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Report on the first IRAM Key Project: Small-Scale Structure of Pre-Star Forming Regions.

This project is devoted to the analysis of the processes which drive the dissipation of the supersonic non-thermal support of molecular clouds, known to be at the origin of their gravitational stability. One of the outcomes of this energy dissipation is expected to be the formation of thermally supported structures. The formation of dense cores, observed in molecular clouds with an internal velocity dispersion close to thermal, and often associated with extremely young stellar sources, is possibly triggered by the local dissipation of supersonic turbulence.

We have therefore mapped several fields of nearby clouds (d<150pc) which all contain a starless dense core of small internal velocity dispersion. The maps include the core (of size tex2html_wrap_inline1342 0.1 pc) or a fraction of it, and extend over a large area (several arc minutes, or several tenths of pc) of their environment, characterized by a non-thermal velocity dispersion. Maps have been completed in five transitions, tex2html_wrap_inline1344 J=1-0 and J=2-1, tex2html_wrap_inline1350 J=1-0 and J=2-1 and tex2html_wrap_inline1356 J=1-0, at high angular resolution (13'' and 24'' at high and low frequency respectively, with a sampling of 7.5'' which is Nyquist sampling for the low frequency maps) and a velocity resolution of 0.05 km s tex2html_wrap_inline1360 . The spatial resolution of the high frequency maps is therefore tex2html_wrap_inline1362 1000 AU.

Between December 1992 and March 1994, 624 hours were scheduled for this project. It was also scheduled as a backup project of the 345GHz receiver run of January 1994 (237 hours) but the efficiency of this period was quite low since only 39.7 hours were used. A total of 338 hours were spent in effective observations. 322 hours were lost for bad weather or zenith optical depth at 230GHz larger than 0.4, and technical reasons. But 92% of this time was lost for bad weather conditions: it means that the weather at Pico veleta degrades very fast and that the limit set on tex2html_wrap_inline1364 (230 GHz) was not a real constraint (either tex2html_wrap_inline1366 (230 GHz)<0.4 or tex2html_wrap_inline1370 (230 GHz)>>0.4). On the average, the time spent on the telescope has been twice the observing time, the additional overhead within this observing time due to calibration and tuning of the receivers being 30%.

The data base consists in 2.7 tex2html_wrap_inline1374 spectra. The final baseline rms noise level ranges between 0.6 and 1.2 K for the tex2html_wrap_inline1376 spectra, 0.5 and 0.9 K for the tex2html_wrap_inline1378 \ spectra, 0.3 and 0.5K for the tex2html_wrap_inline1380 J=1-0 spectra and 0.3 and 1K for the tex2html_wrap_inline1384 spectra.

The data set, because of its size, and the multiplicity of the lines observed, provides several new results, as follows:

(1) despite their low average column density at the parsec scale ( tex2html_wrap_inline1386 ), all the fields observed appear highly structured in all the lines, including those of tex2html_wrap_inline1388 . Unresolved structure is still present in all the fields and all the lines, in the dense cores and in their lower column density environment. It corresponds to size scales < 0.008 pc,

(2) the texture and velocity dispersion of the gas bright in tex2html_wrap_inline1392 \ and weak in tex2html_wrap_inline1394 and tex2html_wrap_inline1396 are significatively different from those of the gas bright in tex2html_wrap_inline1398 or tex2html_wrap_inline1400 . The former exhibits filamentary structure with, in some cases, unresolved transverse dimensions, and aspect ratios tex2html_wrap_inline1402 . Its velocity dispersion is also much larger than that of the latter,

(3) the uniformity of the excitation temperature ratio of the two lowest CO rotational transitions in the three fields, from the brightest to the weakest detected lines, across the whole profiles and for both tex2html_wrap_inline1404 and tex2html_wrap_inline1406 isotopes, is remarkable,

(4) the optical depths of the tex2html_wrap_inline1408 and tex2html_wrap_inline1410 lines reach very large values, though most of the line profiles are neither flat-topped nor self-reversed.

We deduce from these well-defined properties that the uniformity of the CO line excitation is a very robust property of non star-forming molecular clouds. The large range of optical depths and line temperatures over which it is met, imposes a line formation mechanism confined to regions smaller than a few 100 AU and denser than tex2html_wrap_inline1412 for a kinetic temperature T=20K, yet denser in colder gas. Furthermore, the line shapes impose a weak level of radiative coupling among these regions, a condition met in macroturbulence.

It is interesting to stress the outcomes of this program which are specific to its key-project status:

- on scientific grounds

The large size of the maps and the good signal-to-noise level in tex2html_wrap_inline1414 have allowed the discovery of a filamentary structure, hard to detect because it is small-scale structure in almost transparent gas. Furthermore such structures are simultaneously small (narrow width) and large scale (long length) and cannot be recognized in either small maps or large scale maps at low angular resolution. It is only the systematic mapping of low brightness and large areas which allowed the detection of such filamentary structures which are thought to play a role in the dense core formation.

Statistical work on the distribution of line centroid velocities and the power law relations is under progress. The statitical noise is considerably reduced due to the large size of the maps.

The signal to noise achieved was a compromise between the dynamic range of the maps and the total time devoted to the project. It is good enough to allow some work to be done on the line shapes and line ratios of individual spectra.

- on instrumental grounds

A long term monitoring of the time-dependent variations of the line calibration at the telescope has been achieved. We have taken advantage of the various time scales over which the monitoring was performed to correct for the non-thermal contribution to noise in the line calibration and our method reduces the line calibration error by a factor 1.5 at 2.6 mm and a factor 2 at 1.3 mm.

Six hardware and software problems have been discovered, in particular in the frequency settings, and have now been corrected.


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Next: Observation reports Up: IRAM Newsletter 27 (July Previous: Call for Observing Proposals