2005A&A...431..253O

We have produced wide-field (> 0.1 deg²) images of the molecular gas around the dense cores observed by Ladd et al. (1994ApJ...433..117L) in the Perseus cloud complex in various CO (CO(1-0), ¹³CO(1-0), C¹⁸O(1-0)) and CS (CS(2-1), C³⁴S(2-1)) isotopomers, and N₂H⁺(1-0), using the 16-element focal plane array operating at a wavelength of 3mm at the Five College Radio Astronomy Observatory. We also performed mosaic observations in the N₂H⁺(1-0) line and in the adjacent 3mm continuum with the OVRO interferometer. Only within one of the observed cores we unambiguously detected a 3mm continuum compact source with the interferometer. The single-dish large-scale maps of the densest gas, which in Perseus is concentrated within two large filamentary structures roughly aligned along a NE-SW axis, allowed us to analyse the spatial and kinematical properties of the cores and of the surrounding ambient gas. In the PER4/PER5 and PER7 regions we find that the large-scale and core velocity gradients have the same sign and similar magnitudes. In at least three cases we then find pairs of nearby cores with differences in the CS and N₂H⁺ emission and in the line profile, which may have been caused by evolutionary effects. The small fraction of cores with compact continuum sources, the chemical differentiation and the inward motions observed suggest that we are observing objects in a phase preceeding the collapse and the formation of stars.