2.2.3. Narrow Magnetized Features
2.2.3.1 NARROW MAGNETIZED ISOLATED FILAMENTS Nonthermal
(synchrotron) emitting filaments have been found near a wavelength of 92 cm
(Wieringa et al., 1993)
in the interstellar medium, with the Westerbork Synthesis Radio
Telescope in Holland. These filaments have a width
10% of their
length, with sharp edges.
They were found at a distance from Earth of ~ 0.2 kpc to 1 kpc,
with length ~ 10 pc and width ~ 1
pc. There is virtually no change in the angle of linear
polarization along the filaments. Some
may come in systems of almost parallel filaments. These
observations were interpreted as
small clouds of gas with gas density ~ 0.2 cm-3, concentrated
along the local magnetic field lines.
An intriguing 'lens' or filament has been found near the W 5 complex HII region, in continuum emission at a wavelength of 21 cm (e.g., Gray et al. 1998) with the Dominion Radio Synthesis Telescope near Penticton, B.C., Canada, being apparently unrelated to the thermal emission (the boundary of the HII region coincides with the diaappearance of the polarization structure). This 'lens' has a remarkably smooth structure in which the position angle of the linear polarization swings continuously from the center to the edge, possibly due to line-of-sight dependent Faraday rotation effects in a foreground screen.
Thermal (free-free) emitting filaments have been found near large dust clouds. In the Auriga filament (e.g., Puget 1991) the gas velocity changes slowly along the major axis of the filament, suggestive of a physical link such as a magnetic field to maintain the spatial correlation over a length of 1 pc. In the long (12 pc) thin (1 pc) filament near L204 (e.g., Heiles 1988b), a magnetic field was measured by the Zeeman effect to be near 12 µGauss.
2.2.3.2 NARROW MAGNETIZED OUTFLOWS FROM CLOUDS When two high-velocity (~ 100 km/s) outflows emerge in opposite directions from a recently-formed protostellar disk, these outflows push out a shell of molecular gas and magnetic field, often extending to ~ 1 pc, with a lateral width of ~ 0.1 pc. Magnetic stresses at the cavity's surface may affect the evolution of the outflows. Observationally, Simonetti and Cordes (1986a) have determined the RM of background galaxies and QSO near and through the outflows in 2 cases, deriving a magnetic field of ~ 100 µGauss in the outflow walls. Crutcher (1991) used the Zeeman effect to derive a magnetic field near ~ 1000 µGauss (toward the observer) in the north lobe of S106 and ~ 600 µGauss (away from the observer) in the south lobe - thus the magnetic field reverses direction from one lobe to the other in S106. A recent theoretical review of outflows is given in Königl and Ruden (1993).
2.2.3.3 NARROW MAGNETIZED EDGES OF DUSTY MOLECULAR CLOUDS (B ~ n1) A magnetized molecular cloud edge, near an adjacent HII region containing one or more O-type star(s), has been modeled as having successive compressed layers: first an atomic HI layer nearest the HII region, then a molecular H2 layer further inside the cloud,
In these HI/H2 layers, the carbon atoms are ionized into
CII. Radio observations of CII recombination lines (e.g., near
6cm) in cloud edges were
compared with observations of CO
lines (e.g., near 3mm)
deeper into the clouds, indicating a CII layer with the following
parameters: depth ~ 0.1 pc, surface side ~ 1 pc, total gas density
~ 105 cm-3, magnetic field ~
100 µGauss, a weak
compression factor between 2 and 10 due to the presence of magnetic
fields; and these shocked molecular cloud edges display the
B ~ n1.0 behavior (e.g., Table 2 in
Vallée 1989a).
Results were also obtained for the adjacent but deeper layer containing sulfur, by comparing the sulfur recombination lines SII with the CO lines from the cloud cores: depth ~ 1 pc, surface side ~ 1.5 pc, total gas density ~ 106 cm-3, magnetic field ~ 180 µGauss (e.g., Table 3 in Vallée 1989a; Table 4 in Vallée 1989b).
The linear polarization of the 21 cm HI line from the molecular cloud edge of Orion B was studied closely by van der Werf et al. (1993), using the Very Large Array in its D configuration. They found 3 absorbing layers in front of the continuum HII region, with the outermost (A) layer having a total gas density n ~ 103 cm-3 and a magnetic field B ~ 30 µGauss, the next one having a total gas density n ~ 104 cm-3 and magnetic field B ~ 40 µGauss, and the innermost one having a total gas density n ~ 105 cm-3 and a magnetic field B ~ 60 µGauss.