Chemical and physical small-scale structure in a pre-stellar core

Aims. We present a comparative study of several molecular lines and of the dust contiunuum at 1.2mm in a pre-stellar core that is embedded in the Galactic cirrus cloud MCLD123.5+24.9. Previous studies found that the core is gravitationally stable and shows signs of inward motion. Methods. Using the Owens Valley (OVRO) and Plateau de Bure (PdB) interferometers we obtained high-angular resolution maps of the core in the carbon monosulfide CS (2 -> 1) and the cyanoacetylene HC(3)N (10 -> 9) transitions. Together with CS (5 -> 4), C(34)S (3 -> 2), and bolometer data obtained with the IRAM 30 m telescope, we analyse the excitation conditions and the structural properties of the cloud. Results. On the one hand, the new CS (J = 2 -> 1) observations reveal significant substructure on a scale of about 7 '', i.e., the beam size, corresponding to about 1050 AU at an adopted distance of 150 pc. On the other hand, the interferometric observations in the HC(3)N (J = 10 -> 9) transition shows just one single well resolved clump in the inner part of the core. This core is well described by an intensity profile following from a centrally peaked volume density distribution. We find no evidence for depletion of CS onto dust grains. The inward motion seen in the CS (2 -> 1) occurs one-sided from the middle of the filamentary cloud towards the HC(3)N core.