A&A 473, 149-162 (2007)
DOI: 10.1051/0004-6361:20077733
F. Comerón1 - N. Schneider2
1 - ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei
München, Germany
2 -
SAp/CEA Saclay, 91191 Gif-sur-Yvette, France
Received 27 April 2007 / Accepted 4 July 2007
Abstract
Context. The IRAS 16362-4845 star-forming site in the RCW 108 complex contains an embedded compact cluster that includes some massive O-type stars. Star formation in the complex, and in particular in IRAS 16362-4845, has been proposed to be externally triggered by the action of NGC 6193.
Aims. We present a photometric study of the IRAS 16362-4845 cluster sensitive enough to probe the massive brown dwarf regime. In particular, we try to verify an apparent scarcity of solar-type and low-mass stars reported in a previous paper (Comerón et al. 2005, A&A, 433, 955).
Methods. Using NACO at the VLT we have carried out adaptive optics-assisted imaging in the
bands, as well as through narrow-band filters centered on the Br
and the H2
lines. We estimate individual line-of-sight extinctions and, for stars detected in the three
filters, we estimate the contribution to the
flux caused by light reprocessed in the circumstellar environment. We also resolve close binary and multiple systems. We use the K luminosity function as a diagnostic tool for the characteristics of the underlying mass function.
Results. IRAS 16362-4845 does contain young low-mass stars. Nevertheless, they are far less than those expected from the extrapolation of the bright end of the K luminosity function towards fainter magnitudes. We estimate a total stellar mass of 370 .
Nearly all the cluster members display L' excesses, whereas
excesses are in general either absent or moderate (<1 mag). We also detect an extremely red object with
,
likely to be a Class I source.
Conclusions. The fact that solar-type and low-mass stars are present in numbers much smaller than those expected from the number of more massive members hints at an initial mass function deficient in low mass stars as compared to that of other young clusters such as the Trapezium. The origin of this difference is unclear, and we speculate that it might be due to external triggering having started star formation in the cluster, perhaps producing a top-heavy initial mass function. We also note that there are no detectable systematic differences between the spatial distributions of bright and faint cluster members. Such absence of mass segregation in the spatial distribution of stars may also support external triggering having played an important role in the history of the RCW 108 region.
Key words: ISM: HII regions - ISM: individual objects: RCW 108 - stars: luminosity function, mass function - open clusters and associations: IRAS 16362-4845
The RCW 108 HII region in the Ara OB1 association has long been regarded as one of the best case studies illustrating the eroding action of newly formed clusters containing massive stars on the molecular gas in their environments (Shaver & Goss 1970; Straw et al. 1987; Comerón et al. 2005). In visible-light images of the region (see e.g. Petersen 2001) RCW 108 appears as a bright rim nebula (SFO 79 in the catalog of Sugitani et al. 1991) on a size scale of several 10 arcmin, defining a sharp boundary between an extended molecular cloud in the west and a region predominantely filled with ionized gas in the east. The rim represents the ionization front, produced when the ultraviolet radiation from O-type stars of the neighboring cluster NGC 6193 hit and progressively destroy the molecular cloud.
Infrared sources indicating active star forming sites are common among externally ionized molecular clouds (Sugitani et al. 1989, 1991, 1995; Sugitani & Ogura 1994), which has been interpreted as evidence for star formation triggered by radiation-driven implosion of dense cores (e.g. Bertoldi 1989; Miao et al. 2006). This mechanism may well be at work in RCW 108, as noted by the indications of triggered star formation in the region recently discussed by Urquhart et al. (2004) and Comerón et al. (2005, hereafter CSR05).
The most conspicuous star forming site in RCW 108 is the
compact HII region IRAS 16362-4845, first noted by Shaver
& Goss (1970). It is deeply embedded in the dense
molecular cloud and associated with a cluster of IR-sources
discovered by Straw et al. (1987). Its stellar contents
has been more recently studied by Urquhart et al. (2004), based on the 2MASS catalog (Skrutskie et al. 2006); by CSR05 by means of dedicated deeper,
higher resolution
imaging; and by Wolk et al. (2007) using X-ray emission as a tracer of young
stellar populations. The study of CSR05 indicates that the
IRAS 16362-4845 embedded aggregate is a Trapezium-like cluster
containing at least one late O-type star, in consistency with the
visible spectrum of the heavily obscured HII region. The mass of the
cluster was estimated by CSR05 to be
210
,
with a
rather large uncertainty. Furthermore, that study noted as an
intriguing feature of the color-magnitude diagram an apparent lack
of stars fainter than
and with amounts of
foreground reddening in the range covered by the brightest stars in
the cluster, which might be indicative of a peculiar mass function.
While the result hinted at by CSR05 may be potentially relevant to understand the build-up of the initial mass function (IMF) in clusters dominated by massive stars, some practical limitations of the observations presented in that work advised a further analysis based on material of higher quality. First and foremost, though the observations were deep enough to penetrate well into the area of the color-magnitude diagram where the lack of the stars was noted, the completeness of the census in that range was difficult to assess. This was due to the presence of bright nebulosity pervading the cluster with large brightness variations over small angular scales. Secondly, although the observed stellar images have a full-width at half maximum (FWHM) below one arcsecond, the combination of crowdedness and nebulosity still hampered the detection of faint members relatively close to brighter stars. For these reasons, higher quality observations were needed to place our tentative conclusion of a deficit of faint members in IRAS 16362-4845 on a firm standing.
In this paper, we present new observations of the IRAS 16362-4845 cluster carried out using adaptive optics near-infrared imaging at the Very Large Telescope (VLT), which provide a far deeper and sharper view of the cluster than previously available. This new material allows us to reassess the stellar contents of the cluster, and address questions related to possible peculiarities of its IMF, the frequency of infrared excesses among its members, the abundance of massive binary stars, or the spatial distribution of high- and low-mass members, also providing some further insights on the structure of the associated nebula.
Our observations were carried out in service mode using NACO, the
adaptive optics near-infrared camera and spectrograph at the VLT
(Rousset et al. 2002, Lenzen et al. 2003), in
imaging mode. Broad-band images were obtained through the J(1.26 m), H (1.66
m),
(2.18
m), and L'(3.80
m) filters, as well as through two narrow-band filters
centered on 2.12
m and 2.17
m respectively sampling the
nebular emission in the H2
and
Br
lines. The brightest star in the field in
-band
images of the cluster, star #12 from CSR05 (hereafter
CSR-012A
) has
and was used
for wavefront sensing, using the near-infrared wavefront sensor in
NAOS, the adaptive optics module of NACO. Different available
dichroics were chosen, depending on the band of the observations:
the K dichroic for the J and H observations (90% transmitted
light in those bands), the
dichroic for the L' observations
(also 90% transmission in that band), and the N20C80 dichroic for
the
,
2.12
m, and 2.17
m observations (80%
transmission). These choices provide the best compromises between
the signal from the wavefront reference star needed for a good
adaptive optics correction and the light transmitted to CONICA, the
camera of NACO. We selected the wide-field camera optics yielding a
pixel scale of 54 milliarcseconds per pixel for the images in the J, H, and
bands and the narrow-band. The resulting field
size, 54'', matches well the angular size of the IRAS 16362-4845
cluster (
30''), whereas the location of CSR-012A near the
northern border of the cluster allows us to sample a portion of the
dark cloud located to the north of IRAS 16362-4845. For the L' band observations the maximum field size of the field attainable
with NACO is 27'', at a scale of 27 milliarcseconds per pixel. Our
observations through that filter thus sample only the densest part
of the cluster centered on CSR-012A.
The observations were obtained on six different nights between
29/30 June and and 28/29 July 2006. The images were performed
through two series of exposures in each of the J, ,
L',
2.12
m, and 2.17
m filters, and one series of exposures in
the H filter. We used the common technique of stacking a number
of frames, each with a detector integration time
,
centered on a number
of closely spaced telescope pointings on
a random dither pattern, for which we used an amplitude of 10''.
The image resulting from each exposure was constructed by combining
the sky-subtracted individual stacks after correcting for the
telescope offset between pointings. This procedure was also used for
the L' band observations, rather than the alternative chop-and-nod
technique. Table 1 gives the log of the observations,
including the individual exposure parameters. Sky subtraction from
our on-target images was performed by stacking intercalated images
obtained around a separate sky position located 3' away from
IRAS 16362-4845, as the bright nebulosity pervading the cluster
prevented us from using for that purpose the median-filtered
on-target frames uncorrected for telescope offsets, as is normally
done on uncrowded, nebulosity-free fields. The detector readout mode
was selected for each filter as the best compromise between the
necessary sensitivity and the dynamic range available. For the
filters in which two images were obtained on separate observations,
one combined frame was produced and used for further analysis.
Figure 1 shows a composite of the final frames
obtained through the
filters.
Table 1: Log of observations.
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Figure 1:
A color composite of the frames of IRAS 16362-4845
obtained through the J (blue), H (green) and ![]() ![]() ![]() ![]() ![]() |
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Our observations in the
bands were calibrated using the
infrared photometry of non-saturated sources in common with those
listed in CSR05. The L' photometry was calibrated using as a
reference the stars in the field observed in that same band by Straw
et al. (1987).
The intrinsic difficulty of performing stellar photometry in
observations using adaptive optics, due to the noticeable variation
of the point-spread function (PSF) across the field, is compounded
in our case with the additional complication of a variable nebular
background pervading the area of the cluster, which makes it
difficult to accurately estimate the contribution to the measurement
of the extended wings of the PSF. The strong and complicated
variation of the PSF, particularly in the J band where the
adaptive optics correction is poorer and degrades fastest with the
distance to the wavefront reference star, led us not to consider PSF
fitting as a suitable method. Instead, we obtained better results by
performing aperture photometry at the position of each detected
object. Source detection was carried out automatically using the
DAOFIND task in the DAOPHOT package layered on IRAF (Stetson 1987). The choice
of parameters of the task, in particular the sharpness and roundness
cutoff parameters, was made by successive trials by comparing the
results of the automatic detection with a careful visual inspection
of the frames. We identified the sources that were left undetected
by DAOFIND as well as the false detections, relying on the fact that
the human eye ultimately provides the best discriminant between real
stellar sources and detection artifacts.
The radius of the aperture for photometry was chosen as five times
the FWHM of the PSF of a bright, non-saturated star located 15
away from the wavefront reference star, which provided an average
PSF of the field. We verified that the relatively large number by
which the FWHM of the PSF of that star was multiplied ensured a
negligible aperture correction elsewehere in the field. The counts
within this aperture were computed by dividing the aperture into
concentric rings each with a width of one pixel. Pixel values
deviating by more than
from the average value within each
ring normally denoted the existence of a close companion to the star
being measured, and were thus replaced by the average of the other
pixels of the ring. For some faint stars located in regions of a
strongly variable background this procedure prevented us from
obtaining reliable magnitude measurements. Table 2 gives
the photometric measurements obtained for all the stars in our
images, and is available in full electronically.
The magnitude completeness limits of our observations are not
straightforward to assess due to the same factors that complicated
the photometric measurements. On average the better adaptive
optics correction obtained near the wavefront reference star, which
leads in principle to deeper detection limits, is partly offset by
the fact that this star lies near the peak of the surface brightness
of the nebula. In the H and
band images, where the adaptive
optics correction is best and improves image quality over a larger
field, the deepest detection limits are actually obtained near the
edges of the frames, where stellar images are not as sharp as near
the centers but the background nebulosity is nearly absent.
Since knowledge of the completeness of the stellar census of the
cluster is of great importance for the ensuing discussions on the
luminosity function of the cluster, we performed numerical
simulations on the reduced frames using artificial stars. To this
end, stars at different positions of the field were used as local
PSF references, and artificial stars of various magnitudes and the
same PSF were added within a radius of 10
from each local PSF
reference. The same detection procedure and parameters used for the
detection of the real sources were then applied to the image
containing the artificial stars. The completeness limit was then
adopted as the magnitude of the artificial stars for which at least 95% of them were recovered by DAOFIND in the surroundings of any of
the local PSF reference stars used to define them. The location of
such reference stars in all representative regions in terms of
nebular brightness and adaptive optics corrections ensures that any
star brighter than the completeness limit thus defined (except those
in the close proximity of the brightest stars) are indeed detected
and incorporated to our census of members of the IRAS16362-4845
cluster. Nevertheless, it was found, as expected, that many regions
of the cluster provided considerably deeper detection limits, and
our census thus contains numerous stars below the completeness
threshold. Table 3 lists both the completeness and 5
detection limits, taking into account that the latter
apply only to limited areas of the region imaged and are generally
brighter elsewhere where the nebulosity is more intense.
Table 3: Completeness and detection limits.
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Figure 2:
A comparison between the images obtained through
the narrow-band filters centered on the Br![]() ![]() ![]() |
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Figure 1 presents an overall view of the
structure of the cluster and the associated nebulosity, and
Fig. 2 gives a more detailed view of the nebula through
the narrow-band filters centered on the line emission of Brand H2. At near-infrared wavelengths the cluster is dominated by
the likely O-type star CSR-012A (=IRS29, in Straw et al. 1987), near the Northern edge of the HII region. Our new
observations reveal it to be a tight concentration of five stars
within
of the primary (Fig. 3). As already noted
by Straw et al. two other stars to the East of the cluster, CSR-18
and CSR-20 (=IRS19 and IRS20, in Straw et al.), become dominant in
the L' band, and other bright members appear elsewhere in the
cluster. The close correspondence between the cluster and the bright
nebulosity is well apparent from Fig. 1,
although some cluster members appear projected beyond the boundaries
of the nebula. The approximate diameter of the cluster is 40'',
corresponding to 0.25 pc at the distance of 1.3 kpc (distance
modulus 10.6) that we adopt in this paper (see Arnal et al. 2003).
The appearance of the nebulosity is reminiscent of a cocoon
surrounding most of the stars, except towards the East where the
nebulosity fades away without a border as well defined as towards
the other directions. This is well seen in Fig. 2, where
the H2 nebulosity wisps tracing the edges of photodissociation
regions at the interface between the molecular cloud and the HII region are absent towards the East, and where the Br
also
brightens near the Western edge. The edge is actually the brightest
feature of the HII region at longer wavelengths, as shown by the
image obtained with the Spitzer Space Observatory using the IRAC
camera at 8
m, available from the Spitzer archive, where
emission is dominated by PAH molecules (see Fig. 4). The
overall morphology of the nebula may be due to the overpressure of
the cluster gas originated by its ionization having been released by
an outflow towards the East. Both the molecular-line and the H
observations presented in CSR05 support this
interpretation. As noted in that work, the cluster and HII region
are not positionally coincident with the peak intensity of the
molecular emission, which lies to the West, indicating that the
expansion of the HII region finds less resistance in the Eastern
direction. Furthermore, the CO intensity contours presented in CSR05
show a prominent pinching just East from the cluster suggestive of a
sharp decrease in column density. Finally, H
radial velocity
maps show a rather sharp change at the same point (see Fig. 8 of
CSR05), superimposed on a shallower, larger-scale gradient in the
Southwest to Northeast direction seen in both H
and
molecular line observations, as would be expected from an outflow
with a component in our direction.
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Figure 3: L'-band image of the region around CSR-012A, the brightest star in the K-band, showing the rich concentration of cluster members around it. |
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Figure 4:
A wider-field view of the IRAS 16362-4845 region obtained
with the IRAC camera on board of the Spitzer space observatory at a
central wavelength of 8 ![]() |
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Wider-field images including the surroundings of IRAS 16362-4845
show extended nebular emission towards the East (see Fig. 2 of
CSR05), clearly related to the cluster and probably tracing the less
dense regions of the ionized outflow, but disconnected from the main
body of the compact HII region by a dark patch due to a dense,
foreground cloud. The Western border of this cloud appears near the
left edge of our NACO images. Interestingly, our H and images show a strip roughly
across crowded with
numerous faint, lightly reddened stars located between the cluster
and the dark foreground cloud (see Sect. 3.2). The very
confined location of those stars leads us to discard the possibility
that they may be cluster members; if they were, most of them would
be brown dwarfs based on their faintness and their relatively blue
colors. Instead, we consider far more likely that they are
background stars seen through a low column density gap just East of
the cluster. Although the dust column density in their direction is
far lower than that estimated from the molecular gas column density
obtained from 13CO
observations of CSR05
(AV reaching 70 mag near IRAS 16362-4845, as compared to AV as
low as
10 mag from the colors of those sources), a low
extinction hole of the size given above is not ruled out by the
13CO
molecular-line observations at 45''angular resolution.
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Figure 5:
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The color-magnitude diagram of the cluster is presented in
Fig. 5. In it we have made a distinction between the
sources located in the Western region of high extinction on the
background and the Eastern region where, as discussed in the
previous section, the extinction appears to be much lower allowing
the detection of numerous background sources. For simplicity we have
separated the sources in these two categories according to their
position with respect to the line
and we base our discussion on the cluster contents on the Western
region alone. The exclusion of the Eastern region, which occupies 20% of our images, is likely to exclude as well some cluster members, but simple visual inspection of
Fig. 1 indicates that the vast majority of the
cluster members lie in the Western region where contamination by
background sources is very low.
The overall appearance of the cluster
,
diagram
presented in Fig. 5 shows a broad distribution in
color which can be due both to variations of the
foreground extinction along the line of sight towards each star, and
to the varying amounts of infrared excess produced by the
circumstellar material associated to each object. Given the deeply
embedded nature of the cluster and its youth we expect both causes
to significantly contribute. The existence of sources with large
amounts of circumstellar dust reprocessing the light of the central
object is confirmed by the (J-H),
diagram
(Fig. 6) and, most clearly, by the
,
diagram (Fig. 7), where many
stars (in the latter diagram, most of them) lie to the right of the
reddening vector delimiting the region of the diagrams accessible to
normal photospheres obscured by different amounts of extinction; see
Sect. 3.4. For reference, the solid line in those
diagrams is the unreddened 1 Myr isochrone using the evolutionary
models of Palla & Stahler (1999), complemented with the
colors computed for them by Testi et al. (1999)
and shifted to our adopted distance modulus DM = 10.6.
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Figure 6:
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Figure 7:
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The isochrone plotted in Fig. 5 demonstrates the
sensitivity of our observations to the entire range of stellar
masses and even to massive brown dwarfs. The upper end of the
isochrone shown in this figure corresponds to a mass of
30 ,
whereas the lowest end marks the position of a
0.01
brown dwarf. Because of the narrow range of infrared
colors covered by normal stars, the main effect of assuming a
different cluster age is to shift the isochrone vertically in the
color-magnitude diagram, thus changing the mass corresponding to an
object of a given absolute magnitude in the sense of the mass being
lower for a younger assumed age. Although the precise masses of the
intrinsically faintest objects detected in our images critically
depend on the age of the cluster, we can confidently state that the
completeness limit of our observations probes the brown dwarf regime
for any plausible age of the cluster. Indeed, a star at the
hydrogen-burning limit (M = 0.078
)
reaches an absolute
magnitude MK = 6.5 (corresponding to our completeness limit K =
18.1 minus the adopted DM = 10.6 of the cluster, obscured by a
foreground extinction of AK = 1 mag) at an age of 6 Myr (Baraffe
et al. 2003). The age of the IRAS 16362-4845 cluster is
likely to be much less than that (see Sects. 9 and 4).
It is interesting to compare the results obtained here with the
claim made in CSR05 about the apparent lack of cluster members in
the
,
diagram lying below an extinction vector having
its origin near the position of a main-sequence A0 star. Given the
severe incompleteness of the observations presented in CSR05 above
in the area of the cluster due to the bright
nebulosity, the region of the color-magnitude diagram that was
claimed to be devoid of stars in that work is effectively delimited
by the polygon joining the points
,
and
(1.3, 13.6), where only the likely
foreground star CSR-013 is detected in their observations. Our
present work reveals four stars in that area, whose non-detection in
CSR05 is readily explained by examining their location: one of them,
CSR-012E, is a faint companion just 1
1 from the bright star
CSR-012A. The second is CSR-008B, very close (0
22) to the blue,
likely foreground star CSR-008A. The third is CSR-008A itself, to
which CSR05 allocated an infrared excess that our new observations
show to be actually due to the presence of the much redder
component B. The true
color is consistent with it being a
foreground, lightly reddened star when component B is excluded. Finally, the
fourth star CS-104, is faint and near CSR-012A, only 1
5. The
depopulation of that area of the color-magnitude diagram in CSR05 is
thus real, as only close companions to other stars populate it in
the present study.
Near-infrared observations, most notably in the
band,
present well known advantages for the observational characterization
of the population of a cluster: they allow one to probe down to very
low masses given that most of the luminous output of the least
massive stars and brown dwarfs lies in the near-infrared, and they
reduce the effects of extinction with respect to shorter wavelength
bands. Nevertheless, the
band also probes a spectral region
where circumstellar emission can significantly contribute to the
luminosity of star and even dominate over the photosphere, and the
greater transparency of dust at that wavelength increases the level
of contamination of background sources for clusters residing in
clouds of low or moderate column density. Although the large dynamic
range in luminosity covered by our observations is in principle a
very useful resource to study the stellar population of
IRAS16362-4845 over a wide range of masses, the particular
conditions of the cluster require special care in both deriving and
interpreting the properties of its members.
To approximately correct the observed magnitudes for the effects
of extinction and circumstellar emission, let us write the observed
magnitude of a star in each of the J, H, and bands
as
![]() |
(1) |
![]() |
(2) |
![]() |
(3a) |
![]() |
(3b) |
![]() |
(3c) |
where J0i, H0i, KS0i are the magnitudes
given by the models at the isochrone point i. The solution to
system (3) yields the values of the three unknowns ,
AK, and
EK. It may be noticed that a similar procedure has been recently
used by Figuerêdo et al. (2005) in their analysis of
the cluster associated to the giant HII region G333.1-0.4.
They correct for infrared excess assuming that it is noticeable only
in
,
and then correct of extinction by dereddening along the
limiting reddening vector in the (J-H),
diagram marking
the boundary of the region accessible by objects free of infrared
excess. As those authors point out, this procedure yields a lower
limit to EK, as well as an overestimate of AK, and is
equivalent to setting
in Eq. (2). However, both models
and observations suggest that the excess at H cannot be neglected
in general. Furthermore, the method that we have used does not use
the limiting reddening vector, but an actual reddening vector having
its origin at the photospheric colors of the best fitting
theoretical model instead.
Table 4: Objects with infrared excess.
For the stars having measured magnitudes in the J, H, and filters we have solved the system (3) for
,
taking
as the best solution the one for which
,
i.e., for
which the best fitting model is an interpolation between consecutive
points along the isochrone. As may be noticed in Fig. 5,
isochrones within a certain range of young ages present a kink
giving rise to a small mass interval in which stars of a given mass
are brighter than those slightly more massive, due to their slower
evolutionary rate. In those narrow intervals it is possible to find
more than one solution to the system (2) with
.
This is of little practical concern, as the values of AK and
EK, and of J0, H0, KS0 are very similar for all
solutions. The derivation of the K luminosity function (see
Sect. 3.5) is thus hardly affected by the choice of the
solution in those kinks.
The fitting procedure described above yields unphysical solutions
for some objects, with slightly negative values of either AK or
EK that may be due to photometric errors, to the simplifications
adopted for the reddening and infrared excess corrections, or to
deviations between the colors predicted by the models and those of
the actual stars. We have dealt with the cases where the solution to
Eqs. (3) yields EK < 0 by solving the system again by least
squares, setting EK to zero. On the other hand, the only case in
which the solution to Eqs. (3) yields AK < 0 corresponds to the
object farthest to the right of the limiting reddening vector in the
(J-H),
diagram. For this star we have assumed that all
the excess lies in the
band and we have solved Eqs. (3) again,
now replacing the coefficient 0.39 in Eq. (3b) that accounts for the
H-band excess by zero, thus obtaining solutions in which both
AK and EK are positive. This might be an object possessing a
large central hole devoid of the hot dust component that provides
the main contribution to the infrared excess at the shorter
wavelengths.
Finally, many of the fainter objects in our sample have
measurements in only two bands, generally H and .
We have
assumed that they do not have infrared excess and have solved the
relevant subset of Eqs. (3) for
and AK. Since our results
for the objects with measurements in J, H, and
show that
somewhat more than half do not require infrared excess, and that for
those that do the appropriate value of EK is generally small (see
Sect. 3.4) we do not expect that the assumption of EKfor objects with only one measured color introduces a significant
bias.
The list of stars for which an infrared excess in the K band is
needed to obtain a good fit to the colors predicted by the models
using Eqs. (3) is given in Table 4. Taking into
account that the total number of sources having
measurements
in our sample is 44, our results imply that infrared excesses need
to be assumed for 45% of the stars in order to fit their
colors. This fraction is uncertain and the derived infrared excess
may be spurious in some cases, as the values of EK often are of
the order of the combined uncertainties arising from the photometry
and the modeling of the circumstellar emission. Indeed, even in the
cases where the fit suggests the presence of infrared excess we find
EK < 0.75 (implying that most of the emission arises from the
photosphere rather than from reprocessing by the circumstellar
environment) for 15 out of 21 stars, and only 4 objects require EK
> 1.0. It must be noted that the limitation of this analysis to
stars detected at J, H, and
leads us to consider objects
that have cleared most of their circumstellar envelopes and are thus
detectable at those wavelengths. Observations at longer wavelengths
reveal few additional objects that have not reached that stage, and
are discussed in Sect. 3.7.
The luminosity function in the K band is frequently used in studies of young clusters as a diagnostic tool of the mass function and the star formation history of their stellar populations. Pioneering work on the interpretation of the K luminosity function was presented by Zinnecker et al. (1993). More recently, Muench et al. (2000) have carried out extensive modeling of the K luminosity function (hereafter KLF) showing its dependence on factors such as the cluster age, the spread of star formation over time, and the choice of theoretical pre-main sequence evolutionary tracks.
To derive the KLF of
IRAS 16362-4845 we have derived intrinsic
magnitudes using the
method described in the previous Section for all the stars detected
in at least two of the
bands. We have excluded a few sources
with
,
as they are most likely foreground stars. Also,
we have considered components of close binary systems as separate
stars only when reliable photometry could be obtained for each of
them (see Sect. 3.6). Since we are excluding the
Easternmost area of the imaged field due to the apparent extinction
hole in that region, as described in Sect. 3.2, we expect the
population analyzed here to be strongly dominated by cluster
members, as the thick dust column provides an efficient screen
against the background population even at the
band. A few
sources have very red colors and may be background to the cluster,
but as discussed in Sect. 3.7 this is unlikely to be the
case even for the reddest sources.
The KLF that we derive is potentially affected by several
biases that must be carefully taken into account. In addition to the
varying detection thresholds across the field discussed in
Sect. 2.3, the varying level of foreground extinction
implies that the sampled volume of the cluster becomes progressive
smaller for intrinsically fainter sources, as the most obscured
objects of a given intrinsic luminosity fall below the detection
limit. On the other hand, some faint members that would have
remained undetected if their emission were purely photospheric may
be brought above the detection threshold by their infrared excess.
However, based on the moderate or altogether absent -band
excesses that we generally obtain for the stars having
photometry, we estimate the influence of infrared excesses to be
small in the derivation of the KLF.
To minimize the bias due to the variable sampling of the cluster
volume due to extinction, we have obtained a KLF by considering only
stars having
AV < 23.8 and MK < 4.0. Both constraints are
related by the condition that the intrinsically faintest and most
obscured stars in such sample are at the overall completeness limit
of our observations (see Table 3). Indeed, a star
with MK = 4.0 obscured by
AV = 23.8 mag would have H=19.1,
and would thus be detected anywhere in the cluster, as
would be any star with a brighter absolute magnitude and reddened by
a smaller amount. Our choice of the limiting absolute magnitude and
extinction is a compromise between the need to include a
statistically representative sample of the cluster population (which
requires a limiting extinction as high as possible) and to probe the
KLF down to low enough masses. Concerning the latter, MK = 4.0corresponds to a mass
at an age of 1 Myr,
the precise mass being only approximate due to the large
uncertainties in low-mass evolutionary tracks at such early ages
(Baraffe et al. 2002). Our magnitude-limited KLF,
presented in Fig. 8, thus covers nearly the whole range of
stellar masses.
![]() |
Figure 8: The K luminosity function (KLF) of the IRAS16362-4845 embedded cluster, expressed as number of stars per 1 mag bin. The solid line corresponds to the extinction-limited sample that considers members with line-of-sight extinctions AV < 23.8, which is complete down to MK = 4.0. An unrestricted KLF including all the cluster members (as well as possibly a few very reddened background stars) is represented by the dashed line. This latter KLF is virtually complete for the brightest bins, but becomes progressively incomplete as fainter stars become undetectable due to extinction. Finally, the KLF derived by Muench et al. (2002) for the Trapezium cluster is shown for reference, scaled by an arbitrary factor for convenience in the representation. The absolute magnitude scale for the Trapezium has been set by assuming its distance modulus to be 8.0, as used by those authors. No correction for extinction has been applied to the Trapezium KLF. Given that the vast majority of the Trapezium members have extinctions below AK = 0.5 (see Fig. 4a of Muench et al. 2002), a mean extinction correction would shift the Trapezium KLF by less than one bin towards the left. |
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The most intriguing feature of the KLF of the IRAS 16362-4845 cluster is its flatness over virtually the entire absolute magnitude range covered by our observations, which supports the hints of a top heavy luminosity function reported in CSR05. Similar indications are found in the KLF independently derived by Wolk et al. (2007) for the X-ray selected population of RCW 108, where the central region containing IRAS 16362-4845 has a KLF remarkably flatter than that of the surrounding area. This is in contrast with the results of most other studies of embedded clusters in the literature, which obtain KLFs rising towards fainter absolute magnitudes, with a particularly sharp increase in number counts in the 0 < MK < 2 interval. For reference, we show in Fig. 8 the shape of the KLF of the Trapezium cluster (Muench et al 2002). Similar results are obtained on the clusters embedded in galactic giant HII regions (Figuerêdo et al. 2005, and references therein), as well as for other embedded clusters containing massive stars (e.g. Leistra et al. 2006; Massi et al. 2006; Fujiyoshi et al. 2005; Muench et al. 2003, for recent work). Although very low mass stars and even brown dwarfs appear to be present in the cluster coexisting with its most massive members, they do so in numbers far smaller than expected from a normal IMF (e.g. Kroupa 2001).
It is hard to explain the flat shape of the extinction-limited KLF as a result of incompleteness or incorrect assumptions solely. The steepness of the KLFs of other clusters is well detected at MK < 2, which is two full magnitudes brighter than the limit of our extinction- and magnitude-limited sample. The flatness of our KLF that we obtain thus cannot be attributed to the possible inaccuracy of the limiting magnitudes listed in Table 3.
On the other hand, an underestimated infrared excesses would lead
to a derived absolute magnitude that is brighter than the actual
one, as both the extinction and the photospheric flux are then
overestimated. Therefore, the possibility exists in principle that
objects populating the brighter bins of Fig. 8 may actually
be intrinsically fainter objects with strong infrared excess that
has not been properly taken into account. The required size of the
discrepancy between the actual and the derived infrared excesses
should be significantly larger than the bin size of our KLF in order
to have a noticeable impact on its shape. However, such large
infrared excesses would imply negative extinctions for many of the
objects of the cluster, and we can thus rule out this as a
significant contributing effect. A systematic underestimate of the
infrared excess may also have taken place among the objects
undetected at J, as we have assumed EK = 0 for them. However,
those tend to be faint objects at H and
contributing to the
faintest magnitude bins of the KLF, which may thus contain objects
that would have been excluded had the infrared excess been properly
taken into account. Since the lack of J measurements is more
common towards fainter magnitudes, the systematically neglected
infrared excesses among such objects should lead to our KLF being
steeper than the actual one, and thus cannot account for the
derived flatness. Finally, a systematic underestimate of the
luminosity among the brightest objects may also result if the value
of
that we have adopted in Eqs. (2) were smaller than the
actual one, leading to the underestimate of the excess at H and
the invalidity of our assumption
.
We note
nevertheless that a larger value of
would imply that the
dust responsible for the infrared excess should be significantly
hotter than the dust sublimation temperature in order to produce
colors similar to those of the underlying objects, which we consider
highly implausible. We are thus inclined to consider the remarkably
flat shape of the KLF of IRAS 16362-4845 as a real feature of
the cluster, and we discuss its implications in
Sect. 4.
The mass of the IRAS 16362-4845 cluster was crudely estimated by
CSR05 to be about 210 .
The new observations allow us to
refine that estimate by providing a deeper and more accurate census
of its members, based on the more detailed estimate of individual
stellar parameters described in Sect. 3.3. The cluster
mass derived from stars detected in at least two of the
bands amounts to 370
.
The difference is due to the fact
that CSR05 based their estimate on the number of stars above the
absolute magnitude of a main sequence A0V star and assuming that the
cluster population follows a log-normal Miller &
Scalo (1979) IMF, rather than on the individual masses.
Applying the same method to the new observations presented here
taking into account the stars more massive than 1
(the
dashed line in Fig. 5) we obtain a cluster mass of only
120
.
The lower masses obtained with this method are easily
explained by the fact that the mass function underlying the KLF is
flatter than the log-normal Miller & Scalo (1979) IMF,
and that the average mass of the stars above a certain threshold is
therefore greater than the average mass expected from the assumed
IMF. We note by passing that our new determination, 370
,
changes little due to the incompleteness of the census at very low
masses, since their contribution to the cluster total mass is very
small given the flatness of the IMF. It is also fairly insensitive
to the assumed age, as the massive stars dominating the mass of the
cluster evolve very rapidly towards the main sequence.
It is a well known observational fact that binaries with mass ratios close to unity are very common among massive stars (e.g. Garmany et al. 1982; Mason et al. 1998; Preibisch et al. 1999; García & Mermilliod 2001). Close binarity among massive stars actually places important constraints on their proposed formation mechanisms (e.g. Bonnell et al. 1998; Yorke & Sonnthaler 2002; Bonnell 2005; Beuther et al. 2007) and has been usually linked to the fact that virtually all massive stars are formed in dense cluster environments, where dynamical interactions among cluster members and with the accreting gas can dominate their early evolution. Such interactions favor the formation of bound systems, while subsequent accretion causes high mass ratios and orbital evolution towards shorter separations between their members (Bate et al. 2003; it may be noted however that a high fraction of wide binaries with low mass ratios has been observed in the young but dynamically evolved cluster NGC 6611 by Duchêne et al. 2001). Observations of dense clusters containing very young massive stars, or even massive stars still deeply embedded in their parental material, suggests that close binarity arises very early in their evolution (Bosch et al. 2001; Apai et al. 2007).
The diffraction-limited quality of our observations in the L' band allows us to search for visual pairs among the bright members
of the IRAS 16362-4845 cluster with angular separations below
,
corresponding to a projected distance of 130 AU, and
photometry of the individual components can be obtained for pairs
separated by more than
.
We constrain ourselves to
the central area of the cluster covered by the L' images
(Sect. 2.1), as the image quality in that band (and
therefore the shortest separations that can be measured) is nearly
uniform across the entire field. We also consider exclusively the
brighter pairs in which both components are detected at L', since
crowding at fainter magnitudes in the H or
bands makes it
impossible to separate true wide binaries from chance alignments or
unrelated sources. We have set the upper limit of the separation for
stars to be considered as close pairs to
(1430 AU projected
distance), as this is the distance of the farthest star of the
distinct subcluster of five members around CSR 012A, where the
stellar density in our images peaks (see Fig. 3).
Unfortunately our results are of limited value in order to determine
the physical characteristics of the detected systems, given the
frequent existence of excess emission in the L' band
(Sect. 3.2) and the lack of a precise age determination of the
cluster, which prevent us from deriving masses and mass ratios.
However, our findings should include all the massive stars in the
cluster with separations in the
range, and can thus form a
basis for future statistical studies of its massive binary star
content.
Table 5: Close pairs in the L' band.
The multiple systems that we detect are listed in
Table 5. These systems include the CSR 012 cluster of five
members, the rather loose possibly triple system CSR 010, and the
likely casual arrangement of unrelated objects CSR 008. The bluest
component of the latter, CSR 008A, is the brightest star in the
nebula at visible wavelengths and the very light reddening indicated
by its visible and infrared colors suggests that it is a foreground
star. However, it was noted in CSR05 that its infrared photometry
shows a clear -band excess hinting at true cluster membership.
The superior resolution of the observations presented here offer the
solution to the puzzle, showing that CSR 008A has a redder companion
at only 0''22. Although such close chance alignment is highly
unlikely, we believe that the widely different infrared colors of
both components convincingly argues for true membership in
IRAS 16362-4845 of component B only, leaving component A as an
ordinary foreground star. Finally, we include in our list star
CS-093, which appears elongated in the
-band images and shows a
faint, short tail to the South in the L'-band images. We consider
this to be most likely due to a fainter, marginally resolved
companion at
(80 AU projected distance) from the
primary star.
Excluding the likely foreground source CSR 008A and considering
CS-093 as two separate stars, the results listed in
Table 5 indicate that 18 of the 42 sources detected in the
L' band reside in 7 multiple systems with projected separations
from the primary between 80 and 1430 AU, out of which 5 are binary
systems, corresponding to a multiplicity fraction of 0.43. As noted above, our criterion
of detection of both components in the L' band is rather loose in
terms of the lowest masses represented in Table 5, as some
components may be detected thanks to their infrared excess rather
than to the intrinsic brightness (and mass) of the central star.
Assuming that the frequency and amount of L' excesses are the same
among components of multiple systems and single stars, the
multiplicity fraction that we have derived above should nevertheless
remain unaffected by this caveat. In principle such assumption may
not be taken for granted, as multiplicity can affect the timescale
for dissipation of the inner disks responsible for the L' excess.
However, we find no evidence for any systematic differences between
members of multiple systems and single stars when considering their
positions in the
,
diagram
(Fig. 7).
As illustrated by the discussion of the observations of NGC 6611 by Duchêne et al. (2001), completeness corrections in binarity studies require that one carefully takes into consideration instrumental effects, contamination by unrelated sources, extrapolations beyond the range of separations probed by the observations, and assumptions on the statistics of orbital parameters. Comparisons among different studies are furthermore hampered by the different observing techniques, instruments, and wavelength regions used. Leaving aside the difficulties of an unbiased comparison, we can say that our results indicate a high multiplicity fraction among the brightest members, and the lower limit that we find (at least 43% of the massive stars in the cluster residing in binaries) suggests that, like in other clusters, most of the massive stars in IRAS 16362-4845 may be part of multiple systems.
Table 6:
Very red objects detected at .
The bulk of the stars projected on the area of the nebula and its
surroundings have near infrared colors indicating extinctions in the
10 < AV < 30 magnitude range. However, a few objects listed in
Table 6 display
colors in excess of 2.5 mag,
indicating extinctions near or in excess of
.
Of
these, three (CS-010, 014, and 042) lie in the low extinction zone
to the East of the cluster noted in Sect. 3.1
and excluded from our analysis of the cluster, and are probably
distant luminous stars for which most of the foreground extincion is
unrelated to the RCW 108 complex.
Of the remaining four, CSR-006 has a derived extinction of AV =
72 mag which is in good agreement with the extinction produced by
the molecular cloud on the foreground in that direction, as derived
from the 13CO maps presented in CSR05. However,
Fig. 1 shows that the star lies precisely behind
a narrow lane of dust that also obscures the nebula in that
direction. It may thus be that the star is actually a cluster
member, as supported by its also red
color suggestive of
infrared excess. CS-070 and CS-141 are both faint objects with
rather uncertain H-band measurements that may have led to an
overestimate of the extinction in their direction, especially in the
case of CS-070 which is close to our threshold for the
identification of possible background objects. Finally, CS-094 is
also very red in
and may actually be a low-mass object
with substantial infrared excess, possibly also in the
-band.
Since it is not detected at J we derived the extinction by
assuming that the emission at
is photospheric
(Sect. 3.3), which leads to an overestimate of the
extinction if that assumption is incorrect. We can summarize our
results regarding very red sources by stating that we find no strong
evidence for any of these four objects being a background star, thus
confirming that the dense column of molecular gas located behind the
cluster provides an effective screen against such sources.
With only one exception, all the sources detected at L' are also
seen in the
images. The exception is CS-109, a source well
detected with
L' = 11.53 projected outside the boundaries of the
bright nebulosity to the North of CSR-012 subcluster. The darkness
of the background in that region and the good adaptive optics
correction that is attained so close to the wavefront sensing star,
which is only 10
away, allow us to set a very stringent limit of
at its position, implying
.
It is
difficult to account for such extremely red color by assuming that
CS-109 is a background star with normal colors obscured by the full
column of dust in the molecular cloud. The necessary extinction is
AV > 150 mag, which is over twice that derived from the 13CO
maps but is not ruled out on small areas given the large beam size
of the millimeter observations. However, the absolute magnitude of
an object reddened by such amount should be brighter than
ML' =
-7.9. The limit assumes that the object is placed just behind the
cloud at the distance of RCW 108, and that it is just below the
detection limit in our
images. While all this is in principle
possible (such an absolute magnitude could be attained by a bright
Mira variable or a red supergiant), we deem the confluence of
enumerated factors leading to its detection extremely unlikely.
An alternative possibility is that CS-109 is a protostellar object
still deeply embedded in its parental core. To investigate it we
have examined archive Spitzer images of the region obtained with the
IRAC camera between 3.6 and 8.0 m, which should help in
extending or constraining the spectral energy distribution beyond
the L' band. The object is faintly detected at 4.5
m and
marginally detected at 5.8
m. The non-detection at 3.6
m
is in consistency with the flux measured in our L'-band images,
and an upper limit is obtained at 8
m. The measurement of
fluxes in the two bands where the object is detected is made
difficult by the steep brightness gradient of the nebulosity at its
position, which prevents us from accurately determining the
background emission level. Our measurements are listed in
Table 7. CS-109 appears as a point source in our
images despite their excellent resolution, implying the absence of
extended structure with a linear size larger than about 70 AU.
Table 7: The candidate embedded protostar CS-109.
Figure 9 shows the spectral energy distribution of this
source. Despite the large uncertainty in the IRAC fluxes, it seems
clear that the distribution rises towards longer wavelengths over
the entire interval covered by our observations, consistent with it
being a Class I or flat spectrum source, commonly interpreted as an
embedded protostar (e.g. Adams et al. 1987). We must note
however that it is unusual for Class I sources to be unresolved down
to the scale probed by the L' observations (Whitney et al. 1997; Kenyon et al. 1998; Haisch et al. 2006). Deep diffraction-limited VLT observations in
the 4-10 m range should would be very valuable in confirming
the true nature and spectral energy distribution of CS-109.
![]() |
Figure 9: Spectral energy distribution of the protostellar candidate CS-109. |
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IRAS 16362-4845 is a relatively nearby example of the earliest evolutionary stages of a massive star forming cluster. Its youth is supported by its location embedded in the RCW 108 molecular cloud, the evidence for warm circumstellar material around a sizeable fraction of its members, the existence of one likely embedded protostar, and the fact that the HII region produced by the hottest stars of the cluster still has not excavated the surrounding molecular cloud beyond the boundaries of the cluster. However, perhaps the most important characteristic of the cluster is the possibility that it is an example of externally triggered star formation, for which additional tentative evidence in the region has been reported by Urquhart et al. (2004) and CSR05. It is thus interesting to discuss our findings in the light of this possibility.
The most intriguing feature of the IRAS 16362-4845 cluster found in the present study, which we have discussed at length in Sect. 3.5, is the unusually flat KLF. Translating the shape of the KLF to a shape of the IMF requires a number of assumptions (Muench et al. 2000), mainly about the age and star formation history of the cluster that are at present largely unknown. However, the flatness of the KLF strongly suggests a top heavy IMF with a deficit of low-mass stars. A second, and possibly related, unusual feature of the cluster is that, contrarily to what is observed in most clusters containing massive stars, the most massive members are not at the center. At near-infrared wavelengths the cluster is dominated by CSR-012A and its subgroup of companions, but several other bright stars for which we find similar or even brighter absolute magnitudes are scattered over the whole area of the cluster: most notably, CSR-006 and CSR-020 are candidate O-type stars based on their absolute magnitudes, in addition to CSR-012A, and are far removed from it.
The central location of the most massive stars in most clusters is thought to be of primordial nature due to their preferential formation at the bottom of the cluster potential well, rather than a result of dynamical evolution (Bonnell et al. 2007). This may play an important role in determining the shape of the IMF as a function of the distance to the cluster center in the competitive accretion scenario, leading to the preferential formation of low mass stars in the peripheral regions of the cluster. Low-mass stars might also be depleted at the centers due to coalescence to form more massive stars. We observe no hints of such mass segregation among the detected members of IRAS 16362-4845. Low-mass stars appear all across the cluster, but they do so in small numbers as implied by the shape of the K luminosity function (Fig. 8). Furthermore, it is obvious from Fig. 1 that the CSR-012 subcluster does not occupy a central position, but is rather located near the northern edge. The overall lack of low-mass stars might be interpreted in terms of competitive accretion as a result of the relatively widespread presence of massive stars, which would hamper the formation of low mass stars everywhere in the volume of the cluster rather than in the central regions only, contrarily to the more common case of clusters that have their most massive members at their centers.
Modern theoretical studies of the build-up of the IMF, based on the turbulent fragmentation and gravitational collapse of an isolated molecular cloud (see e.g. Bonnell et al. 2007, and references therein), have proven to be quite successful in reproducing both the shape of the IMF and the mass segregation evidences observed in many clusters. However, the mass spectrum resulting from externally triggered star formation might be markedly different from the one resulting from those studies, as noted by Zinnecker (1989), and remains largely unexplored. Indirect evidence for significant differences in the IMF resulting from triggered star formation, leading to the preferential formation of intermediate- and high-mass stars, has been reported (Sugitani et al. 1991; Dobashi et al. 2001; Getman et al. 2007). A recent study by Negueruela et al. (2007) of the young cluster NGC 1893, which contains both both early-type O stars and evidence for triggered star formation, also reports indications of a top-heavy IMF. Clearly, more modeling work is needed in order to obtain more quantitative predictions on the IMF resulting from triggered star formation (Elmegreen 2007). However, it may be noted that already the early modeling of the sequential star formation scenario by Elmegreen & Lada (1977) predicted the preferential formation of massive stars. The reasons for this are related to the warm temperature of the gas in the shocked layer located ahead of the ionization front produced by the triggering massive stars, to turbulence induced by Rayleigh-Taylor instabilities in this shocked layer, and to possible coalescence of unstable fragments in this layer. Observations like the ones reported here may provide the strongest constraints on future theoretical work.
The new observations of IRAS 16362-4845 presented in this paper represent a great improvement over previously existing ones, in terms of both depth and resolution, and give access to a more detailed study of its stellar population. The main conclusions of our work can be summarized as follows:
Acknowledgements
We are pleased to acknowledge the advise of Dr. Lowell Tacconi-Garman at the ESO User Support Department in the preparation of our Service Mode observations. The Paranal Science Operations staff is warmly thanked for the careful execution of this program. We also thank Dr. Francesco Palla for making available to us the evolutionary tracks used in this paper, and Dr. Hans Zinnecker for useful comments on an early draft of this paper. The constructive comments of the referee, Dr. August Muench, helped improve the paper and are greatly appreciated.
Table 2: Photometry of sources detected in the IRAS 16362-4845 cluster.