As introduction and background information for the following sections, an overview will be given of the W 3 region. First the early detections and models will be noted, then the three individual sources will be discussed. A sketch of the region is shown in Fig. 1 (click here); it consists of the Giant Molecular Cloud (GMC) core containing IRS4 and IRS5, with a smaller core containing the W 3(OH) and W 3() clumps located to the south-east.
Figure 1: Middle: a high resolution IRAS map (prepared by P. Roelfsema, SRON Groningen) of the W 3 region. The condensation in the center is the core of the giant molecular cloud. The condensation in the lower left corner is the interface between the W 3 and the W 4 region. The core is shown in more detail in the cartoon at the top. The near-infrared sources IRS 2 to 7 and the extent of the dense material traced in the submillimeter continuum are indicated. Below a cartoon of the second condensation is shown, with the compact HII region W 3(OH) and the ``hot core'' W 3() indicated. The coordinates for the three sources studied in the survey are (B1950.0): IRS4 ; IRS5 ; W 3()
The W 3 sources have been popular research objects for many years. Since the discovery of its radio continuum radiation by Westerhout in 1958, W 3 has been the subject of many radio studies. Several HII regions were discovered (e.g. Wynn-Williams 1971; Harris & Wynn-Williams 1976; Colley 1980) and identified with the near-infrared sources of Wynn-Williams et al. (1972). While some of the infrared sources clearly coincide with ionizing radiation of their HII regions, others are hardly associated with any free-free continuum radiation at all. One of these sources (IRS5) has a very deep silicate absorption band at (Willner et al. 1982), as well as solid water and methanol bands (Allamandola et al. 1992). Not only is this a clear sign of a very young source, but it is also indicative of large amounts of dust and molecular gas in front of the luminous source. The far-infrared continuum of the W 3 core has been measured e.g. by Werner et al. (1980), Jaffe et al. (1984) and most recently by Ladd et al. (1993). It is now unambiguously clear that IRS5 is one of the most luminous sources, with an output of radiated primarily in the far-infrared. Submillimeter continuum studies (Richardson et al. 1989; Oldham et al. 1994; Ladd et al. 1993) show that the mass of the GMC is concentrated in the core and divided about equally among IRS5, IRS4 and a source 20 south of IRS4. The nature of the last source is somewhat enigmatic since it is not associated with any of the near-IR sources, and it will not be discussed further.
Early molecular line studies of the entire cloud (Dickel 1980; Dickel et al. 1980; Hayashi et al. 1989) showed that the extent of the GMC core is (2.7 pc ). Attempts were made to derive accurate column densities from the low-J CO, CS and HCN millimeter lines, but these studies suffer from large optical depths in the lines. The small velocity gradient present over the core has been interpreted as evidence for collapse (Dickel 1980), but recent higher resolution maps in optically thin lines by Tieftrunk et al. (1995) suggest rotation of the core as a whole in the sense of Galactic differential rotation.
Another result from the molecular line mapping is that at the interface region of the W 3 and W 4 clouds the density appears to be rising. At this interface, a second site of high-mass star-formation is located, the compact HII region W 3(OH). Close to W 3(OH) (), at the place of the water masers, Turner & Welch (1984) found a strong, compact object in their millimeter interferometer data, called W 3(). Both sources are embedded in a core of warm and dense molecular material (Mauersberger et al. 1988; Wilson et al. 1991), as are the infrared sources in the main W 3 core.
IRS4 is a luminous (near-)IR source (Ladd et al. 1993), associated with a concentration of molecular material and dust of (Oldham et al. 1994). The molecules found toward this source are simple species containing 2-4 atoms, with the exception of methanol (see Paper I), while the lines are narrow with a typical line width of around . As shown in Paper I, the cloud is dense () and warm (). Taken together, a picture of a relatively unperturbed, somewhat warm molecular cloud emerges. This is strengthened by the fact that, in contrast with other pre-main sequence objects, no clear outflow signature has been found (e.g. Hasegawa et al. 1994, HMMT hereafter; see also Sect. 5.1). Also, the characteristic or OH masers seen in other sites of star-formation are missing.
Close to the peak of the near-infrared radiation, a shell-like structure is found in the radio continuum by Colley (1980). Together with the high infrared luminosity, this has been interpreted in Paper I as a blister structure arising at the back-side of the cloud, indicating that IRS4 is a more evolved object of spectral type O9 (Colley 1980) which has already broken free from its parent cloud. This explanation is not generally accepted, since IRS4 could also be at the earliest evolutionary stages where the outflow has yet to emerge (Tieftrunk et al. 1995).
The most luminous source in the W 3 cloud core is IRS5. Its energy output has long been thought to be due to a single, young O-star heating its environment. Recently Claussen et al. (1994) showed the existence of several small radio-blobs at the position of IRS5. Each ``blob'' has the ionizing radiation comparable to that of an early-B star. Near-infrared observations of Megeath et al. (1996) show that there are infrared counterparts to at least 4 of these radio sources. Moreover, they provide evidence that these sources have masses larger than each. Together with these high-mass stars, a dense cluster of less massive stars has been detected in the IRS5 clump, giving a star-formation efficiency of more than 20%.
The activity in this region was traced by earlier observations of molecular lines. The CO, CS and HCN lines (Dickel 1980; Dickel et al. 1980; Hayashi et al. 1989; Mitchell et al. 1991, 1992; Choi et al. 1993, HMMT) all show broad wings characteristic of outflowing gas. This gas was also detected in the ro-vibrational CO lines seen in absorption toward IRS5 at near-infrared wavelengths by Mitchell et al. (1990, 1991). However, whereas the (sub-)millimeter emission lines are all centered at , the infrared lines show absorption ranging from -100 to . Mitchell et al. were able to derive temperatures and column densities for the absorbing components (see HMMT for the most recent values). In Paper I the outflowing gas was tentatively linked to the high temperature emission found toward IRS5. At the place where the outflow runs into the ambient medium the temperature is expected to rise. Subsequent high-temperature chemistry efficiently converts sulfur into sulfur dioxide.
The majority of the gas is, however, at lower temperatures (, Paper I) than the and at the same density as IRS4. It is this gas that is expected to be traced by most other molecular lines. In general, lines toward IRS5 have a typical width of , although some show wings due to the outflowing gas. Deep self-absorptions are seen in the optically thick lines of CO, indicating the presence of colder, lower density foreground gas.
Most earlier observations have concentrated on the very bright, compact HII region W 3(OH), mainly because the OH masers provide excellent tools to study kinematics (see Bloemhof et al. 1992 for an overview). Subsequent work at high spatial resolution showed that the source of the masers is much stronger in lines of molecules such as HCN, and (Wink et al. 1994; Mauersberger et al. 1988; Turner & Welch 1984). The line widths are typically , although the outflow can be seen in the CO lines. Using millimeter aperture synthesis, it was shown by Turner et al. (1994) and Wilner et al. (1995) that there is a very compact core (< 1'') of line and continuum radiation, which most likely hosts an early B-star. Turner & Welch already argued that the heating of the W 3() clump cannot be provided by W 3(OH), so that it must be a site of star-formation itself. This is confirmed by measurements of the spatial and kinematical changes in the maser positions by Alcolea et al. (1992) and Reid et al. (1995). Reid et al. also showed that synchrotron emission is coming from this clump and that it is likely that the jets, producing the synchrotron emission, drive the outflow from this source.
Three components of molecular gas can be distinguished in the direction of W 3(). The first is the warm and dense core in which both W 3() and W 3(OH) reside. On the basis of submillimeter continuum maps (Sandell 1995, private communication) the size of this core is estimated to be , larger than our JCMT beam. The second are the 5-10 condensations seen in the interferometer map of by Wink et al. (1994), and the third is the very dense clump of seen in by Wink et al. and in the continuum by Wilner et al. (1995). In general the emission lines occur at a of . Absorbing gas is found only in the direction of W 3(OH) but at a velocity of , indicating the presence of a less dense envelope surrounding the core. We will argue that most of our observed single-dish emission comes from the dense core surrounding W 3() and W 3(OH), and from condensations within it.
Although the three sources originate from the same parent molecular cloud, they have very different chemical characteristics (Paper I), and the question arises if this can be explained by different evolutionary stages.
Toward IRS5, there are clear signs of large amounts of molecular material frozen on the grains, and the overall kinetic temperature may still be rising with time. Toward W 3(), the large abundance of methanol provides evidence for a ``hot core'' chemistry, in which species like methanol and formaldehyde have recently evaporated from the grains and drive a complex organic chemistry. Toward IRS4, a quiescent chemistry with simple molecules was found. This is in agreement with the current scenarios of the evolution of gas and dust in high-mass star-forming regions. We therefore interpreted the phenomena seen in Paper I with an evolutionary sequence in which IRS5 is the youngest, IRS4 is the oldest and W 3() is somewhere in between. This hypothesis will be further investigated through the more detailed, complete set of observations presented here and through quantitative chemical models by Helmich et al. (1997).