THE IRAM KEY-PROJECT - SMALL-SCALE STRUCTURE OF PRE-STAR-FORMING REGIONS - I - OBSERVATIONAL RESULTS

This paper presents the observational results of the first IRAM key-pr oject and a straightforward interpretation of the most salient feature s of the data. The project is devoted to the analysis of the processes which drive the dissipation of the non-thermal support of molecular c louds, a mandatory step toward the formation of almost thermally suppo rted cores. The selected fields therefore all contain a starless dense core of small internal velocity dispersion. The maps include the core (of size similar to 0.1 pc) Or a fraction of it, and extend over larg e areas of their environment (several are minutes, or several tenths o f pc at the distance of the clouds, d < 150 pc). Maps have been comple ted in five transitions, (CO)-C-12(J=1-0) and (J=2-1), (CO)-C-13(J=1-0 ) and (J=2-1) and (CO)-O-18(J=1-0), at high angular resolution (22 '' and 11 '' at low and high frequency respectively, with a sampling of 7 .5 '') and a velocity resolution of 0.05 km s(-1). The spatial resolut ion of the high frequency maps is similar to 1700 AU. The data set, be cause of its size, the good signal-to-noise ratio of the spectra and t he multiplicity of the lines observed, provides several new results, a s follows: (1) there is little unresolved structure left in the maps o f line integrated emission, but unresolved structure is still present in the channel maps of all the fields and all the lines. The velocity gradients involved reach values as large as 10 km s(-1) pc(-1), implyi ng large accelerations never observed before at small scale in non sta r-forming clouds, (2) the texture and velocity dispersion of the gas b right in (CO)-C-12 and barely detected in (CO)-C-13 are significativel y different from those of the gas bright in the three isotopes. The ga s bright in (CO)-C-12 only exhibits filamentary structure with, in som e cases, unresolved transverse dimensions, and aspect ratios larger th an similar to 5. Its velocity dispersion is much larger than that of t he latter. Unexpectedly, it is in the more opaque transitions and in t he gas component of larger velocity dispersion that the smallest scale structure has been observed, (3) the dense cores are not isolated str uctures but are connected, in space and velocity, to another kind of f ilamentary structures, bright in (CO)-C-13 and (CO)-O-18, (4) the brig htness temperature ratio of the two lowest CO rotational transitions i s remarkably uniform: R(2-1/1-0)=0.65+/-0.15 for 80% of the data point s in the three fields, from the brightest to the weakest detected line s, across the whole profiles and for both (CO)-C-12 and (CO)-C-13 isot opes, (5) the (CO)-C-13 lines reach intensities as large as those of t he (CO)-C-12 lines, though the line profiles are in general neither fl at-topped nor self-reversed. From these well defined spectral properti es, we infer that the lines have to form in very small cells, weakly c oupled radiatively to one another, optically thick in the (CO)-C-12 li nes and that the line shapes are governed mostly by the spatial and ve locity dilution of the emitting cells in the beam. Under the simple as sumption that the cells are statistically independent, we estimate tha t they are smaller than similar to 200 AU with H-2 densities n(H2) sim ilar to a few 10(3) cm(-3) in the gas component barely detected in (CO )-C-13, and are up to two orders of magnitude denser in the component bright in the three isotopes. We also notice an anticorrelation betwee n the intensities of the (CO)-C-13 lines and their linewidths which we interpret as a signature of a gradual loss of the non-thermal support which increases the phase-space radiative coupling of the cells.