The material surrounding such embedded young stars, presumably related to the dense condensation existing prior to star formation, is confined to a very small region, of the order of a few seconds of arc (0.1 pc typically), and its presence may be overshadowed by larger structures. The precise problem depends on the observing technique: (i) in the radio continuum by the extended HII regions generated by older generation massive stars; (ii) in molecular lines by larger scale low density (but optically thick) molecular clouds; and finally (iii) in the near and far IR by radiation of more luminous nearby stars in the same complex. Hence, high resolution and selective molecular tracers are needed to distinguish the emission from the environment of the youngest stars from that emanating from the surrounding medium.
We believe that H2O masers
(of star forming type, to distinguish them from those associated
with late type stars) represent an
excellent tracer of the earliest stages of star formation.
The advantage of using H2O masers as tracers is twofold: firstly
their positions are known with very high accuracy (0.1
)
and hence one can limit the region to be studied to a small
field around the masers.
Secondly, several observations
suggest that H2O masers are closely related to
the earliest high density phases of the formation of massive stars,
prior to the development of an ultracompact (UC) H II region
(Tofani et al. 1995 hereafter TFTH;
Hofner & Churchwell 1996;
Cesaroni et al. 1997a; Cesaroni et al. 1997b;
Testi et al. 1998;
Plume et al. 1992; Plume et al. 1997;
Codella et al. 1997).
H2O masers of star forming type have been
found in the surroundings of HII regions (Wood & Churchwell
1989a;
Churchwell et al. 1990).
However, it is becoming more and more evident that they are not
associated with the diffuse ionised gas of evolved
HII regions, but with dense molecular clumps present in the same area,
which in some cases contain embedded UC HII regions
(e.g. Cesaroni et al. 1991). Recently, observations with high
spatial resolution () have shown that
there are several cases where no UC HII region
is associated with the H2O masers (Hunter et al. 1994; TFTH).
Considering that an H2O maser requires a neighbouring stellar
source in order to excite it (Elitzur et al. 1989),
the lack of a compact radio
continuum source suggests that the embedded massive star
is not capable of exciting an observable UC HII region.
This might be because the UC HII region
is so optically thick and highly self-absorbed to be undetectable at
radio wavelengths and thus very dense and very young.
The complex scenario that is emerging from more detailed studies of selected star forming regions suggests that different and probably independent episodes of star formation, each with its own HII region, may have occurred in the same complex and that H2O masers are associated with the most recent episodes (Hunter et al. 1994; Felli et al. 1997; Schreyer et al. 1997).
At the same time, in several objects, the H2O maser
spots show an excellent positional agreement with molecular clumps revealed in
several transitions and with far IR or sub-mm continuum
peaks (i.e. dust), but not with radio continuum peaks.
This situation is for instance
seen in W3(OH) (Turner & Welch 1984; Wink et al. 1994;
Wyrowski et al. 1997) where the
molecular emission comes from the position of the H2O maser,
which is offset by (0.06 pc)
from a strong UC HII region.
In Orion itself, the H2O masers are not associated with the
diffuse gas ionised by the Trapezium Cluster, but with the
molecular peak in the BN/KL region and with a very weak
unresolved radio source (component I: Churchwell et al. 1987;
Felli et al. 1993; Gaume et al. 1998).
VLA observations in the (4, 4) inversion transition of ammonia towards other UC HII regions (Cesaroni et al. 1994) lead to similar conclusions: both H2O masers and NH3(4, 4) emission arise from the same position, well apart from continuum peaks. Cesaroni et al. (1994) conclude that the H2O - NH3 clumps are likely to be the site of massive star formation. Methyl cyanide seems to trace the same high density clumps (Olmi et al. 1993, 1996, 1996b) and one concludes that these are examples of the "hot core'' phenomenon, where the presence of a nearby luminous star causes the evaporation of dust grain ice mantles in the surrounding medium and consequently enhances the abundance of several molecular species (see Millar 1997 and Ohishi 1997).
![]() a Maser component with higher peak flux from TFTH. |
With this in mind, we have selected a sample of 12 H2O maser sources
that radio continuum
observations with high spatial resolution and sensitivity (TFTH)
have shown to be well separated () from the closest HII region.
A few of these objects have already been observed in some molecular lines
(see Krügel et al. 1987; Serabyn et al. 1993) and most of them
have been detected in the CS(7-6) and CO(3-2) transitions
(Plume et al. 1992, 1997).
Our goal is that of obtaining a full picture
with high single dish resolution (
)
of the molecular environment of the water masers.
For this purpose, we have used IRAM 30-m observations of
the C34S(2-1), (3-2) and (5-4) transitions to estimate
the size and density of the associated molecular clump
(cf. Cesaroni et al. 1991). We have also observed the HCO+(1-0) and
HCN(1-0) lines with the aim of studying the dynamics and ionisation degree
of the molecular gas. Note that the HCO+(1-0) line
may be a good tracer for infall (see e.g. Welch et al. 1987 and
Rudolph et al. 1993), as well as of outflow
(Cesaroni et al. 1997b).
Observations of the CH3OH(3-2) and (5-4) transitions and CH3CN
(8-7) and (12-11) transitions
were also carried out with the aim of detecting the "hot core'' if
present, as well as to determine the clump density and temperature.
Methyl cyanide and methanol have advantages from this point of view
as one can observe several transitions of differing excitation within
one bandwidth. The relative intensities of these lines are thus
essentially independent of calibration errors. Moreover,
we have available LVG statistical equilibrium codes
which permit us to interpret the results (see e.g.
Bachiller et al.
1998; Walmsley 1987;
Olmi et al. 1993)
Finally, 13CO(2-1) and CS(3-2) were also mapped,
in order to obtain
information on the larger scale
molecular gas distribution.
Our survey does indeed show that all our targets are associated with molecular clumps centred on the H2O maser spots. In this respect, our results are analogous to those of Plume et al. (1992, 1997), who searched for high density molecular tracers in several transitions of CS and C34S towards a large number of H2O masers. The essential difference is that we have carried out a more detailed mapping of a restricted number of objects, whereas Plume et al. merely observed towards the water maser position.
Some of the results presented here have been covered in earlier publications but, for completeness, here we present the whole sample observed with the 30-m here. Partial results for S235A-B were reported in Felli et al. (1997). A detailed analysis of the survey observations for IRAS20126+4104 as well as comparison with Plateau de Bure interferometric observations and near IR observations were presented by Cesaroni et al. (1997a). These two objects are fairly typical of the complete sample.
In Sect. 2 of this article, we summarise the properties of the sample and give the water maser coordinates from TFTH. In Sect. 3, we explain the procedures used during the observations and data reduction. In Sect. 4, we present the data giving both line parameters towards the central positions and maps. We also discuss the properties of individual sources in Sect. 5. Finally, in Sect. 6, we summarise our conclusions.
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