The photodissociation of HCN and HNC: effects on the HNC/HCN abundance ratio in the interstellar medium

10 pags., 7 figs., 2 tabs., 2 apps.The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross sections of HCN and HNC isomers have been calculated in the 7-13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need to consider non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental value. It is found that HNC is photodissociated faster than HCN by a factor of 2.2 for the local interstellar radiation field and 9.2 for the solar radiation field, at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field, it is necessary to use different photodissociation rates for each of the two isomers, which are obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). We also acknowledge support by the Ministerio de Economía e Innovación under grants Nos. CSD2009-00038, AYA2012-32032, AYA2016-75066-C2-1-P and FIS2014-52172-C2. M.A. thanks funding support from the Ramón y Cajal programme of Spanish MINECO (RyC-2014- 16277). The calculations have been performed in the CCC-UAM and trueno-CSIC computing centers.Peer Reviewe