Spectral line surveys are a very powerful method to obtain a detailed
physical and chemical overview of star-forming regions. The
best-studied example is provided by the Orion-KL object, where various
surveys have revealed a very rich chemistry and significant changes in
physical and chemical conditions over small (
10
(0.02 pc))
scales (Sutton et al. 1995; Blake et al. 1984;
Sutton et al. 1985; Blake et al. 1986, 1987;
Jewell et al. 1989; Ziurys & McGonagle
1993; Turner 1991; Greaves & White 1991;
Groesbeck 1994). Another well studied object, which also shows a
remarkable number of lines and a rich chemistry, is the Sgr-B2 Giant
Molecular Cloud (GMC) (e.g., Cummins et al. 1986; Sutton et
al. 1991; Turner 1991; Hjalmarson & Bergman
1992). This region, however, has the disadvantage of its
large distance and its location in the southern hemisphere. Sgr-B2 is
close to the Galactic Center and thus 18-19 times farther away than
Orion, so that the linear resolution in single-dish observations is much
lower. For distant high-mass star-forming regions like W 49A the
situation is even worse.
With the availability of large-aperture submillimeter telescopes, the more recent line surveys have shifted to the higher frequency atmospheric windows. The main advantage over the earlier lower frequency data is that only the warmer and denser gas is sampled by the higher excitation lines, so that there is less confusion with the colder, more extended molecular cloud material. Also, the beam sizes are smaller, only 15-20'', compared with >1' in the early work, so that the observations are much more sensitive to the chemistry on the smallest scales.
The 345 GHz window is a good region in which to perform such surveys, because of its high frequency coupled with good atmospheric transmission. The number of completed projects in this window has recently increased considerably and includes a number of high-mass star-forming objects such as Orion-KL, Orion-S, G34.3 and NGC 6334 (Groesbeck 1994; Sutton et al. 1995; Schilke et al. 1996; Macdonald et al. 1996; McCutcheon et al., in preparation). In addition, more selected settings covering about half the window have been performed for lower-mass objects like IRAS 16293-2422 (van Dishoeck et al. 1995; Blake et al. 1994). Due to the improved sensitivity of receivers and stability of backends it is now possible to do such surveys almost routinely down to low noise levels. Because a large frequency range is covered, these data allow a fairly complete census of the molecules present in the gas, especially of the heavier linear and asymmetric rotor molecules. The surveys automatically cover the optically thin lines of the rarer isotopomers and often contain several transitions of the molecule, so that both the excitation and the column densities of these species can be determined accurately. Combination with lines from lower or higher frequencies can lead to a detailed analysis of the physical parameters of the gas. The disadvantage of the well-studied Orion-KL and Sgr-B2 objects is that the line crowding is so large that an easy identification of the lines is often not possible, and that the contribution of different lines to a blend is hard to estimate, especially in double side-band spectra. CLEANing or maximum entropy techniques are needed to extract the best information out of these spectra.
On the chemical modeling side, there has also been considerable advancement in recent years. The survey data have led to a better appreciation of the importance of gas-grain interactions in star-forming regions, since large abundances of complex organic molecules are difficult to form by ion-molecule gas-phase reactions alone. The preferred current picture is one in which molecules freeze out onto the grains during the cold collapse phase, and are released back into the gas phase (perhaps in modified form) after the star has formed due to radiative heating and shock disruption of the grains. The evaporated molecules subsequently drive a rapid gas-phase chemistry leading to complex organic molecules for a limited amount of time (Blake et al. 1987; Millar et al. 1991; Charnley et al. 1992; Caselli et al. 1993; Shalabiea & Greenberg 1994). These models have been tested against millimeter observations of gas-phase species. Unfortunately, no information on the composition of the ice mantles for the same lines of sight is available.
We present here a 345 GHz line survey obtained with the JCMT of three
sources in the W 3 Giant Molecular Cloud: IRS4, IRS5 and W 3(
).
Our main motivation for choosing these objects stems from the fact that
IRS5 and IRS4 are sufficiently bright at near- and mid-infrared
wavelengths to permit ground-based and ISO absorption line observations
of solid state features. Thus, information on the chemical composition
of both the gas phase and the ices will be available for the first
time to constrain the models. A second motivation is that the three
sources originate from the same parent cloud, and are observed in
similar detail with the same telescope. Thus, evolutionary effects can
be studied much more accurately. Another advantage is that although W 3
is five times farther away than Orion (at 2.3 kpc; Georgelin &
Georgelin 1976), it is still much closer than Sgr-B2, which is almost
twenty times more distant than Orion. The 
JCMT beam
corresponds to 
pc, keeping the linear scales within
reasonable bounds. Finally, the line blending for these sources is not
as severe as in Orion, allowing easier identification and line fitting.
The first results of this project were presented in Helmich et al.
(1994); Paper I hereafter). The analysis of this survey
actually goes one step further than that of previous surveys.
Specifically, the detailed excitation processes of each molecule are
considered, rather than just its excitation temperature. Thus
information on the kinetic temperature, density and source size for each
species is obtained. Observations of a few molecules (
, 
and 
) showed that the three sources have very different chemical
and physical characteristics. Helmich et al. tentatively linked these
to the evolutionary stage of the regions. In this paper, the work of
Paper I is extended, and all molecules detected toward the three sources
are discussed and analyzed. These data support the original conclusions
of Paper I, although specific questions about the evolutionary state of
the objects remain. Part of the survey data toward W 3 IRS5 have also
been analyzed by de Boisanger et al. 1996) to determine the
ionization fraction of this source. The observations and analysis of
the HDO lines are presented separately in Helmich et al. (1996).
The paper is divided as follows. In Sect. 2, a log of the observations and technical details are given. In Sect. 3, background information on the three sources is presented. In Sect. 4, the method of analysis is presented, and the results are analyzed per molecule in Sect. 5. Sect. 6 discusses the general trends, whereas conclusions are given in Sect. 7. The tables and figures with the observed lines are found at the end of the paper.