The rotational excitation of the interstellar HNC by para- and ortho-H2

Abstract

Rotational excitation of the interstellar HNC due to collisions with H(2) is investigated. We present a new four dimensional (4D) potential energy surface for the HNC-H(2) collisional system. Both molecules were treated as rigid rotors. Interaction energy was obtained from the electronic structure calculations using a single and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The five atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the HNC center of mass and the center of mass of H(2) for a better description of the van der Waals interaction. Close coupling calculations of the inelastic integral cross sections of HNC in collisions with para-H(2) and ortho-H(2) were calculated for kinetic energies up to 800 cm(-1). After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 100 K. Significant differences exist between para- and ortho-H(2) results. The strongest collision-induced rotational HNC transitions are the transitions with Δj = 1 for collisions with para-H(2) and with ortho-H(2). The new rate coefficients should induce important consequences on the determination of HNC abundance in the interstellar medium. In particular, we expect that they will help to solve the interstellar problem of relative abundance of the HCN and HNC isomers.

The interstellar gas-phase chemistry of HCN and HNC

“…The HCN/HNC abundance ratio is always above 1 and as high as 3. The determination of the HCN/HNC ratio using non-LTE analyses are likely to be unreliable due to the use of incorrect collisional excitation rates for HNC (Sarrasin et al 2010, Dumouchel et al 2010, Dumouchel et al 2011. Taking into account the new collisional excitation rates will lead to smaller HNC abundances, almost certainly leading to a HCN/HNC ratio above 1 in agreement with our simulations.…”

“…The rotational excitation rate constant with H 2 being deduced from the one with He. However it has been shown that this approximation leads to a notable overestimation of the rotational rate constant for HNC (Sarrasin et al 2010, Dumouchel et al 2010, Dumouchel et al 2011. The use of the new rotational rate constant will lead to revised HNC/HCN abundance ratios derived from observations from values >1 to values ≤1 (Dumouchel et al 2010).…”

“…The abundances relative to H 2 are directly deduced from line intensities (assuming local thermodynamic equilibrium conditions, LTE) and the HCN and HNC abundances are deduced from 13 C isotopomers abundance using a 12 C/ 13 C ratio equal to 65. The observations of (Dickens et al 2000) also use H 13 CN and HN 13 C as a proxy and they compute the abundances from line intensities using a statistical equilibrium model using the same collisional rates for HNC and HCN, which is known to lead to an overestimation of HNC abundances (Sarrasin et al 2010, Dumouchel et al 2010, Dumouchel et al 2011. (Dickens et al 2000) found higher HCN and HNC densities attributing their differences with (Swade 1989) to the optical depth estimation as Swade assumed optically thin emission, potentially underestimating the column densities.…”

“…HCN and HNC abundances do not take into consideration the new rotational excitation cross sections (Sarrasin et al 2010, Dumouchel et al 2010, Dumouchel et al 2011 and are determined using H 13 CN and HN 13 C with a 12 C/ 13 C ratio equal to 68 (Milam et al 2005). Values denoted by * have been deduced from an LTE analysis considering optically thin absorption.…”

Influence of collisional rate coefficients on water vapour excitation

“…CO by Wernli et al (2006); HC 3 N by Wernli et al (2007a); SiS by ; ; H 2 CO by ; HNC by Dumouchel et al (2011); CN − by ; SO 2 by Cernicharo et al (2011); HF by Guillon & Stoecklin (2012); HDO by Faure et al (2012)). Except in the CN − case, a common conclusion is that the o-H 2 CRC are larger than for p-H 2 .…”