The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium

Abstract

The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross section 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 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 of considering 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 one. 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, obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.

A new radio molecular line survey of planetary nebulae

“…There exists no dependence of the HNC/HCN ratio on the type or presence of an X-ray source. For the HNC/HCN line ratio to decrease steadily with UV luminosity, high energy photons must be either heating the gas within the PDR or selectively photodissociating HNC (Aguado et al 2017). Under the former scenario, as CSPN UV luminosity rises, the resulting CSPN-induced ionization evidently raises the local temperature to exceed the activation energy for conversion of HNC into HCN+H.…”

“…Another potential mechanism that might drive the HNC/HCN ratio is the difference in photodissociation rates of HNC and HCN. Calculations by Aguado et al (2017), taking into account the photodissociation cross sections of each molecule as well as variation in the incident UV radiation field, find that selective photodissociation can increase the rate of destruction of HNC by a factor of ∼2-10 relative to that of HCN. While high effective temperature sources, such as CSPNe, have fewer lowenergy photons and thus a smaller effect on photodissociation rates than cool sources, the interaction between the strength of the evolving UV source and the photochemical reaction rates of HNC and HCN could be sufficient to drive the observed trend.…”

Nobeyama 45 m mapping observations toward Orion A. III. Multi-line observations toward an outflow-shocked region, Orion Molecular Cloud 2 FIR 4

“…As we mentioned above, there are at least two effects discussed for reducing the [HNC]/ [HCN] ratio (Hirota et al 1998;Colzi et al 2018;Aguado et al 2017 (2018) estimated the dimensionless FUV strength G 0 from the Herschel 70 µm data and they found that the G 0 parameter of the FIR 4 region exceeds 100 and peaked at around FIR 3 (∼ 7000) and…”

Origin band of the first photoionizing transition of hydrogen isocyanide

“…The photoelectron spectrum of HNC isomer is calculated using a wave-packet method similarly to that applied to calculate the photodissociation of HCN42,43 and photodetachment of OHF – . 39 In the present approach, the transition dipole moment is neglected, and the initial wave-packet is the rovibrational wave function of the neutral HNC isomer.…”