is very surprising for a positively charged species such as HeH⁺ that is a stable diatomic ion whose spectral signatures are well known in the laboratory.

Aims. This non-observation, even in hydrogen rich regions, could imply that HeH⁺ is not a proper target and that alternatives have to be considered, such as small HeH_n⁺ clusters. The present study aims at finding whether the leading term HeH₃⁺ fulfills the conditions required.

Methods. We addressed the question with state-of-the-art numerical simulations. We determined a two-dimension ab initio potential energy surface (PES) of the HeH₃⁺ cluster along the He...H₃⁺ and HeH⁺...H₂ reaction coordinates. The calculations rely on complete active space configuration interaction followed by a second order perturbation treatment (CAS-PT2). This surface was used for the evaluation of the two radiative associations rate constants by means of a quantum treatment of the collision between the interacting fragments.

Results. These calculations show unambiguously that HeH₃⁺ is the most stable point on the corresponding global PES. Then, we determined the rate constants of the radiative associations HeH⁺ + H₂ and He + H₃⁺ leading to HeH₃⁺.

Conclusions. Significative values were obtained that reach up to 2 × 10^-18 cm³ s^-1, which should stimulate new tentatives to detect molecular helium in astrophysical objects.