Search for hydrogen-helium molecular species in space

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.