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Astron. Astrophys. 332, L5-L8 (1998)
2. Observations of the Rosette filaments
We have secured a 30 min B -band exposure of a Rosette elephant
trunk (Fig. 1a), centered at ,
, epoch 2000.0, with the 2.6 m Nordic Optical
Telescope (NOT), La Palma, Spain. The effective scale of the utilized
TEK1024 CCD-detector is 0.176 arcsec per pixel. The resolution, as
measured from stars in the field, is 1.0 arcsec , corresponding to
1600 au at the adopted distance. It is to be noted that this is much
better than the resolution normally available for the study of
molecular clouds. The reason for this is that the fairly homogeneous
background illumination from the Rosette nebula provides a rare
opportunity to study internal structure of a dark cloud with a
resolution determined by the telescope resolving power, not the
density of background stars. The dark trunks, seen in Fig. 1b, seem to
form an interconnected structure. Notice that the orientation of the
trunks varies from running tangentially around the center of the
nebula to being oriented radially towards it.
The elephant trunk shown in Fig. 1a is almost radially directed
with respect to NGC 2244 and has a thickness of about one arcmin
which at the adopted distance corresponds to .
The NOT-image resolves the cylindrical trunk into a great number of
filaments, many with sinusoidal or serpentine-like structures. They
seem to be preferentially situated in the outer parts of the trunk,
where their wavelength is about 7-9 times the trunk radius. In the
darker southern regions of the trunk the sinusoidal structure is
complemented by knots. At some locations braids formed by thin
subfilaments can be seen.
Most of the outer filaments seem to form helices around the trunk
axis. The observed increase in absorption could correspond to
structures oriented along the line-of-sight, thereby contributing to a
larger extinction. At some positions in the southern part of the
trunk, filament crossings give rise to an enhanced extinction. We
conclude that the sinusoidal filaments are, in fact, projected
helices.
It is striking how the helicity appears on different scales; from
the majestic slow twist of the trunk as a whole, down to the braiding
of subfilaments with diameters of 1-4 arcsec (1600-6400 au). The
presence of subfilaments on scales down to the limit of resolution
could advertise even smaller entities.
The southern part of the trunk is notably dark. This is not due to
any increase in extinction, but rather a consequence of the decrease
in background illumination. Actually, the filaments display comparable
amounts of extinction throughout the trunk; the extinction
was extracted against the background intensity
of about per and is
shown in Fig. 2. Most filaments and knots have an extinction of
0.5-1.0 mag , but a few knots exceed 1.5 mag .
![[FIGURE]](img15.gif) |
Fig. 2. Extinction map of the NOT field where the gradient of the bright background has been compensated for. The contours show = 0.25, 0.5, 0.75, and 1.25 mag .
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© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998
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