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

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1. Introduction

The archetypical "cometary" HII region G34.3+0.2, first characterized by Reid & Ho (1985), presents a prominent system with a bright "coma" and a parabolic-shaped ionisation front with an extremely sharp "upstream" edge and a weaker tail extending for about [FORMULA]. The cometary morphology of some UC H II regions can be explained in a variety of ways. Initially, Reid and Ho suggested that the cometary morphology could be caused by the relative motion of the exciting star and the surrounding molecular material. In particular, they noted that a large expanding shell of gas associated with the stellar precursor to the W44 supernova might supply a significant component of the relative motion. Detailed bow-shock models have been elaborated by Van Buren et al. (1990) and Mac Low et al. (1991). These authors postulate that for a cometary UC H II region the ionised and molecular gas approach each other with a speed of order 10 [FORMULA] and their interaction forms a molecular bow shock supported by the wind from the ionising star(s). Alternatively, other authors suggested a "champagne flow" model (Tenorio-Tagle 1979; Bodenheimer, Tenorio-Tagle, & Yorke 1979; Tenorio-Tagle, Yorke & Bodenheimer 1979; Bedijn & Tenorio-Tagle 1981; and Yorke, Tenorio-Tagle & Bodenheimer 1983). In this model, an UC H II region is embedded in a molecular cloud with a large pressure gradient, and the ionised material expands rapidly in the direction of lower pressure. Finally, Gaume et al. (1994) suggest that the structure of the cometary HII region in G34.3+0.2 may be affected by ionised outflows from two other stars, G34.3+0.2A and B, embedded in the complex.

In a previous paper (Zheng, Moran & Reid 2000, hereafter Paper I), we discussed the distribution in space and velocity of masers in an "arc" paralleling the head of the cometary HII region based on earlier VLBI observations. Since magnetic fields play an extremely important role in star forming regions and may affect the dynamical properties of the region G34.3+0.2, we re-observed this source with the Very Long Baseline Array (VLBA) to map the polarized OH emission with sufficient angular resolution of ([FORMULA] mas) to resolve most maser clusters. These observations yielded a total of 120 maser features, from which we found 15 Zeeman pairs with the position differences between oppositely polarized masing features of less than 2 mas. This letter presents the magnetic field structure of two HII regions in the G34.3+0.2 complex.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000