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

Abstract

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$\to$1) and the cyanoacetylene HC 3 N (10$\to$9) transitions. Together with CS (5$\to$4), C 34 S (3$\to$2), and bolometer data obtained with the IRAM 30 m telescope, we analyse the excitation conditions and the structural properties of the cloud. Results. On the one hand, the new CS (J = 2$\to$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$\to$9) transition shows just one single well resolved clump in the inner part of the core. This core is well described by an intensity profile following from a centrally peaked volume density distribution. We find no evidence for depletion of CS onto dust grains. The inward motion seen in the CS (2$\to$1) occurs one-sided from the middle of the filamentary cloud towards the HC 3 N core.