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Astron. Astrophys. 357, L37-L40 (2000)

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3. Discussion

3.1. Spatial distribution

The high spatial resolution and dynamic range of our image allow us to obtain several new results: First, all OH maser spots seem to be well aligned along the parabolic ionisation front and the distribution of OH maser spots delineates the shape of the shocked gas. Second, we find that there is a strong OH maser cluster near the apex of the arc. The angular distance between the cluster and the continuum peak is [FORMULA], corresponding to a projected distance of [FORMULA] cm. The cluster lies on the symmetry axis, which passes through the peak or "head" of the cometary HII component (component C). There are 55 OH maser spots in the cluster including the strongest maser components in the right circularly polarized emission of the 1665 MHz transition and both right and left circular polarized emission of the 1667 MHz transition. The expanded view of the cluster near the apex (see the inserted boxes in Fig. 2) show possible filamentary or "sheet-like" structures in the shocked gas. In the 1667 MHz transition one of these structures appears as a long, thin series of maser features, but we did not detect a systematic velocity gradient as would be expected were this an edge-on, rotating, disk-like object. The thickness of the shocked molecular gas is [FORMULA] mas, or [FORMULA] cm for a source distance 3.8 kpc. Third, the OH maser spots along the arc tend to cluster in clumps rather than uniformly distributed; this is particularly distinct in the northern part of the arc. The separations of adjacent clusters are greater than 600 mas and less than 1500 mas. Speculations as to the origins of this clumping include fluid instabilities in the shocked gas shell (e.g., Gwinn 1994, Garcia-Segura & Franco 1996) and the separations between the clusters might be related to a scale of corrugations of C-shocks (Wardle 1990).

3.2. Magnetic field structure

Fig. 3 is a plot of the magnetic field strengths and orientations in the G34.3+0.2 HII region complex. Eight Zeeman pairs associated with HII region B exhibit an average field 4.2 mG. While seven of the eight pairs indicate a magnetic field pointed away from us, there is considerable variation in the strength of the magnetic field and, indeed, one Zeeman pair indicating a field oriented towards us. However, overall this region does contain a partially ordered magnetic field structure.

[FIGURE] Fig. 3. Structure of the magnetic field in the G34.3+0.2 region. The numbers near the Zeeman pairs show the magnetic field strength. The conversion from the velocity separation of the RCP and LCP components to the field strength assumed 0.590 and 0.354 km s-1 mG-1 for the 1665 and 1667 MHz transitions, respectively. Positive (negative) magnetic field values indicate that field points away (toward) from the observer.

Turning now to the cometary HII region masers, we note that the northern part of the arc has magnetic fields of -4 mG. However, at the vertex and in the southern part of the cometary HII region, magnetic fields are generally small ([FORMULA] mG) when measured. The bow shock model in its simplest form would suggest an axially symmetric field configuration. This is not indicated by the data. The velocity distributions in the southern and northern maser clumps are not uniform, nor are the velocities seen in recombination lines for the ionized component (see Paper I for a more complete discussion). This complex distribution of magnetic field strength and velocities suggests a significantly anisotropic medium.

3.3. Polarization in OH masers

Very high circular polarization in OH maser emission has been recognized in many star-forming regions (Davies et al.  1966; Moran et al.  1978; Reid et al.  1980: Garcia-Barreto et al.  1988). Polarization in OH masers is caused by the presence of a directed magnetic field. The Zeeman effect splits the OH spectral lines, producing pairs of right and left circularly polarized ([FORMULA]-component) lines. In a small number of sources one also detects linearly polarized [FORMULA]-components.

For the best studied source W3(OH), Garcia-Barreto et al. (1988) found 65 VLBI maser spots ([FORMULA]% of the total number) which were nearly 100% circularly polarized. However, only five pairs of oppositely circular polarized spots were detected within a small fraction of the spot sizes. The small number of Zeeman pairs detected could be due to gradients in velocity and magnetic field strength over the amplification length, which shift the line frequency by an amount comparable to the maser line width (see Cook 1966; Moran et al. 1978). Alternatively, maser amplification is non-linear, and small changes in the magnetic sub-level populations can lead to significantly different amplification for one component of a Zeeman pair compared to the other component.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000