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

2. Observations and results

The observations were conducted on 3 June 1995 with the VLBA operated by NRAO.  ¹ We observed the OH emission in both the 1665 and 1667 MHz transitions, in both right and left circular polarization. A 250 kHz passband centered on  km s^-1 was divided into 256 spectral channels, providing a Doppler velocity spacing of 0.176 km s^-1.

The calibration procedures are similar to those discussed in Paper 1, except for some extra steps needed to register the maps in both transitions and polarizations. Prior to normal calibration steps, we corrected the data for feed orientation changes (i.e., parallactic angle rotation). A strong and simple point source at an LSR velocity of 58.8 km s^-1 in the 1667 MHz RCP spectrum was selected as a phase reference feature. Fringe-fitting was performed on this feature to determine the residual fringe phase as a function of time. This was applied to all spectral channels in both polarizations in transitions. The calibrated visibilities were Fourier transformed and deconvolved using the AIPS task IMAGR. A arcsec (E N) field was mapped with a pixel size of mas and the synthesized beam was mas at a position angle East of North. Individual channel maps had a noise level of about 10 mJy per beam. We identified maser features when they were detected over at least three adjacent spectral channels. For these features the position offsets relative to the reference feature, the FWHM and the peak intensity were fitted with a two-dimensional Gaussian brightness distribution.

Fig. 1 shows the spatial distribution of the identified OH maser features projected onto a contour plot of 1.3 cm continuum (Argon at al. 1999). Fig. 2a and 2b show the locations of the 1665 and 1667 MHz maser features, respectively. The appearance of the 1665 MHz maser features is similar to that observed by Gaume and Mutel (1987). These OH masers can be grouped as associated with two different newly formed stars: one group is clustered near the HII region source B in the north-east corner of the map near offset-coordinates (400,2000) mas. The source B cluster contains the strongest 1665 MHz masers in the entire region. This cluster is probably related to a young ionizing star that is physically distinct from the more energetic star that ionizes the cometary HII region. All other OH masers, including the 1667 MHz masers shown in Fig. 2b, are aligned along an arc that parallels the head of the cometary HII region. Maser spots in the northern section of the arc are less numerous but possibly better aligned with the limb-brightened parabolic shell than those in the southern part of the arc, which display a greater spread perpendicular to the arc. Masers in both sections may lie in a thin shell and the spread could result from projection effects.

Fig. 1. A map of the distribution of OH maser features superimposed on the 23 GHz continuum (Argon et al. 1999). The absolute alignment between masers and continuum is taken from the VLA study of Argon et al. The continuum contour levels are 6, 12, 17, 24, 34 and 48 mJy. Solid dots show the maser features in the 1665 and 1667 MHz transitions in both right and left circular polarization. The rectangles delineate regions which we mapped and found OH emission.

Fig. 2a and b. Maps of the relative positions of the OH maser features. (a ) The RCP and LCP components of the 1665 MHz transition. (b ) The RCP and LCP components of the 1667 MHz transition. The solid and dashed lines delineate the ionisation front and the contact discontinuity expected by the bow shock model. The asterisk indicates the position of the continuum peak.

The RCP and LCP components from a single physical condensation or cloudlet are usually referred to as a Zeeman pair (e.g., Moran et al.  1978, Reid et al.  1980, Garcia-Barreto et al.  1988, Baudry & Diamond 1996). However, one rarely finds Zeeman pairs where the RCP and LCP components coincide to a small fraction of the spot size. We examined all possible pairings of oppositely circularly polarized features and identified 15 Zeeman pairs which are listed in Table 1. The polarized masing components identified as Zeeman pairs comprise 25% of all masing features detected. Most of the 90 masing features which could not be identified as part of a Zeeman pair are located in the region close to the vertex of the cometary HII region. Possibly a combination of magnetic field and velocity gradients results in greater amplification of one component of a Zeeman pair compared to the other, resulting in the detection of only one component.

Table 1. Zeeman pairs in G34.3+0.2
Footnotes:
^a Position offsets in right ascension and declination
were determined by the Gaussian fitting in the amplitude peak channels.
^b The velocity of the amplitude peak of the features.
^c Angular distance between two features in a Zeeman pair.
^d Inferred magnetic field strength. The conversion from the velocity separation of the RCP and LCP components to magnetic field assumed 0.590 and 0.354 km s^-1 mG^-1 for the 1665 and 1667 transitions, respectively. Positive (negative) magnetic field values indicate that field points away (toward) from the observer.

The values in column 11 of Table 1 indicate the angular separations between the RCP and LCP components. These are among the best matched Zeeman pairs for any interstellar OH masers. The measurement uncertainty was typically less than 1 mas for the angular separations We required that the spatial separation of a Zeeman pair be less than  cm clustering scale found for W3OH by Reid et al.  (1980). For G34.3+0.2 at a distance of 3.8 kpc this corresponds to about 50 mas. However, the separations between RCP and LCP components average only 2 mas, or cm. The uncertainty in the velocity difference was typically less than about 0.2 km s^-1 resulting in a magnetic field error of less than about 0.5 mG.

The magnitude of the magnetic fields, calculated from the velocity separation of the Zeeman pairs, ranges from -7.8 mG to mG. The sign of the value indicates the line-of-sight direction of magnetic field, with positive sign corresponding to the RCP component having a higher velocity than the LCP component and the field pointing away from the observer. The vast majority of the Zeeman pairs associated with HII region B have positive magnetic field values. However, the pairs associated with the cometary HII region tend to have negative magnetic field values, further supporting the physical independence of these two sources.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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