![[*]](/icons/foot_motif.gif){kind=icon}
A&A 383, 540-547 (2002)
DOI: 10.1051/0004-6361:20011772
T. Preibisch1 - M. D. Smith2
1 - Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121 Bonn, Germany
2 -
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
Received 24 October 2001 / Accepted 30 November 2001
Abstract
The S140/L1204 cloud contains a deeply embedded
region of star formation and a powerful molecular outflow.
In this paper, we present images of the S140 region
obtained in the light of the 2.12 
m molecular hydrogen emission line and
adjacent continuum.
Our images reveal several knots of H2 line emission
originating from shocked material close to IRS1 as well as
further out.
Strong H2 shock emission is found
north-east of IRS1 (at position angle of 
20
-30),
as well as to the south-west of IRS1 (at position angles around
190-220), clearly demonstrating the presence of
outflow activity in the north-east/south-west direction.
We also find patches of H2 emission
several arcminutes away from IRS1 at a position angles of 150
and
,
i.e. in directions consistent with the previously known
north-west/south-east molecular outflow.
Our results therefore provide evidence for the existence of two distinct
bipolar outflow systems originating simultaneously from IRS1.
We also discuss
general aspects of the star formation process in the
S140 region. An inferred high ratio of stellar to gas mass
suggests that the outflows have dispersed most of the cloud mass.
Key words: stars: individual: S140 IRS1 - stars: individual: S140 IRS3 - stars: formation - stars: winds, outflows
S140 is an HII region at the southeast edge of the L1204 dark cloud
at a distance of 900 pc (cf. Crampton & Fisher 1974).
It is part of a cloud complex located at the edge
of a prominent infrared
emission ring, known as the Cepheus ring. This ring is probably the
result of supernova explosions and stellar winds from massive stars
in the open cluster NGC 7160 at the center of the
ring (cf. Kun et al. 1987; de Zeeuw et al. 1999).
The S140 cloud
contains a small cluster of highly obscured, optically invisible
infrared sources, originally detected by Rouan et al. (1977).
The 20 m observations by Beichman et al. (1979) revealed
three individual infrared sources in this region.
The brightest source is called IRS1, and the two other sources
IRS2 and IRS3 are located
17'' north and 9'' east of IRS1.
The infrared spectra of these sources are
rising steeply between 10 m and 50 m (Lester et al. 1986),
demonstrating that they are deeply embedded young stellar
objects associated with circumstellar material.
The luminosities of IRS1, 2, and 3 were estimated at
5, 3, and 2
respectively (Lester et al. 1986),
suggesting stellar masses between 
6
and
10.
S140 IRS is the source of a strong
molecular outflow. A bipolar CO outflow was first detected by
Blair et al. (1978) and later studied in more detail, e.g. by
Hayashi & Murata (1992), Minchin et al. (1993), and
Wilner & Welch (1994).
S140 IRS1 lies just in the middle between the
blue- and red-shifted outflow lobes which have position angles of
and
,
respectively.
While the morphology of the molecular line emission in all these observations
is consistent with the idea that IRS1 is the driving source of the outflows,
the spatial resolution of the maps
is far too coarse to exclude the possibility that IRS2 or IRS3
also contribute to the outflow activity.
First high-resolution infrared observations of IRS1 were obtained
by Hoare et al. (1996) and revealed diffuse emission extending to the
south-east of the source. In recent high-resolution
studies of the S140 star forming
region we have presented bispectrum speckle interferometric
K-band images of IRS1 (Schertl et al. 2000; Weigelt et al. 2002).
We found a bright elongated clumpy structure
pointing away from IRS1
in a direction (
)
very consistent with that of the blue-shifted
CO outflow lobe.
We interpreted this feature as the clumpy inner edge of a cavity
in the circumstellar envelope around IRS1, which
has been excavated by the south-eastern
wing of the strong
molecular outflow.
We also found three
arc-like structures to the north-east of IRS1 at position angles
of 15-25
and suggested that these structures
represent cavities created by material flowing away from IRS1 in
north-eastern directions.
These high-resolution data revealed unique insights
into the detailed morphology of the circumstellar structures around
IRS1. However, as they had to be taken in a broad-band K'-filter,
they did not allow us to discriminate between
continuum emission (e.g. scattered stellar light from IRS1 and
thermal dust emission from the inner circumstellar matter) on the one hand
and line emission from shocked gas on the other hand.
Therefore, the interpretation
of the observed structures was not conclusive.
Outflows and jets from young stellar objects strongly
interact with the surrounding material.
Shocks are generated where the flow, ejected from the
protostar, impacts with the dense environment.
The 2.12 m 
S(1) ro-vibrational emission line of
molecular hydrogen is a convenient tracer of these
shocks (e.g. Smith 1993; Eislöffel et al. 2000).
Images taken in the light of the molecular hydrogen emission line
can therefore reveal the structure and morphology of the outflow,
and provide important constraints on the nature of the flow collimation
and variability.
Motivated by these ideas, we have imaged the S140 IRS region
in the molecular hydrogen line emission and adjacent continuum.
In Preibisch et al. (2001) we presented a study combining
high-resolution speckle
images and molecular hydrogen line emission images of S140 IRS3.
In this paper we present our H2 results on IRS1 and consider
general aspects on the star formation process in the S140 IRS region.
The infrared data presented here were obtained
in October 2000 with the Omega Prime wide-field
near-IR camera (Bizenberger et al. 1998)
on the Calar Alto 3.5 m telescope. The camera uses a
pixel
HgCdTe array. At 0.4'' per pixel, the field-of-view is
.
Images were taken through the NB2122 filter, a 1% filter
centred on the v = 1-0 S(1) line of the H2 molecule at 
m,
and through a broad-band K' filter (
m).
We took images at 6 different dither positions with positional shifts of
40''; at each position a series of 10 images with 30 s (1.677 s for
the K' filter) exposure time
was taken.
We also obtained images through a standard J-band filter.
The weather conditions during our observations were good, the
seeing (as measured by the FWHM of the PSF in our images)
was
1.2'' - 1.6''.
Standard data reduction techniques were used to sky subtract, flat field,
and mosaic the data (cf. McCaughrean et al. 1994).
Then, the NB2122 filter image was carefully aligned with the
K' filter image via several point sources in the outer parts of the image.
Finally, we subtracted the K' filter image from the NB2122 filter image
in order to create the H2 emission line image.
The field-of-view of our final mosaic image is
.
The H2 emission related to S140 IRS3 has already been investigated in a previous paper (Preibisch et al. 2001); here we will therefore focus on the H2 emission related to IRS1.
![\begin{figure}
\par\includegraphics[width=10.6cm]{MS2033f1col.eps} \par\end{figure}](/articles/aa/full/2002/08/aa2033/img22.gif) |
Figure 1:
The color image shows our K'-band image of the
S140 IRS region with a rainbow color-table; the central point source IRS1
is saturated.
The black contour lines show the continuum-subtracted m H2 line
emission image. Contour levels are
10, 15, 20,2 5, 30, 35, and 40 
above the median in the H2 image.
In the immediate vicinity of IRS1,
our H2 emission map is slightly affected by small
artifacts due to imperfect continuum subtraction. We therefore will
only discuss features more than 4'' away from IRS1.
The eight most prominent H2 knots are marked with numbers 1
to 8.
The position of the deeply embedded infrared source IRS2, which is invisible
in the near-infrared, is marked by the red asterisk. (This figure is available in color in the online version.) |
| Open with DEXTER | |
![\begin{figure}
\par\includegraphics[width=10.6cm]{MS2033f2col.eps} \end{figure}](/articles/aa/full/2002/08/aa2033/img23.gif) |
Figure 2:
Color-composit image of a
area around S140 IRS.
The red color plane represents the K' band image, the blue color plane the
continuum-subtracted m H2 line emission image.
The most prominent patches of H2 emission around S140 IRS are marked.
The three white asterisks mark the positions of IRS1, IRS2, and IRS3. (This figure is available in color in the online version.) |
| Open with DEXTER | |
Our image reveals many patches of H2 line emission
in and around the S140 IRS region.
The strongest H2 emission knots are
found within 15'' of IRS1. Figure 1 shows the central part
of our
K'-band image of S140 IRS onto which the contours of the
continuum-subtracted m H2 line emission are overlaid.
The most prominent knots of H2 line emission are marked with numbers
1 to 8.
Knot 1 is an elongated feature of strong H2 emission north-east of
IRS1. The emission peak
is found 13'' away from IRS1 at a position angle of
28.
The H2 emission is strongly elongated
in the north-south direction and very well aligned with the eastern edge of
the arc-structure north-east of IRS1 in the K' band image.
It is important to note that the peak of the H2 emission does not
coincide with the brightest features seen in the K' band continuum, but
is offset by about 3''.
The weaker extensions of the H2 emission cover the full extent of the
arc structure.
Another weak peak of H2 emission (knot 2) is found 6'' north-east of
IRS1 and is well
aligned with the southern part of the arc-structure.
The area south-west of IRS1 contains a ridge of strong H2 emission,
roughly oriented in the east-west direction. The two strongest maxima are
knot 4, located 6'' south of IRS1 at a position angle of
185,
and knot 5, located 8'' south-west of IRS1 at a
position angle of
.
These H2 knots coincide very well
with the brightest parts of the K' band continuum nebulosity.
The detection of these H2 emission knots provides evidence for
material flowing from IRS1 to the south-western direction.
This can be quite naturally explained as the counterpart of the
outflow.
Another patch of H2 emission (knot 3)
is seen 8'' south-east of IRS1
at position angles in the range 135-160.
This feature is probably related to the well known
molecular
outflow.
Figure 2 shows a larger field around S140 IRS.
The most prominent structure in the H2 line emission image
is the bright rim of the
the S140 HII region, crossing the south-western edge of our image.
This HII region is powered by the B0 star HD 211880, located
8' south-west of S140 IRS. The H2 line emission
results from fluorescent excitation (cf. Tanaka et al. 1989).
In the northern and eastern part of Fig. 1 we find
several patches of H2 line emission at distances between 15'' and
90'' away from IRS1.
We have assigned numbers 9 to 14
to those H2 emission knots in
which the peak surface brightness exceeds the noise level in the continuum
subtracted image by more than 
.
Knot 9 is located 70'' north-west of IRS1 at a position angle
of
.
It is clearly elongated in a direction pointing
towards IRS1. Therefore, we conclude that this H2 emission knot
is related
to the well known
CO outflow from IRS1.
Knot 9 is the first infrared evidence for the receding lobe of the
outflow.
Knot 10 is found 53'' north-east of IRS1 at a position angle of
.
This direction is in very good
agreement with the orientation of the arc complex north-east of
IRS1 described in the speckle images by Weigelt et al. (2002).
Knots 11 (50'' north-east of IRS1 at a position angle of
)
and 12 (30'' north-east of IRS1 at a position angle of
)
are probably also related to the north-eastern outflow.
Knot 13 is related to the bow-shock like nebulosity 90'' north-east
of IRS3.
Its position angle measured from IRS3 is
,
exactly the
direction of the precessing outflow from IRS3 detected by
Preibisch et al. (2001).
Knot 14 is located 90'' south-east of IRS1 at a position angle of
.
It is not clear whether this patch is related to IRS1 or IRS3.
The combination of our H2 data with previous observational results, allows us to draw a comprehensive picture of the outflow activity of S140 IRS1 on a wide range of spatial and temporal scales:
100 AU to
10000 AU.
S(1) transition,
the molecular hydrogen emission illustrates the current
shock-activity in the outflows.
50000 AU
and the long-term effects of the outflows on the ambient material
/340
outflow
The presence of the large scale
outflow
was already well established from radio line observations. In the radio maps,
outflowing gas can be seen at least up to distances of 1' (54000 AU)
from IRS1 (e.g. Minchin et al. 1993).
Our high-resolution speckle observations (Schertl et al. 2000; Weigelt
et al. 2002) revealed the detailed structure of the cavity through which the
material in the
outflow lobe is flowing on scales of 90-1800 AU.
Our molecular hydrogen emission line images show
only two relatively weak H2 knots associated
with this outflow, knots 3 and 9. This suggests that there
are currently not many strong shocks in this outflow, perhaps
because the path of the outflow is already well cleared and
the material is flowing away without producing strong shocks.
However, the detection of H2 shock emission in this outflow is
aggravated due to the inclination of the flow and projection effects:
In the south-eastern part, where the outflow is directed towards us,
H2 emission very close to (
2'' away from) IRS1 would be projected
onto the very bright emission of the cavity.
In the north-western part of this outflow, H2 shock emission might
be present, but
hidden behind the dense circumstellar material around IRS1.
The fact that the high resolution radio continuum observations
presented by Schwartz (1989) and Hoare & Muxlow (1996)
do not show any sign
of the
outflow, supports our assumption
that this outflow is probably currently not very active.
A rough estimate of the kinematic age of this outflow can also be made:
from the projected distance of knot 9, an assumed velocity of
100 kms-1, and assuming an inclination angle of
,
we find a lower limit of
years.
/200
outflow
The existence of a
outflow
has not so far been well established.
Radio line observations had shown a relatively weak
low-velocity wing in the CO and HCO+ emission that indicated
outflowing material in the
direction, but no reliable evidence
for an outflow in the
direction.
The high-resolution radio continuum maps of S140 IRS1 presented
by Schwartz (1989) revealed a strongly elongated jet-like appendage extending
from IRS1 in the south-west direction (position angle
), and
a diffuse radio source (VLA 4) south-west of IRS1 (position angle
)
which may be a radio Herbig-Haro object.
These features were interpreted as evidence for ejection
of material from IRS1 in the south-western direction.
The high-resolution radio observations by Hoare & Muxlow (1996) showed
a strongly elongated structure oriented in the
direction.
It was interpreted as a highly collimated jet, probably being driven
by a disk wind. The 8.4 GHz VLA observations of S140 presented by
Tofani et al. (1995) revealed an interesting S-shape for the
emission within 1'' of IRS1. While the core of the emission is
oriented in the
direction, the position angle
of the north-eastern tip of the 8.4 GHz emission is
,
in very good agreement with the position angle of the outflow cavities
we found in the speckle images.
Our high-resolution speckle observations (Weigelt et al. 2002)
revealed three arc-like structures with position angles of
,
,
and
north-east of IRS1.
The shapes of these arcs could be well reproduced by a
semi-analytical model of jet-driven flows,
in which prompt entrainment occurs at the head of the traveling bow shock.
This strongly suggested a flow of material
in this direction, but could not be considered as a clear proof.
The result that the eastern edge of the supposed cavities is actually
associated with strong H2 emission,
confirms our previous interpretation that these cavities have been
created by outflowing material from IRS1
in the 20
direction.
Our H2 line emission image reveals numerous knots along the paths
of the
outflow:
knots 1, 2, 10, 11, and 12 are related
to the
flow,
knots 4 and 5 to the
flow.
These knots span a range of position angles from
to
,
which is consistent with the orientation of the elongated radio emission
immediately around IRS1.
The numerous H2 shocks in these directions indicate that
the outflow currently interacts very strongly with its surroundings.
Perhaps the outflow is currently in the process of clearing its way through
the ambient material.
The rather wide range of position angles of the individual H2 knots
north-east of IRS1 suggest a pulsed nature for the outflow and directional
variability. A wiggling jet in the NE/SW direction
is also suggested by the radio continuum and
maser emission on the scale of
(Tofani et al. 1995), which
extends
in the 45
direction i.e. towards H2 knot 2.
The temporal variability of the outflow direction may be caused
by the precession of the circumstellar disk around the outflow source
which is a member of a non-coplanar binary system (see Weigelt et al. 2002
for details; for the theoretical background of this model
see e.g. Papaloizou & Terquem 1995; Terquem 1998; Bate et al. 2000).
A rough estimate of the kinematic age of this outflow, based on the
distance of knot 10, an assumed inclination angle of
and a velocity 100 kms-1, yields
years for the
outflow. This
is nearly three times younger than the lower limit to the kinematic age of
the
outflow.
![\begin{figure}
\par\includegraphics[width=8.5cm]{MS2033f3.eps}\end{figure}](/articles/aa/full/2002/08/aa2033/img45.gif) |
Figure 3:
Color-magnitude diagram of stars in the S140 IRS region
(solid dots). We have included
the individual objects resolved in our speckle images; for these, no
colors can be determined and we assume each component to have the
same color as reported for the unresolved systems.
All objects for which only K'-band magnitudes could be derived
are plotted as small crosses at J-K'= 5.8.
The thick solid line in the left part of the diagram shows theoretical
colors and magnitudes based on the PMS models of Palla & Stahler (2000)
and Bernasconi & Maeder (1996) for an age of
yr for
masses from
to
 .
Tick marks are shown for
masses of 0.1, 0.4, 1, 2, 3, 4, 6, and
.
The long arrow shows the reddening path for AV = 25 mag starting
at the location for a
yr old
star. |
| Open with DEXTER | |
Near-infrared images (e.g. Fig. 2; see also Hodapp 1994) show a small
cluster of fainter point sources around the nebulosity surrounding IRS1.
It seems very likely that most of these objects are associated
with the S140 star forming region for two reasons:
first, the extinction of the S140 molecular cloud
was estimated to be up to
mag based on the determination
of
by Minchin et al. (1995);
it therefore provides an essentially opaque
shield for the stellar background, and we can assume that all stars
we see in this region are either foreground stars or associated
with the S140 cloud. Second, the optical images of the region show only very
few stars within a few arcminutes of the position of IRS1, clearly
demonstrating that the foreground population cannot be very large.
Our deep J- and K'-band data offer a good opportunity to examine the
stellar population of the S140 star-forming region. For this, we
used DAOPHOT to locate point sources in our deep J- and K'-band images
and performed aperture photometry.
The central 20'' region had to be excluded from this analysis because
of the extremely strong diffuse emission in the
image center. In the circular region with radius 4' around IRS1,
photometric data could be determined for 388 stars in the
K'-band image and for 272 stars in the J-band image.
The magnitudes were calibrated
via observations of other regions and comparison with the corresponding
magnitudes from the 2MASS catalogue.
Due to the strong diffuse emission in the S140 IRS region the accuracy of the
photometry is limited to 0.2 mag.
Nevertheless, this is sufficiently accurate to consider the
positions of the stars in the color-magnitude diagram.
Our color-magnitude diagram (CMD) is shown in Fig. 3.
The comparison with the expected colors and magnitudes for pre-main sequence
stars shows that our sample of K'-band sources should be complete
for
stars suffering up to 25 mag of visual extinction
(expected magnitude
). The number of stars for which the NIR data
are consistent with being cluster members with
is 39 (this includes 20 stars with K' < 12.5 for which we could only measure
K'-band magnitudes.)
Four out of these 39 stars, namely IRS1, IRS2, IRS3a, and IRS3b,
are probably more massive than
.
We can now compare these numbers with the expectation from the
field star IMF. In the IMF representation by Scalo (1998),
which has the form
![$[5 \dots 10]\,M_\odot$](/articles/aa/full/2002/08/aa2033/img52.gif){kind=small}
range
would imply a total number of 83 stars with masses between
and
.
The IMF representation given by
Kroupa (2001)
which has the form
![$[1{-}10]\,M_\odot$](/articles/aa/full/2002/08/aa2033/img55.gif){kind=small}
range.
Our detection of 39 stars above
is lower than both
predictions. However, we have to take into account that
we certainly miss some fraction of low-mass members,
for two main reasons: first, our photometry does not cover the central
20'' area due to extremely strong diffuse emission.
Second, our sample can be expected to be complete only up to
mag,
while the total cloud extinction near the center is probably
mag.
This suggests that our images probably reveal only about half of the
low mass members in the cloud.
Considering this, we conclude that the number of stars we see in our image
is not inconsistent with the assumption that the cluster mass function
follows the field star IMF.
We can estimate the total mass of the
stars in the cluster by assuming that the mass function
is the same as the field star IMF (as demonstrated above).
In that case, the total mass of all stars (
)
in the cloud would be
340
based on the Scalo (1998) IMF and
240
based on the Kroupa (2001) IMF.
We can estimate the total mass of the stars in the cluster in the
following way:
For a very conservative lower limit, we consider only the objects
IRS1-IRS7. The sum of the estimated masses of these objects
is 40.
The mass of the gas and dust in the S140 IRS cloud has been determined
in various radio and submm studies. For example, Hayashi & Murata (1992)
derived a mass of
within a radius of r = 0.15 pc (
34'').
Van der Tak et al. (2000) found
in r = 0.15 pc from
submm maps.
Bally & Lada (1983) derived a total mass of 64
within
r = 0.46 pc (
1.7') from CO observations.
From these numbers one can see that the mass of the stars is considerably
higher than the cloud mass.
A similar result is obtained for the center of the starforming region:
Considering only the objects IRS1-IRS7, the sum of their
estimated masses is 40.
On the other hand, the mass
of the three submm cores within 25'' of IRS1 is only
29
(Minchin et al. 1995).
These considerations make it quite clear that the inferred total mass in young stars exceeds the mass in gas significantly. This implies that a high fraction of the original cloud mass has been converted to stars or that the formation of a protostar leads to the escape of a much larger mass.
In S140, there is clear evidence for the ejection of a large fraction of the cloud. Numerous outflows are found here in the warm shocked molecular gas, which reveals that even the present outflows contain sufficient thrust to eject material from the cloud. This is not surprising given that 10-30% of the mass accreting from the dense envelope around a core must be ejected into the jets observed from Class 0 protostars (e.g. Smith 2000) and the jet speed is of order 100kms-1. The jets' thrust is thus sufficient to gravitationally unbind roughly ten times more mass than goes into the protostar. Hence, the total mass of the initial cloud out of which the stars formed may have been much larger than the sum of the star + gas mass presently found.
Given this picture, outflow activity must have influenced the cloud for its full lifetime. We could expect perhaps 100 outflows within a few million years. This would still be detectable in the CO rotational lines which contain a long-term record of outflow activity. And, indeed, the CO high-velocity gas possesses a complex distribution, consisting of numerous widely spread clumps, but bordered in the south and west by high density gas.
A problem with jet/outflow feedback scenarios
is that a well collimated outflow only
influences its immediate surroundings and not the
cloud as a whole. It is not plausible to
assert that an outflow can provide turbulent
support to a cloud since turbulence decays faster than it
could spread laterally, from an outflow, across a
cloud given the rapid decay of turbulence found
in MHD simulations (e.g. Mac Low 1999).
Hence, even though the momentum inherent in the
outflows may be sufficient (Yu et al. 2000), they will not
be present to support the cloud as it evolves.
A large number
of outflows, however, distributed over space and the
cloud life, may still be effective in dispersing
the cloud with the following argument. If each outflow is
represented as a cone with a half opening angle of just
10,
then 103 outflows would
fill the cloud volume. Hence, over the cloud lifetime,
and given random outflow directions over the cloud
lifetime, a large fraction of the cloud can be ejected.
In contrast,
the H2 gas is observed from hot shocks which are
driven by the latest energetic outflows. Even so, we have
found here evidence for several outflow directions, related to
at least three outflows.
The kinematic ages of the outflow have been estimated to be just
a few thousand years.
We suspect that the outflows are much
older and penetrate far into the surrounding lower density halo
where they are not detectable because of both the low density and
possibly a low molecular fraction. Therefore, deep optical H
imaging
in the NE region beyond 4' from IRS1 could reveal Herbig-Haro
objects.
Triggered star formation by external
compression
might also lead to an enhancement of the star formation efficiency.
This could also contribute to the high ratio of stellar mass versus gas mass
we found above for the S140 region.
The S140 region is probably a good example for triggered star formation. A sequence of triggered star formation events in the Cepheus bubble has been suggested by Patel et al. (1998): initially, a cluster containing high-mass stars formed in a molecular cloud. The winds and the ionizing radiation of the massive stars then created an expanding shell around this cluster. Gravitational instabilities in the expanding ring lead to fragmentation, cloud collapse and the formation of a second generation of stars, which now constitute the Cep OB2 association, which has a presumed age of about 5-7 Myr. The massive stars in Cep OB2 now affect the dense gas remaining from the parent shell and seem to trigger the formation of the third generation of stars in the dense cloud cores, including S140, along the edge of the bubble (see also Ábráhám et al. 2000).
The shape of the S140 cloud strongly suggests that the gas is being compressed by external pressure from the south-west direction. One source for this compression is the B0 star HD 211880, the exciting source of the S140 HII region, at a projected distance of 2.5 pc from S140 IRS. Another factor may be the general expansion of the Cepheus bubble, the center of which is located to the south-west of the S140 cloud.
![\begin{figure}
\par\includegraphics[width=8.5cm]{MS2033f4.eps}\end{figure}](/articles/aa/full/2002/08/aa2033/img63.gif) |
Figure 4:
Radial distance distribution of the stars around IRS1.
All objects with
J - K' > 1.5 (corresponding to objects with strong
infrared excess or suffering from
at least 3 mag of visual extinction) are considered to be cluster
members. In the two innermost bins our sample is incomplete due to
the very strong diffuse nebulosity, hampering the detection of faint
point sources. |
| Open with DEXTER | |
It is interesting to consider the spatial distribution of the cluster
members. In Fig. 4 we plot the number of objects with
J - K' > 1.5, i.e. objects that either show strong infrared excess
or suffer strong extinction and therefore can be considered cluster member
candidates, in several bins of radial separation from the center.
One can see that the radial distribution of these stars is more or less
uniform.
The interpretation of this uniform radial distribution is not fully
conclusive. We could
interpret the embedded stars as objects which have formed
at a uniform rate within a very dense central core over the star formation
lifetime of S140 and which form an expanding spherical distribution.
Or, the stars formed in a rapid burst with a linear distribution
of escape speeds. Such a burst, followed by dispersal of a large
fraction of dense ambient material (reducing the confining gravitational
force), would result in the free expansion of the young stars. A
third possibility is that the stars formed in situ with
an efficiency related to both the local density and the compression
of a passing shock wave which would sweep up material into an inner
clumpy ring, possibly as now observed in CS with a radius of
30
(Hayashi & Murata 1992). In this case, the stars could be younger.
The age of the embedded stars could be tested by searching
for excess H-K band emission (the H-band filter failed during the
run described here).
The main results of our study are as follows:
We find strong H2 shock emission around the 10
protostar
S140 IRS1. Some of these shocks are related to the well known
160/340
CO outflow, but the strongest shocks
are found in the
outflow, which seems to be currently
very active.
These results provide evidence for the existence of two distinct
bipolar outflow systems originating simultaneously from IRS1.
The 8-10
protostars IRS1-3 are associated with a cluster
of fainter
sources, presumably lower-mass young stellar objects.
The total mass in stars seems to exceed the mass of the remnant
gas and dust in the S140 region significantly. This might indicate that
the strong outflows of the protostars have expelled most of the
gas from the region.
Acknowledgements
We would like to thank the Calar Alto staff for their support during our observations and Thomas Stanke for assistance in the data reduction. MDS is grateful to G. Weigelt for support and discussions during visits to the MPIfR.
