Astron. Astrophys. 336, 991-1006 (1998)
5. Conclusions
We present and (2,2) maps sampled at a
beam spacing with a FWHM beamwidth of
( pc) over a 120
square-arcminute region toward the high density layer of the W 3
GMC. The map in- cludes the W 3 Main and W 3(OH)
star-forming regions. We find that strong
emission ( K km s-1 ) is
detected toward 9% of the surveyed region. It is only detected toward
regions of high-mass star formation, namely the W 3 Main and
W 3(OH) clouds, and toward a newly detected core
to the southeast of W 3 Main, which we
denote W 3 SE.
Toward W 3 West, we find cool, = 45
K, am- monia gas in a region of 1 pc diameter outlined by many other
molecular tracers. We find N( =
cm-2 and a virial mass of 1100
for this core. This yields an
relative abundance of X(
= , consistent with predictions based on
chemical models.
Toward W 3 East we find only weak
and (2,2) emission, but relatively strong (3,3) emission. From the
and lines, we
determine K and
N(H2) = cm-2 . The
resulting X( = is an
order of magnitude lower than the relative
abundance found toward W 3 West or the
relative abundance predicted by chemical models. Since
W 3 East is the most active star-forming core, we propose
that the lack of emission toward the stellar
cluster may be due to the enhanced rate of
destruction in an environment dominated by the energy input of the
emerging young stellar sources.
For the newly detected W 3 SE core, we find a
= 25 K, N(
cm-2 and a virial mass of
. W 3 SE has a
relative abundance of ,
similar to that of W 3 West. The core appears to be
marginally resolved in our data, suggesting a core size comparable to
the beamsize (0.5 pc). Star formation may be occurring inside of
this core as indicated by an elongated, jet-like nebulosity emanating
from W 3 SE apparent in our -band
images. Recent observations show that this nebula is tracing
vibrationally excited emission excited by UV
radiation or an outflow.
Toward W 3(OH) we find an extended and cool
component with = 27 K,
which is not substantially heated by the embedded young stars in
W 3(H2O) and the W 3(OH) ultracompact
H II region. The strong
emission is extended over an area of pc,
demonstrating that the W 3(OH) core is much larger than
previously thought. The core seems to be composed of a component
centered on the W 3(H2O)/W 3(OH) ultracompact
H II region and a plume which extends as far as
1 pc from the ultracompact H II region.
Toward the central component of W 3(OH), we calculate on average
N( = cm-2
and a virial mass of 1500 . We find
X( = for the cool
extended gas, an order of magnitude lower than than the
relative abundance predicted for the hot
central core in W 3(OH).
We find that the plume of ammonia reaching from W 3(OH) toward
the northeast is a much more extended structure than indicated by
previous VLA maps in Using the virial theorem,
we find that the plume holds a mass of at least 400
of molecular gas. We find that the plume extends
toward two clusters of stars, one of which is located at the edge of
the plume. These clusters may be previously unknown sites of star
formation associated with the W 3(OH) molecular cloud and merit
further investigation. The stellar clusters are found in the
westernmost component of a chain of nebulosities which extends
eastward toward the W4 H II region.
We detect weak emission over one fourth of
the surveyed region. Examination of a representative region suggests
that the weak emission is tracing
cm-3 gas, which may be in bound
clumps. In some cases, the small size of the observed emission regions
indicates that beam dilution may also contribute to their weak
signal.
We have compared the mass in the surveyed region traced by the CO
(1-0) transition to the total mass of the cores
detected in the W 3 GMC. We find that the
comprises % of the mass traced by CO. This is
comparable to the mass fraction of dense cores in Orion B derived by
Lada (1992). Although the total mass of dense gas detected in the
Orion B survey of Lada (1992) is similar to the total mass of the
cores in W 3 GMC, we find that the dense
gas in W 3 is concentrated into fewer, more massive cores.
Following the observation by Lada (1992) that star formation is
concentrated in the most massive cores, we suggest that the higher
core masses in the W 3 GMC may be the reason for the higher rate
of high mass star formation observed in the W 3 GMC.
Although all sites of recent star formation appear to be associated
with the cores on a parsec scale, on a
sub-parsec scale we find an anticorrelation between the
cores and the sites of recent star formation.
This anticorrelation is particularly apparent toward W 3 Main
where we find no evidence for NIR clusters or H II
regions toward the centers of W 3 West and W 3 SE.
One explanation is that the NIR clusters remain hidden in the cores
until the cores are dispersed. However, in one case, we find that the
anticorrelation is due to a lower relative abundance of the
molecule. We propose that this "thinning out" is
due to the preferential destruction of molecules
by the stars embedded in the cores.
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998
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