Context :

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 ^ { \prime \prime } , 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 .

Conclusions :