The IRAM key-project: small-scale structure of pre-star-forming regions. I. Observational results
E. Falgarone, J. F. Panis, A. Heithausen, M. Perault, J. Stutzki, J. L. Puget, F. Bensch;
AaA, 1998, 331, 669
ABSTRACT:This paper presents the observational results of the first IRAM key-project and a straightforward interpretation of the most salient features of
the data.
The project is devoted to the analysis of the processes which
drive the dissipation of the non-thermal support of molecular clouds, a
mandatory step toward the formation of almost thermally supported cores.
The
selected fields therefore all contain a starless dense core of small internal
velocity dispersion.
The maps include the core (of size ~ 0.1 pc) or a fraction of
it, and extend over large areas of their environment (several arc minutes,
or several tenths of pc at the distance of the clouds, d<150pc).
Maps have
been completed in five transitions, ^{12}COJ=1-0e\ and (J=2-1),
^{13}COJ=1-0e\ and (J=2-1) and C^{18}OJ=1-0e, at high angular resolution (22'' and
11'' at low and high frequency respectively, with a sampling of 7.5'') and a
velocity resolution of 0.05km s^{-1}.
The spatial resolution of the high
frequency maps is ~ 1700 AU.
The data set, because of its size, the good
signal-to-noise ratio of the spectra and the multiplicity of the lines observed,
provides several new results, as follows: (1) there is little unresolved
structure left in the maps of 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 10km s^{-1}
pc(-1) , implying large accelerations never observed before at small scale in
non star-forming clouds, (2) the texture and velocity dispersion of the
gas bright in ^{12}CO and barely detected in ^{13}CO are significatively
different from those of the gas bright in the three isotopes.
The gas bright in
^{12}CO only exhibits filamentary structure with, in some cases, unresolved
transverse dimensions, and aspect ratios larger than ~ 5.
Its velocity
dispersion is much larger than that of the latter.
Unexpectedly, it is in the more
opaque transitions and in the gas component of larger velocity dispersion
that the smallest scale structure has been observed, (3) the dense cores are
not isolated structures but are connected, in space and velocity, to
another kind of filamentary structures, bright in ^{13}CO and C^{18}O, (4) the
brightness temperature ratio of the two lowest CO rotational transitions is
remarkably uniform: R(2-1/1-0)=0.65+/-0.15 for 80% of the data points in the
three fields, from the brightest to the weakest detected lines, across the
whole profiles and for both ^{12}CO and ^{13}CO isotopes, (5) the ^{13}CO
lines reach intensities as large as those of the ^{12}CO lines, though the
line profiles are in general neither flat-topped nor self-reversed.
>From these well defined spectral properties, we infer that the lines have to
form in very small cells, weakly coupled radiatively to one another,
optically thick in the ^{12}CO lines and that the line shapes are governed mostly
by the spatial and velocity dilution of the emitting cells in the
beam.
Under the simple assumption that the cells are statistically independent,
we estimate that they are smaller than ~ 200 AU with H_2 densities n_{H_2} ~
a few 10(3) cm^{-3} in the gas component barely detected in ^{13}CO, and
are up to two orders of magnitude denser in the component bright in the three
isotopes.
We also notice an anticorrelation between the intensities of the ^{13}CO
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.
KEYWORDS: turbulence, ism: structure, ism: clouds, ism: kinematics and dynamics, ism: molecules, radio lines: ism
PERSOKEY:turbulence, co, h2, submillimeter, ,
CODE: falgarone98