2007A&A...472..537C

The hypothesis that the cyanomethyl anion CH₂CN^– is responsible for the relatively narrow diffuse interstellar band (DIB) at 8037.8±0.15Å is examined with reference to new observational data. The 0₀⁰ absorption band arising from the ¹B₁-∼X¹A' 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 8037.78Å. 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 33km/s that depends on the specific sightline. Convolution of the calculated CH₂CN^– transitions with the optical depth profile of interstellar TiII 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 K_a=1 of ortho CH₂CN^– near 8025 and 8050Å 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 K_a'' levels approaching a Boltzmann distribution near 3K.