The CHCN^- molecule: carrier of the 8037 diffuse interstellar band?

¹ School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK e-mail: m.cordiner@qub.ac.uk
² Astrophysics Research Centre, School of Mathematics and Physics, Queen's University, Belfast BT7 1NN, UK

Received: 23 February 2007
Accepted: 4 July 2007

Abstract

The hypothesis that the cyanomethyl anion CH₂CN^- is responsible for the relatively narrow diffuse interstellar band (DIB) at  Å is examined with reference to new observational data. The absorption band arising from the  transition from the electronic ground state to the first dipole-bound state of the anion is calculated for a rotational temperature of 2.7 K using literature spectroscopic parameters and results in a rotational contour with a peak wavelength of 8037.78 Å. By comparison with diffuse band and atomic line absorption spectra of eight heavily-reddened Galactic sightlines, CH₂CN^- is found to be a plausible carrier of the λ8037 diffuse interstellar band provided the rotational contour is Doppler-broadened with a b parameter between 16 and 33 km s^-1 that depends on the specific sightline. Convolution of the calculated CH₂CN^- transitions with the optical depth profile of interstellar results in a good match with the profile of the narrow λ8037 DIB observed towards HD 183143, HD 168112 and Cyg OB2 8a. The rotational level populations may be influenced by nuclear spin statistics, resulting in the appearance of additional transitions from of ortho CH₂CN^- near 8025 and 8050 Å that are not seen in currently available interstellar spectra. For CH₂CN^- to be the carrier of the λ8037 diffuse interstellar band, either a) there must be mechanisms that convert CH₂CN^- from the ortho to the para form, or b) the chemistry that forms CH₂CN^- must result in a population of levels approaching a Boltzmann distribution near 3 K.