The CH2CN- molecule: Carrier of the lambda8037 diffuse interstellar band?

Abstract

The hypothesis that the cyanomethyl anion CH2CN- is responsible for the relatively narrow diffuse interstellar band (DIB) at 8037.8 +- 0.15 Angstroms is examined with reference to new observational data. The 0_0^0 absorption band arising from the ^1B_1 - X ^1A' 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 Angstroms. 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 lambda8037 diffuse interstellar band provided the rotational contour is Doppler-broadened with a b parameter between 16 and 33 km/s that depends on the specific sightline. Convolution of the calculated CH2CN- transitions with the optical depth profile of interstellar Ti II results in a good match with the profile of the narrow lambda8037 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 8050 Angstroms that are not seen in currently available interstellar spectra. For CH2CN- to be the carrier of the lambda8037 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 3 K.