The framework of the IRAM key-project has allowed us to build a unique data set on the environment of quiescent low mass dense cores, because of the multiplicity of the lines observed simultaneously, because of the large size of the maps with respect to the resolution, and because of the good signal-to-noise ratio of the spectra, despite the moderate strength of the lines. The scientific goals of this project were manyfold. One of our primary objective was to determine the dynamical characteristics of the turbulent gas which is the placental medium of thermally supported dense cores, and more specifically to study the transition region over which the dissipation of the non-thermal kinetic energy takes place. Our maps were not centered on some bright source at large scale but on weakly CO emitting regions containing low mass condensations of dense gas, with little amount of non-thermal support. A thorough interpretation of the data is beyond the scope of this first paper and we have focused here on the presentation of the salient features of the observational results and on a straightforward interpretation of these results.
Despite their low average column density at the parsec scale, all the fields appear highly structured in space and velocity. Maps of integrated line emission barely exhibit unresolved structures, but channel maps do so. lines, in particular, reveal a remarkable filamentary structure with, in some cases, unresolved transverse sizes ( ), and aspect ratios . These filaments have a much larger velocity coverage than the gas bright in or . Interpreted as velocity gradients, this velocity coverage corresponds to gradients as large as 10 km s-1 pc-1 across the structures. Unexpectedly, the quiescent dense cores are surrounded by a gas component which exhibits quite large accelerations.
The uniformity of the brightness temperature ratio of the two lowest CO rotational transitions, in the three fields, from the brightest to the weakest detected lines, across the entire profiles and for both and isotopes, is remarkable. 80% of the data points fall within the range R(2-1/1-0)=0.65 0.15. Deviations from this general behaviour are also visible. In the Polaris and L1512 fields, the line profiles may be decomposed into a line-core and a line-wing component. The line-wing component is bright in but barely detected in while in the line-core component the lines reach temperatures as large as those of . The spatial structure of the line-wing component is quite different from that of the line-core component although both are present along most lines of sight. It is the gas component which emits in the line-wings of the emission and has the broadest velocity coverage which exhibits the highest level of observed small scale structure. The line-wing emission is characterized by a line ratio R(2-1/1-0) 0.6, constant down to the weakest line intensities while in the line-core emission the line ratio R(2-1/1-0) increases from 0.65 to 0.8 with the line temperature. Last, in spite of the low isotopic line ratio observed in the line-core emission, line profiles are neither flat topped nor self-reversed, except in the L134A field.
We interpret these well-defined properties as a signature that the lines form in macroturbulent conditions, i.e. emission arises in phase-space cells with little radiative coupling with one another. Under the simple assumption that the cells in a beam are statistically independent, we infer an upper limit to their size, 200 AU. A preliminary analysis of the line properties (isotopic line ratios and rotational line ratios) in the framework of macroturbulence, suggests cell densities a few , for the line-wing emission and up to 100 times larger in the line-core emission. We find that, in the Polaris field which has the largest velocity dispersion, the and line intensities increase as their linewidth decreases. It may be interpreted as an increase of the radiative coupling of the cells as the cell to cell velocity gradually drops along a well-defined filament. In the L1512 field, the and lines are yet brighter and narrower, and we speculate that the dissipation process there has already proceeded further.
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998