  We report on high-angular resolution observations of a dense core in the
cirrus cloud MCLD123.5+24.9 obtained with the PdB and the OVRO interferometers.
Our maps show substructures down to the scale of the beam (~ 1000 AU). The
chosen molecules CS(2-1) and HC_3N(10-9) trace different regions in the core.
This can be explained by time-dependent chemical evolution of the cloud and
therefore provides constraints on the timescales of the fragmentation and core
formation processes. Our data demonstrate that the chemical evolution plays the
decisive role in our attempt to interpret observational data of different gas
density tracers.
