Astron. Astrophys. 336, 991-1006 (1998)

1. Introduction

The W 3 giant molecular cloud complex (GMC), situated in the Perseus arm at a distance of kpc, contains several spectacular regions of high and low-mass star formation. The total mass of the W 3 GMC has been estimated to be   (Lada et al. 1978). Of that mass, 70% is found in a layer of strong ¹²CO and ¹³CO emission which is commonly referred to as the high density layer (hereafter HDL). Since the HDL is parallel to the edge of the W4 H II region, Lada et al. (1978) proposed that the HDL had formed from molecular gas swept up by the expanding W4 H II region. Thronson et al. (1985) found velocity shifts between the HDL and the adjacent lower density molecular gas and interpreted this velocity shift as the kinematical signature of the shock wave responsible for sweeping up gas into the HDL.

All known sites of ongoing high-mass star formation in the W 3 GMC are found in the HDL (Thronson et al. 1985). Two of these regions, W 3 Main and W 3(OH), have been the target of extensive observational studies. W 3 Main has received considerable attention due to the cluster of H II regions embedded within this cloud, which appears to be ionized by a recently formed association of O and B stars (Harris & Wynn-Wiliams 1976, Colley 1980, Roelfsema & Goss 1991, Tieftrunk et al. 1997). The sizes of the H II regions range from 0.01 pc to 1 pc. The smallest of these H II regions, with diameters of AU [hereafter referred to as hypercompact H II regions (Tieftrunk et al. 1997)], are associated with the infrared source IRS 4 and the infrared double source IRS 5; these may be examples of the earliest directly detectable stage of high-mass star formation. A number of intense OH and H₂O maser sources (Wynn-Wiliams et al. 1974, Gaume & Mutel 1987, Claussen et al. 1994) and a powerful outflow (Bally & Lada 1983, Claussen et al. 1984, Mitchell et al. 1991, Mitchell et al. 1992, Choi et al. 1993, Hasegawa et al. 1994) are further evidence of ongoing star formation. Far-infrared and submillimeter continuum (Werner et al. 1980, Jaffe et al. 1983, Campbell et al. 1989, Ladd et al. 1993) and millimeter molecular line (Wright et al. 1984, Hayashi et al. 1989, Oldham et al. 1994, Dickel et al. 1996) studies of W 3 Main show two distinct dense molecular cores: south of IRS 4 in the west (W 3 West) and toward IRS 5 in the east (W 3 East). Recent high resolution observations show that the molecular gas in these cores is highly fragmented into dense clumps, including the dense star-forming molecular clumps toward IRS 4 and IRS 5 (Tieftrunk et al. 1995, Roberts et al. 1997). The stellar content of W 3 East has been examined through near-infrared imaging studies by Rayner et al. (1990) and Megeath et al. (1996), who detected a dense cluster of 80 - 240 low-mass stars.

The W 3(OH) region lies SE of W 3 Main. The region is named after the bright OH maser emission (see e.g. Reid et al. 1980, Norris & Booth 1981, Norris et al. 1982, Rickard et al. 1982) associated with a shell-like ultracompact H II region with an angular diameter of (Dreher & Welch 1981, Guilloteau et al. 1985). Strong methanol masers have also been detected toward the H II region (Menten et al. 1988, Menten et al. 1992). The most recent episode of high-mass star formation seems to have occurred in a molecular condensation east of the H II region (Turner & Welch 1984, Mauersberger et al. 1988, Wilson et al. 1993, Wink et al. 1994). This condensation contains a group of H₂O masers (Forster et al. 1977, Genzel et al. 1978) and is commonly referred to as W 3(H₂O). Embedded in W 3(H₂O) is the Turner-Welch (TW) object, a luminous millimeter continuum source which may be the high-mass equivalent of a "class 0 object" (Turner & Welch 1984, Wilner et al. 1995). Proper motions of the H₂O masers indicate a bipolar outflow centered on the TW object (Alcolea et al. 1992). Non-thermal continuum emission has also been detected near the TW object; this emission may arise in shocks due to the outflow (Reid et al. 1995). Near-infrared observations have shown a cluster of low-mass stars neighboring W 3(OH) and W 3(H₂O) (Rayner et al. 1990, Zinnecker et al. 1993). High resolution single dish and interferometric studies have shown that molecular emission is concentrated around W 3(H₂O) (Keto et al. 1987). A number of maps of OH, H₂CO and absorption lines toward W 3(OH) indicate that the dense molecular gas extends to the H II region (Guiloteau et al. 1983, Guilloteau et al. 1985, Reid et al. 1987, Dickel & Goss 1987, Keto et al. 1987, Mauersberger et al. 1988, Wilson et al. 1991, Wilson et al. 1993, Baudry et al. 1993, Wink et al. 1994). Although most recent studies of W 3(H₂O) have concentrated on increasingly higher spatial resolution (e.g. Wyrowski et al. 1997), there are indications that the dense gas may be much more extended. Mapping in by Zeng et al. (1984) indicated an extent of , and more recently, Wilson et al. (1993) found a plume of emission extending extending as far as northeast from W 3(H₂O).

Several studies have examined the extended low and moderate density gas toward the entire W 3 GMC (Lada et al. 1978, Dickel et al. 1980, Thronson et al. 1985); however, the distribution of gas with densities in excess of cm^-3 is not well known. Since gravitationally bound clumps of dense gas, or dense cores, are thought to be sites of star formation in molecular clouds, surveys for such dense cores are critical to relating the properties of the molecular gas to the star-forming properties of the cloud. In an early study of the distribution of dense gas, Zeng et al. (1984) mapped a section of the HDL in including W 3 Main, W 3(OH), and the intervening gas. They detected emission only toward the W 3 Main and the W 3(OH) regions; however, Zeng et al. (1984) had a limited sensitivity and sparse sampling. Since these measurements the telescope efficiency of the 100-m telescope has improved, as has the receiver sensitivity. On this basis, we decided to obtain a new map of the W 3 HDL in 2 metastable inversion lines of with finer sampling and higher sensitivity than previous maps. We also present the results of a near-infrared survey of the region in the -band, and compare the distribution of stellar clusters identified in the -band survey with the distribution of dense cores. We also compare the distribution and mass of these cores with the distribution of the lower density gas toward the W 3 HDL, and with the distribution and mass of the dense CS cores associated with stellar clusters sur- veyed toward the Orion B cloud. Finally, we present evidence for possible variations of the relative abundance within the W 3 GMC.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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