Images of atomic carbon in the interstellar medium

Abstract

ALTHOUGH carbon is the fourth most cosmically abundant element, it exists only rarely as neutral atoms, because carbon that is cool enough to be neutral rapidly combines with other atoms to form molecules such as CO. Atomic carbon (CI) forms when CO is dissociated by ultraviolet photons, but the dissociation energy is close to the ionization energy of CI, so that neutral carbon is easily ionized into C II. At the edges of molecular clouds, carbon will exist mostly in the form of C II, with only a narrow transition zone containing CI covering the interior of the cloud, where CO predominates. We present here observations of the submillimetre line due to the3Pl-3P0 fine-structure transition of C I, from which we construct high-resolution maps of neutral carbon in molecular clouds in the star-forming region Orion A, in the externally illuminated cloud S140, in the edge-on ionization front of M17, and at the Galactic Centre. C I emission is indeed concentrated in a transition zone, but we confirm earlier suggestions that neutral carbon is also present at lower concentrations within the clouds. In the case of M17, our images provide direct evidence for clumpiness in the cloud distribution.