The rotational excitation of HCN and HNC by He: new insights on the HCN/HNC abundance ratio in molecular clouds

Abstract

Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. The present paper focuses on the calculation of rate coefficients for rotational excitation of the hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) molecules in their ground vibrational state in collision with He. The calculations are based on new two‐dimensional potential energy surfaces obtained from highly correlated ab initio calculations. 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 eight first rotational levels of HCN and HNC were calculated for kinetic energies up to 1000 cm−1. These cross‐sections were used to determine collisional rate constants for temperatures ranging from 5 to 100 K. 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. The consequences for astrophysical models are evaluated and it is shown that the use of HCN rate coefficients to interpret HNC observations can lead to significant inaccuracies in the determination of the HNC abundance, in particular, in cold dark clouds for which the new HNC rates show that the j= 1 − 0 line of this species will be more easily excited by collisions than HCN. An important result of the new HNC–He rates is that the HNC/HCN abundance ratio derived from observations in cold clouds has to be revised from >1 to ≃1, in good agreement with detailed chemical models available in the literature.

Hydrogen cyanide and isocyanide in prestellar cores

“…4), we gain information about the high density "core of the core". It is worth noting that our observations also are a test of the collisional rates for HCN and HNC (Sarrasin et al 2010;Dumouchel et al 2010). We illustrate this in the next section.…”

“…12, we show the comparison between the values of the excitation temperatures of H 13 CN and HN 13 C evaluated from the simultaneous fit of the hyperfine components for the three sources examined, revealing that T ex (HN 13 C) is essentially always greater than T ex (H 13 CN) which we presume to be due to the differing collisional rates. In the same plot, two curves trace the values for T ex computed using RADEX (van der Tak et al 2007) which uses the new collisional rates for HNC (Sarrasin et al 2010;Dumouchel et al 2010) that are no longer assumed equal to HCN. These curves assume kinetic temperatures, T kin , equal to 6 and 10 K, and typical values of 2 × 10 12 cm −2 and 0.2 km s −1 for column density and line width, respectively.…”

Dense molecular gas towards W49A: a template for extragalactic starbursts?

“…(a) Tentative detection based on the rms noise in the spectrum, N(DCN) < 3.7 × 10 12 cm −2 for T ex ≥ 10 K and DCN/HCN < 0.008. central gas, but a possible difference in critical density between HCN and HCO + on one hand and HNC on the other may also play a role (Sarrasin et al 2010). Since the HCN, HNC and HCO + main isotopic lines are optically thick, the observed line intensity is not a direct measure of column density, but is rather the product of surface area, filling factor and temperature.…”

“…We adopt the HCN collision rates for HNC, which assumption is a potential source of error since the two species have somewhat different dipole moments (3.0 vs. 3.3 D). Very recently, Sarrasin et al (2010) and Dumouchel et al (2010) have presented collision data for HCN and HNC with He; the impact of these data on the present work will be analyzed in a forthcoming paper.…”

“…We conclude that no single PDR or XDR model is able to reproduce our observations, so that a more complex, multi-component model must be invoked. One caveat is the Meijerink et al models should be updated with the new collision data for HCN and HNC (Sarrasin et al 2010). …”