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Issue A&A
Volume 472, Number 2, September III 2007
Page(s) 537 - 545
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20077358



A&A 472, 537-545 (2007)
DOI: 10.1051/0004-6361:20077358

The CH $_\mathsf{2}$CN- molecule: carrier of the $\mathsf{\lambda}$8037 diffuse interstellar band?

M. A. Cordiner1, 2 and P. J. Sarre1

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 $8037.8\pm0.15$ Å is examined with reference to new observational data. The 000 absorption band arising from the $^1{\rm B}_1-\tilde{\rm X}~^1{\rm A}'$ 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 $\lambda$8037 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 $\ion{Ti}{ii}$ results in a good match with the profile of the narrow $\lambda$8037 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 $K_{\rm a} = 1$ 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 $\lambda$8037 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 $K_{\rm a}''$ levels approaching a Boltzmann distribution near 3 K.


Key words: astrochemistry -- ISM: lines and bands -- ISM: molecules -- ISM: atoms -- ISM: clouds



© ESO 2007


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