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Astron. Astrophys. 332, L5-L8 (1998)

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2. Observations of the Rosette filaments

We have secured a 30 min B -band exposure of a Rosette elephant trunk (Fig. 1a), centered at [FORMULA], [FORMULA], 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 [FORMULA]. 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 [FORMULA] was extracted against the background intensity of about [FORMULA] per [FORMULA] 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] Fig. 2. Extinction map of the NOT field where the gradient of the bright background has been compensated for. The contours show [FORMULA] = 0.25, 0.5, 0.75, and 1.25 mag .
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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998