Far-infrared detection of methylene ^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: epoleham@mpifr-bonn.mpg.de
² Physikalisches Institut, Universität zu Köln, 50937 Cologne, Germany
³ Laboratoire d'Astrophysique de Marseille, CNRS & Université de Provence, BP 8, 13376 Marseille Cedex 12, France

Received: 5 July 2004
Accepted: 1 October 2004

Abstract

We present a clear detection of CH₂ in absorption towards the molecular cloud complexes Sagittarius B2 and W49 N using the ISO Long Wavelength Spectrometer. These observations represent the first detection of its low excitation rotational lines in the interstellar medium. Towards Sagittarius B2, we detect both ortho and para transitions allowing a determination of the total CH₂ column density of cm^-2. We compare this with the related molecule, CH, to determine [ CH/CH. Comparison with chemical models shows that the CH abundance along the line of sight is consistent with diffuse cloud conditions and that the high [ CH/CH₂] ratio can be explained by including the effect of grain-surface reactions.