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Title: 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 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 photodissociationmore » 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.« less

Authors:
 [1]; ;  [2]; ;  [3]
  1. Departamento de Química Física Aplicada (UAM), Unidad Asociada a IFF-CSIC, Facultad de Ciencias Módulo 14, Universidad Autónoma de Madrid, E-28049, Madrid (Spain)
  2. Instituto de Física Fundamental (IFF-CSIC), C.S.I.C., Serrano 123, E-28006 Madrid (Spain)
  3. Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, Cantoblanco E-28049 (Spain)
Publication Date:
OSTI Identifier:
22661253
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 838; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; ASTROPHYSICS; CROSS SECTIONS; DISSOCIATION; HYDROCYANIC ACID; INTERSTELLAR SPACE; MOLECULES; PHOTOLYSIS; PHOTONS; RENORMALIZATION; SOLAR RADIATION; SURFACES; THREE-DIMENSIONAL CALCULATIONS; ULTRAVIOLET RADIATION; WAVE PACKETS; WAVELENGTHS

Citation Formats

Aguado, Alfredo, Roncero, Octavio, Zanchet, Alexandre, Agúndez, Marcelino, and Cernicharo, José, E-mail: octavio.roncero@csic.es. The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA63EE.
Aguado, Alfredo, Roncero, Octavio, Zanchet, Alexandre, Agúndez, Marcelino, & Cernicharo, José, E-mail: octavio.roncero@csic.es. The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium. United States. doi:10.3847/1538-4357/AA63EE.
Aguado, Alfredo, Roncero, Octavio, Zanchet, Alexandre, Agúndez, Marcelino, and Cernicharo, José, E-mail: octavio.roncero@csic.es. Mon . "The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium". United States. doi:10.3847/1538-4357/AA63EE.
@article{osti_22661253,
title = {The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium},
author = {Aguado, Alfredo and Roncero, Octavio and Zanchet, Alexandre and Agúndez, Marcelino and Cernicharo, José, E-mail: octavio.roncero@csic.es},
abstractNote = {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.},
doi = {10.3847/1538-4357/AA63EE},
journal = {Astrophysical Journal},
number = 1,
volume = 838,
place = {United States},
year = {Mon Mar 20 00:00:00 EDT 2017},
month = {Mon Mar 20 00:00:00 EDT 2017}
}
  • Observations of the rare isotopes of HCN and HNC have been used to determine the relative abundance of these two chemical isomers along the central ridge of the Orion molecular cloud. The abundance ratio HCN/HNC decreases by more than an order of magnitude from the relatively warm plateau and hot core sources toward the KL nebula to the colder, more quiescent clouds to the north and south. Even in the cooler regions, however, the ratio is an order of magnitude larger than that found in previous investigations of cold dark clouds. The kinetic temperature in the regions studied is determinedmore » from new observations of methylacetylene (CH3CCH), together with other recent estimates of the gas temperature near KL. The results suggest that the warmer portions of the cloud are dominated by different chemical pathways that those in the general instellar cloud material. 22 references.« less
  • We have used the rare isotopes of HCN and HNC to measure the relative abundance of these two chemical isomers in four giant molecular clouds. The lines of H/sup 13/CN, HC/sup 15/N, HN/sup 13/C, and H/sup 15/NC are all sufficiently weak to be little affected by saturation. A comparison of intensities measured with different beamsizes indicates that the sources are not resolved even with the 42'' beamsize used in the present work. The abundance ratio (HNC)/(HCN) varies by more than an order of magnitude, ranging from 0.015 to 0.40. These low values (which are similar to those found in warmmore » sources from the common isotopes by Wootten et al.) are not consistent with HNC and HCN production solely from various sources of the precursor ion H/sub 2/CN/sup +/, and they indicate that another source of HCN is required in warm, giant molecular clouds. We suggest that neutral atom-molecule reactions may make an important contribution to the HCN production rate in such regions.« less
  • Using the H{sup 13}CN and HN{sup 13}C J = 1–0 line observations, the abundance ratio of HCN/HNC has been estimated for different evolutionary stages of massive star formation: infrared dark clouds (IRDCs), high-mass protostellar objects (HMPOs), and ultracompact H ii regions (UCH iis). IRDCs were divided into “quiescent IRDC cores (qIRDCc)” and “active IRDC cores (aIRDCc),” depending on star formation activity. The HCN/HNC ratio is known to be higher at active and high temperature regions related to ongoing star formation, compared to cold and quiescent regions. Our observations toward 8 qIRDCc, 16 aIRDCc, 23 HMPOs, and 31 UCH iis showmore » consistent results; the ratio is 0.97 (±0.10), 2.65 (±0.88), 4.17 (±1.03), and 8.96 (±3.32) in these respective evolutionary stages, increasing from qIRDCc to UCH iis. The change of the HCN/HNC abundance ratio, therefore, seems directly associated with the evolutionary stages of star formation, which have different temperatures. One suggested explanation for this trend is the conversion of HNC to HCN, which occurs effectively at higher temperatures. To test the explanation, we performed a simple chemical model calculation. In order to fit the observed results, the energy barrier of the conversion must be much lower than the value provided by theoretical calculations.« less
  • The ultraviolet photoabsorption spectra of the HCN and HNC isomers have been simulated in the 7-10 eV photon energy range. For this purpose, the three-dimensional adiabatic potential energy surfaces of the 7 lowest electronic states, and the corresponding transition dipole moments, have been calculated, at multireference configuration interaction level. The spectra are calculated with a quantum wave packet method on these adiabatic potential energy surfaces. The spectra for the 3 lower excited states, the dissociative electronic states, correspond essentially to predissociation peaks, most of them through tunneling on the same adiabatic state. The 3 higher electronic states are bound, hereaftermore » electronic bound states, and their spectra consist of delta lines, in the adiabatic approximation. The radiative lifetime towards the ground electronic states of these bound states has been calculated, being longer than 10 ns in all cases, much longer that the characteristic predissociation lifetimes. The spectra of HCN is compared with the available experimental and previous theoretical simulations, while in the case of HNC there are no previous studies to our knowledge. The spectrum for HNC is considerably more intense than that of HCN in the 7-10 eV photon energy range, which points to a higher photodissociation rate for HNC, compared to HCN, in astrophysical environments illuminated by ultraviolet radiation.« less
  • The N = 1--0 transition of CO/sup +/, with components J = 1/2--1/2 at 117.692 GHz and J = 3/2--1/2 at 118.102 GHz, was searched for but not found in molecular clouds showing strong emission from the J = 1--0 transition of carbon monoxide. Rotational transition rest frequency calculations were made for HCN/sup +/, HNC/sup +/, and CN/sup +/; subsequent interstellar searches were conducted for these ions, but none were detected. 2 tables.