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Astron. Astrophys. 344, 668-674 (1999)

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3. Results

The polarization results for the three sources are listed in Table 1, and the deduced magnetic field structures are discussed briefly here. The standard assumption is used, that the grains are magnetically aligned with the net field perpendicular to the observed polarization angle (but see also Sect. 5.1 below). Some information about the physical properties of the sources is also given, for context (Table 2).


Table 2. Physical properties of the cloud cores, from dust emission maps (Henning et al. 1992; Richardson et al. 1986; Oldham et al. 1994). The dust temperature and gas density are given by T and n(H2). For DR21, the second set of results uses an unpublished JCMT map at 850 [FORMULA], with the methods of Richardson et al. (1986) but a 15" beam instead of 40"; the luminosity is from Harvey et al. (1977). For Mon R2, the models include some cloud emission outside the cores (diameters [FORMULA] 0.1 pc), and the temperature is from NH3 data (Henning et al. 1992).

The Mon R2 cloud core, centred near the IRS1 source, has been mapped in 800 [FORMULA] polarization by Greaves et al. (1995). Our new data for IRS2 and IRS3 support their model. The field directions converge near the dust peak (see Fig. 1 in Greaves et al. 1995), consistent with field lines pulled in during core contraction. There is a significant difference in the [FORMULA] values at 800 and 1100 [FORMULA] for IRS3 which is difficult to explain under the normal assumptions (optically thin dust emission in the submillimetre, so the polarization at each wavelength traces exactly the same aligned grains). The origin of position angle variations is discussed further below.

DR21 has been mapped in 800 [FORMULA] polarization by Minchin & Murray (1994), and the field across the cloud core was found to be quite linear. The new p and [FORMULA] values show consistency between the two datasets. The mean field orientation is 116o, which is intermediate between that of the core major axis ([FORMULA] 170o) and the dominant outflow axis ([FORMULA]). There is as yet no detailed picture for the magnetic field in DR21, but a model for sources with no obvious field alignments has been presented by Minchin et al. (1996).

The W3 cloud has been mapped in 100 and 800 [FORMULA] polarimetry by Hildebrand et al. (1995) and Greaves et al. (1994). The 800 [FORMULA] data suggest an hourglass-shaped field centred on IRS5, and the larger-scale 100 [FORMULA] map indicates that the hourglass field axis lies roughly SE-NW. Near IRS4 (located about 1´ west of IRS5), there appears to be another local distortion in the field, but with a wave-like shape (Fig. 1). The field twists through a right angle at a position between IRS4 and the dust core IRS4S, 20" to the south (Oldham et al. 1994). Zeeman data show a changing gradient in the line-of-sight field strength near IRS4 (Roberts et al. 1993, their Fig. 4a), which also supports the idea of a distortion in the field.

[FIGURE] Fig. 1. 800 [FORMULA] continuum map of W3-IRS4 (from Oldham et al. 1994), with polarization vectors superimposed. The vector length and direction indicate the percentage polarization (see scale) and position angle. Data at 800 [FORMULA] (this work) are shown within circles representing the 14" beam size, and 100 [FORMULA] data (Hildebrand et al. 1995) are shown without circles (beam size of 35"). A suggested field morphology is also shown (dashed lines). IRS4 is located at the centre of the middle circle.

In W3, both the IRS4 and IRS5 sources have a polarization minimum of p(800) [FORMULA] 0.5%, compared to surrounding positions with p(800) [FORMULA] 1-3% (Greaves et al. 1994, and this paper). This effect is consistent with complex field structure at the star-formation sites, resulting in polarization directions that cancel out within the beam (Minchin & Murray 1994). Core formation has strongly affected the field around both IRS5 and IRS4/IRS4S, but the overall field geometries are seen to differ, even though the cores are physically similar (Table 2). This suggests that the fields are affected by the larger-scale environment, such as cloud internal motions or the presence of HII regions adjacent to IRS5 (Oldham et al. 1994).

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999