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).