The rotational excitation of HCN and HNC by He: temperature dependence of the collisional rate coefficients

Abstract

Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. This paper deals with the calculation of rate coefficients for rotational excitation of the HCN and HNC molecules in their ground vibrational state in collision with He. Calculations of pure rotational (de‐)excitation cross‐sections of HCN and HNC by He were performed using the essentially exact close‐coupling method. Cross‐sections for transitions among the 26 first rotational levels of HCN and HNC were calculated for energies up to 3500 cm−1. These cross‐sections were used to determine collisional rate constants for temperatures ranging from 5 to 500 K. The propensity rules of both collisional systems are discussed. A propensity for even Δj transitions is observed in the case of HCN–He collisions whereas a propensity for odd Δj transitions is observed in the case of HNC–He collisions. These propensities become less pronounced at high temperature, although they do not vanish within the temperature range considered in this work. The new rate coefficients will significantly help in interpreting HCN and HNC emission lines observed with current and future telescopes. In particular, the HNC/HCN abundance ratio derived from observations would have to be revised from values >1 to values ≤1.

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.…”

How Dense Is Your Gas? On the Recoverability of LVG Model Parameters

“…For all of our tests we generate artificial lines using RADEX and collision rates from the Leiden Lambda database (van der Tak et al 2007;Dumouchel et al 2010;Yang et al 2010). The specifics depend upon the investigation in question, but the features common to most of the models are as follows.…”