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Astron. Astrophys. 342, 233-256 (1999)
8. Conclusions
-
Molecular line, millimetre/submillimetre/radio continuum, and
mid-IR observations are reported of the opaque fingers which cross the
Eagle Nebula. These are examined, along with optical and near-IR data
to measure the physical environment within the fingers, and to examine
their potential for being future sites of star-formation.
-
The fingers are surprisingly warm when viewed in the CO J =
3-2 lines, with peak temperatures approaching 60 K. The lines are
however relatively narrow, and there is no compelling evidence for any
molecular outflow activity. The masses of the fingertip cores
determined from CO isotopomeric observations range from
10 to 60
, and they contain 55-80% of the total
mass of each of the fingers. The total mass contained in the three
fingers and the nearby extended material is
200
.
-
The velocity fields of the gas are complex and show that the
material is very clumpy. The best evidence for coherent velocity
structure is along the centre finger, where a velocity gradient
1.7 km s-
1 pc-1 is seen. There may be evidence for somewhat
larger velocity gradients, however these are difficult to untangle
from the clumpy structure of the gas.
-
The fingers contain a number of embedded submm continuum cores. The
most intense cores in each finger are found close to the tips of the
fingers. The continuum spectra of these cores suggest that they are
much cooler, 20 K, than the gas
temperatures measured by the CO observations.
-
Detailed modelling shows that the fingers should contain dense cold
cores surrounded by a sheath of warm molecular gas. These
characteristics prove to be an excellent match to the observational
data.
-
The radio and morphological evidence suggest that the UV radiation
field incident on the surfaces of the fingers is
1700 G
.
-
A simple thermal and chemical model of a finger structure was
developed to study their internal physical environment. This model
suggests that the fingers should contain cold
( 20 K) dense cores near their tips,
surrounded by a sheath of warm (
100 K) gas. The model predictions are consistent with all of the
available observations.
-
The fingers appear to have been formed after primordial dense
clumps in the original cloud (which formed NGC 6611) were irradiated
by the light of its own OB stars. These primordial clumps have
shielded material behind it from the photoevaporative dispersal of the
cloud, leading to the formation of the finger structures. The cores
appear to be at a very early stage of pre-protostellar development:
there are no embedded infrared sources or molecular outflows present.
The pressure inside the cores appears to be just less than that of the
surrounding gas, and the consequence is that the material in the cores
is being compressed by the external medium. It is well known that such
characteristics are those shared by the earliest stages of objects
popularly known as `protostars'. The cores in the tips of the Eagle
Nebula's fingers appear to have the characteristics which are expected
for objects in the very earliest stages of (pre-) protostellar
activity .
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998
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