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

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Letter to the Editor

Helical structures in a Rosette elephant trunk *

Per Carlqvist 1, Helmuth Kristen 2 and Gösta F. Gahm 2

1 Royal Institute of Technology, Alfvén Laboratory, SE-100 44 Stockholm, Sweden
2 Stockholm Observatory, SE-133 36 Saltsjöbaden, Sweden

Received 2 December 1997 / Accepted 13 January 1998


We discuss small-scale, helical, interstellar filaments on the basis of optical observations of an elephant trunk in the Rosette nebula. The trunk studied is composed of a number of sinusoidal or serpentine-like dark filaments, preferentially in the outer part of the trunk, where their wavelength is 7-9 times the trunk radius. The diameters are down to the limit of resolution of 1.0 arcsec , corresponding to 1600 au , and ranging up to about 6400 au . At some positions filament crossings give rise to enhanced extinction. We suggest that the sinusoidal filaments are helices lined up by magnetic fields.

We derive average extinctions of 0.5-1.0 [FORMULA] in the filaments, implying molecular densities of [FORMULA]. From existing data on the Rosette H II  region, we conclude that the surrounding kinetic and dynamic pressure and the background radiation field suffice to balance even the denser filaments and to exert drag forces on the trunk as a whole, consistent with evidence of stretching of the trunk. The helical magnetic structures imply the presence of electric currents along the trunk axis. These currents should form a nearly force-free geometry and are consistent with a model consisting of 4-7 helical cables on the surface of a cylinder and which produce the observed wavelength of the helices.

We suggest that the Rosette elephant trunks form an interconnected system of rope-like structures which are relics from filamentary skeletons of magnetic fields in the primordial cloud.

Key words: ISM: clouds – ISM: H ii regions – ISM: individual objects: Rosette nebula – ISM: kinematics and dynamics – ISM: magnetic fields

* Based on observations collected at the Nordic Optical Telescope, La Palma, Spain

Send offprint requests to: Per Carlqvist

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998