Tracing the evolutionary stage of Bok globules: CCS and NH3
  We pursue the investigation of a previously proposed correlation between
chemical properties and physical evolutionary stage of isolated low-mass
star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to
be a potentially useful indicator for the evolutionary stage of cloud cores. We
aim to study its applicability for isolated Bok globules. A sample of 42 Bok
globules with and without signs of current star formation was searched for
CCS(2-1) emission, the observations were complemented with NH3 measurements
available in the literature and 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. The NH3/CCS ratio could be assessed for
18 Bok globules and is found to be moderately high and roughly similar across
all evolutionary stages from starless and prestellar cores towards internally
heated cores harbouring protostars of Class 0, Class I or later. Bok globules
with extremely high CCS abundance analogous to carbon-chain producing regions
in dark cloud cores are not found. The observed range of NH3/CCS hints towards
a relatively evolved chemical state of all observed Bok globules.
