Astron. Astrophys. 318, 931-946 (1997)

1. Introduction

W3 is situated in the Perseus arm at a distance of 2.3 kpc (Georgelin & Georgelin 1976) and has a size of by (Brackman & Scoville 1980, Dickel et al. 1980), corresponding to a linear size of 2.6 pc by 1.9 pc. Intense OH and H₂ O maser emission toward W3 (Wynn-Williams et al. 1974, Forster et al. 1977, Gaume & Mutel 1987) and a powerful outflow observed from the very compact infrared source IRS 5 (Bally & Lada 1983, Claussen et al. 1984, Mitchell et al. 1991, 1992, Choi et al. 1993, Hasegawa et al. 1994) are indicative of ongoing star formation in the W3 core. Representatives of all morphological classes of H II regions (Wood & Churchwell 1989) can be found toward the W3 complex. The H II regions in W3 range in size from 0.01 pc to 1 pc in diameter: the cluster of hypercompact radio continuum regions, presumably a compact O/B association (Claussen et al. 1994) associated with the double-peaked infrared source IRS 5 (Megeath et al. 1995); the ultracompact cometary region W3 C showing evidence for new star formation toward IRS 4, the compact H II region and H₂ O maser center W3 B, the well-developed shell-like H II region W3 A (Hayward et al. 1989), and the diffuse and much more dispersed regions W3 K and J (Wynn-Williams 1971, Wink et al. 1975, Harris & Wynn-Williams 1976, Colley 1980). Extensive studies of radio recombination lines have allowed the determination of electron temperatures of this H II complex (e.g. Reifenstein et al. 1970, Wilson et al. 1979, Roelfsema & Goss 1991) suggesting strong temperature and density gradients and the presence of high density clumps within individual H II sources (van Gorkom 1980). Studies of the W3 IR and FIR continuum emissions (Wynn-Williams et al. 1972, Jaffe et al. 1983, Richardson et al. 1989, Ladd et al. 1993) suggest that a large fraction of the total luminosity may come from radio-quiet sources with strong submillimeter continuum emission. Studies of the compact molecular clouds of the W3 main core (in CO by Claussen et al. 1984, in HCN by Wright et al. 1984, in NH₃ by Zeng et al. 1984, and in H₂ CO,CH₃ OH,SO₂ by Helmich et al. 1994) have revealed dust-to-gas emission gradients as well as temperatures of the molecular clouds. Observations of these clouds with high angular resolution have revealed the clumpy structure of the dense molecular gas associated with the H II regions (in ¹³ CO by Hayashi et al. 1989, and Roberts et al. 1996, in C¹⁸ O by Oldham et al. 1994 and Roberts et al. 1996, and in C¹⁸ O,C³⁴ S by Tieftrunk et al. 1995) and radiative-transfer dust-cloud models have been presented for the compact clumps toward W3 IRS4 and IRS5 (Oldham et al. 1994, Campbell et al. 1995). In addition, the strength of the line-of-sight magnetic field toward the W3 core has been mapped by Roberts et al. (1993) using the H I line.

In this paper we present new results from sensitive high-resolution imaging of the radio continuum sources of W3, using multifrequency, multiconfiguration observations with the Very Large Array (VLA) ¹. Based on our data, obtained from observations with the best available resolution and sensitivity, we refine details of the interaction of the molecular environment with the associated H II regions. We present data of the hypercompact continuum regions and analyze their association with the very dense molecular clumps. From our observations of H66 recombination lines we analyze physical and dynamical characteristics of some of these H II regions and relate these to those of the neutral molecular gas and the submillimeter and infrared sources. We conclude with a section that looks at current models which propose to explain the observed morphology and kinematics of H II sources and the association of the different stages of young stellar evolution observed toward W3.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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