Tracing the evolutionary stage of Bok globules: CCS and NH 3

Abstract

Aims. We investigate a previously proposed correlation between the chemical properties and the physical evolutionary stage of isolated low-mass star-forming regions. The NNH 3/N CCS abundance ratio has been proposed to be a potentially useful indicator of the evolutionary stage of cloud cores, and we study its applicability for isolated Bok globules. Methods. We searched for CCS(2 1-1 0) emission in 42 Bok globules both with and without signs of current star formation. A set of NH 3 measurements was compiled from measurements available in the literature and from our own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models. Results. We determine the NNH 3/N CCS ratio for 18 Bok globules and find that it is moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harboring protostars of Class 0, Class I, or later. We do not find any Bok globules with extremely high CCS abundances analogous to carbon-chain producing regions in dark cloud cores. The observed range of NNH 3/N CCS implies that all of the observed Bok globules are in a relatively evolved chemical state. © 2012 ESO.