A common feature of interstellar clouds of all sizes is that they are organized in substructures of filaments and clumps. Filaments can be remarkably straight, but there are also numerous examples of curved and wavy forms. Magnificent examples of long, twisted structures can be found on HST images of cloud regions, such as the Lagoon nebula (e.g. Caulet 1997). There is growing evidence that some filaments are lined up by helical magnetic fields (Shibata et al. 1991; Carlqvist & Gahm 1992; Hanawa et al. 1993; Lazarian 1993; Nakamura et al. 1995; Gehman et al. 1996; Nakajima & Hanawa 1996; Bally 1989; but see also Heiles 1997). Such fields are compatible with the presence of electric currents, which may be important for forming and maintaining the clouds (Alfvén & Carlqvist 1978; Carlqvist 1988; Verschuur 1995; Carlqvist & Kristen 1997).
Here we want to draw the attention to small-scale helical filaments and associated physical processes. We do so by focusing on optical observations of one of the dark elephant trunks seen in the Rosette nebula (NGC 2237-2246), an H II region at one end of a large molecular cloud (Williams et al. 1995) at a distance of 1600 pc (Pérez et al. 1987). The ionized gas expands at a velocity of about (Smith 1977; Fountain et al. 1979) from the central young star cluster NGC 2244. The prominent dark trunks and drop-shaped globules in the northern and north-western section of the Rosette nebula were noted on early photographs presented by Minkowski (1949) and Bok et al. (1949). These features were further discussed by e.g. Herbig (1974).
The contrast between dark and bright nebulosity makes it possible to distinguish filaments, with diameters down to the limit of resolution of 1 arcsec . The presence of helical structures, which appear on a much smaller scale than most of the molecular filaments referred to above, may have extensive consequences for the understanding of how elephant trunks form and more generally for the physical processes in the interstellar medium.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998