Issue |
A&A
Volume 515, June 2010
|
|
---|---|---|
Article Number | A56 | |
Number of page(s) | 13 | |
Section | Catalogs and data | |
DOI | https://doi.org/10.1051/0004-6361/200912226 | |
Published online | 09 June 2010 |
Recent star formation at low metallicities. The star-forming region NGC 346/N66
in the Small Magellanic Cloud from near-infrared VLT/ISAAC observations
,![[*]](/icons/foot_motif.png)
D. A. Gouliermis1 - J. M. Bestenlehner1,2 - W. Brandner1 - T. Henning1
1 - Max-Planck-Institut für Astronomie, Königstuhl 17,
69117 Heidelberg, Germany
2 -
Armagh Observatory, College Hill, Armagh BT61 9DG, UK
Received 29 March 2009 / Accepted 27 February 2010
Abstract
Context. The emission nebula N66 is the brightest H II
region in the Small Magellanic Cloud (SMC), the stellar association
NGC 346 being located at its center. The youthfulness of the
region NGC 346/N66 is well documented by studies of the gas and
dust emission, and the detection in the optical of a rich sample of
pre-main sequence (PMS) stars, and in the mid- and far-IR of young
stellar objects (YSOs). However, a comprehensive study of this region
has not been performed in the near-IR that would bridge the gap between
previous surveys.
Aims. We perform a photometric analysis on deep, seeing-limited
near-IR VLT images of the region NGC 346/N66 and a nearby control
field of the SMC to locate the centers of active high- and
intermediate-mass star formation by identifying near-IR bright objects
as candidate stellar sources under formation.
Methods. We use archival imaging data obtained with the
high-resolution camera ISAAC at VLT of NGC 346/N66 to construct
the near-IR color-magnitude (CMD) and color-color diagrams (C-CD) of
all detected sources. We investigate the nature of all stellar
populations in the observed CMDs, and we identify all stellar sources
that show significant near-IR excess emission in the observed C-CD. We,
thus, select the most likely young stellar sources.
Results. Based on their near-IR colors, we select 263 candidate
young stellar sources. This sample comprises a variety of objects, such
as intermediate-mass PMS and Herbig Ae/Be stars and possible massive
YSOs, providing original near-IR colors for each of them. The spatial
distribution of the selected candidate sources indicates that they are
located along the dusty filamentary structures of N66 seen in mid- and
far-IR dust emission and agrees very well with that of previously
detected candidate YSOs and PMS stars.
Conclusions. Our study provides an original accurate set of
near-IR colors for candidate young stellar sources. This provides
significant information about the star formation process in
NGC 346/N66, but does not establish the types of these objects,
which requires the construction of complete spectral energy
distributions for individual sources from multiwavelength data. This
would be an important follow-up study to that presented here.
Key words: stars: pre-main-sequence - Magellanic Clouds - open clusters and associations: individual: NGC 346 - HII regions - ISM: individual objects: LHA 115-N66 - catalogs
1 Introduction
Located in the stellar constellation Tucana, the Small Magellanic Cloud (SMC), is
an excellent laboratory for investigating the star formation processes and the associated
chemical evolution of dwarf galaxies. Its present subsolar chemical abundance (Z=0.004;
20% of solar) implies that this galaxy may have characteristics similar to those in
earlier times of the evolution of the universe. Since SMC is so close to our Galaxy, it is
therefore an excellent laboratory among the large collection of dwarf irregulars and blue
compact galaxies for the study of resolved extragalactic stellar populations and star-forming
regions. The young stellar association NGC 346 (RA (J2000) = 00
59
18
,
DEC (J2000) = -72
10'48'') is a large star-forming cluster in the SMC,
located at a distance of about 60.6 kpc from us (Hilditch et al. 2005). It is embedded in the
brightest H II region of the SMC, which is referred to as LHA 115-N66 or N66
(Henize 1956). With an H
luminosity almost 60 times higher than the star-forming
region of Orion (Kennicutt 1984), N66 has a diameter of about 7' corresponding to
approximately 123 pc.
The star-forming region NGC 346/N66 comprises a variety of young stellar populations
(Gouliermis et al. 2006). The stellar association NGC 346, located at the center of the so-called
nebular ``bar'' of N66, hosts the largest sample of O-type stars in the entire SMC (Walborn et al. 2000; Evans et al. 2006; Massey et al. 1989). Studies based on deep imaging with the Hubble Space Telescope
show that the vicinity of the whole region of NGC 346/N66 is also very rich in low- and
intermediate-mass PMS stars, some of which exhibit recent star formation with an age 5 Myr,
while others belong to an older underlying population of age
10 Myr (see e.g., Hennekemper et al. 2008).
Additional evidence of the youthfulness of this region comes from observations with the Spitzer Space Telescope and the discovery of 111 candidate massive young stellar objects (YSOs) with
(Simon et al. 2007). Nevertheless, although these objects should emit in
near-IR bands, no detailed study in such wavelengths exists in the literature.
Pioneering work on the dust and gas content of NGC 346/N66 was performed by Rubio et al. (2000)
and Contursi et al. (2000), who found a correlation between H2
infrared emission and CO lines, characteristic of a photo-dissociated region (PDR).
A PDR develops when the far-UV radiation of the bright OB stars reaches the
surface of the parental molecular cloud. The degree of ionization decreases
outwards, and a thin barrier develops that segregates the ionized from the
atomic gas. While H2 is not fully ionized behind this front but partly
dissociated, CO molecules located little deeper in the cloud are more easily
dissociated by absorbing UV photons. Rubio et al. (2000) and Contursi et al. (2000)
inferred that star formation in NGC 346/N66
has taken place as a sequential process in the ``bar'' of N 66, which these authors
define as the oblique bright emission region extending from southeast to
northwest centered on NGC 346. This process results in several embedded
sources, seen as IR-emission peaks in the 2.14 m H2 line and
the ISOCAM LW2 band (5-8
m). These peaks are alphabetically
numbered from ``A'' to ``I'' (Contursi et al. 2000; Rubio et al. 2000), the association
NGC 346 itself coinciding with peak ``C''.
![]() |
Figure 1:
Mosaic images from the combination of the northern and center
jittered ISAAC frames of the area of NGC 346/N 66 in the a) J-;
b) H-; and c) |
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Studies with the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope (HST) have shown that the PMS stellar content of NGC 346/N66 covers a mass range in the subsolar regime. These studies suggest that recent star formation occurred around 3-5 Myr ago (Nota et al. 2006; Sabbi et al. 2007) there being an underlying older PMS population, which indicates that there were earlier star formation events that occurred about 10 Myr ago (Hennekemper et al. 2008). The PMS population is mainly centrally concentrated apart from the association NGC 346 in a number of subclusters with clustering properties quite similar to those of Milky Way star-forming regions (Schmeja et al. 2009). Furthermore, the intermediate-age star cluster BS 90 (Bica & Schmitt 1995) with an age of about 4.5 Gyr (Rochau et al. 2007) is also located in this part of the SMC, projected in front of N66.
This photometric study of the star-forming region NGC 346/N66 focuses on
identifying young stellar sources, mainly intermediate- and high-mass
PMS stars of the region, stars that have not yet started their lives on the main
sequence. The PMS phase in the evolution of stars with masses up
to 6 M
corresponds to the time between the gravitational
core collapse, which forms the protostar (on the birthline), and the ignition
of hydrogen in the formed star, placing it on the Zero-Age Main Sequence
(ZAMS). During this evolutionary phase, the observed radiation from the
star is affected significantly by circumstellar disks of dust and gas,
formed by matter infalling during the collapse of the rotating core, and
surface activity. Examples of these typical PMS stars, the T Tauri stars,
thus, exhibit prominent optical emission lines, which are
understood to stem from chromospheric heating, periodic fluctuations in
light that indicate rotating star-spots, variability, and excess broadband
flux in UV and IR, and are sometimes associated with molecular
outflows, winds, or accretion (e.g., Lada & Kylafis 1991). Intermediate-mass
(
)
PMS stars are called Herbig AeBe
(HAeBe) stars (e.g., Perez & Grady 1997). Being more massive
analogues of T Tauri stars, they are PMS A- and B-type stars that exhibit
emission lines produced by both strong stellar winds and the cocoons of
remnant gas from which they collapsed. They typically contain circumstellar
disks and therefore have spectral energy distributions (SEDs) of young
stellar objects (YSOs) of class II. They range in age between 0.5 and 5 Myr,
similar to T Tauri stars. Because of the aforementioned
characteristics, PMS stars are visible in near-IR wavebands.
While extensive observations have been performed of young stellar sources in the region NGC 346/N66 at optical (Nota et al. 2006; Gouliermis et al. 2006) and mid- and far-IR (Bolatto et al. 2007; Simon et al. 2007) wavelengths, there has been no comparative investigation in the near-IR. The present study aims to close this gap in the available spectral coverage for this region by acquiring near-IR data to characterize with greater accuracy its young stellar population. This will also aid our understanding of star formation in the low-metallicity environment of the SMC. In this paper, we present our ground-based near-IR photometry derived from observations with VLT/ISAAC of the region NGC 346/N66. We present the observational material used and its reduction in Sect. 2, and we discuss the photometric process in Sect. 3. The various observed stellar populations and the corresponding stellar systems comprised in the observed field are discussed in terms of variations in the constructed color-magnitude diagrams in Sect. 4. We present the constructed near-IR color-color diagram of all detected sources and apply a selection criterion by identifying stars currently forming by means of their near-IR excess emission in terms of their positions in this diagram in Sect. 5. In the same section, the spatial distribution and the nature of the selected young stellar sources is also discussed. Finally, conclusive remarks on this study are given in Sect. 6.
2 Observations and data reduction
Red giants, stars close the end of their life usually have
high mass-loss rates, while stars at the earliest stages of their
formation are embedded into dense molecular clumps and cores. In both
cases, the dust shells or disks around the objects of interest absorb almost all
of the visible radiation, which is re-radiated at longer wavelengths. As a
consequence, absorption decreases very rapidly with increasing
wavelength (e.g., Glass 1999; Joyce 1992), the extinction coefficient
at 2.2 m being approximately 10% of that at 500 nm.
Here we are interested in young stellar sources associated
with circumstellar dusty shells or disks, characterized by bright
IR excess emission.
2.1 Observations
The near-IR images of NGC 346/N66 are obtained
within the ESO Program ID 63.I-0329 (PI: M. Rubio) with
the Infrared Spectrometer And Array Camera (ISAAC), mounted on
the Nasmyth-Focus B of UT1 (Antu) at the Very Large Telescope
(VLT). The images were taken between 24 and 26 Sep. 1999,
under fair seeing conditions (FWHM between 0.6
and 1.0
).
They were used partly to detect YSOs in the
region of N66 (Simon et al. 2007), but have never been presented in their
complete spatial and wavelength coverage. The observations were
performed with the short wavelength arm of ISAAC, using the
1024
1024 HgCdTe Hawaii Rockwell array. The pixel scale of
the Hawaii detector is 0.1484
/pixel, providing a maximum
field-of-view of 152
152 arcsec2. The images were
obtained in the filters J (1.25
m), H (1.65
m), and
(2.16
m).
At wavelengths longer than 2.2
m,
the thermal background is dominated by atmospheric and telescope emission,
leading to a highly variable sky brightness in the infrared. Detector
cosmetics and instabilities also illustrate the need for an accurate sky subtraction.
This is achieved by mean of the technique of
jittering
,
which is available for the short-wavelength and some
long-wavelength imaging templates of ISAAC. A set of 10 to 100 frames are
combined to form one final ``jittered'' frame. At the beginning and end of each
night, twilight flats are taken for each filter. The
following morning, dark images of the detector are taken for all
detector integration times (DITs) used during the previous night.
The area of NGC 346 was divided into two jittered frames, each consisting
of 10 single frames. An additional jittered frame was taken for a control field of the galaxy,
located to the south of the system. For each single frame, the total integration
time was 60s, taken in 5s (DIT) exposures of 12 sub-integrations (NDIT).
The field-of-view is about 2.5
2.5
,
corresponding to about
44
44 pc2 at the SMC (distance from us 60.6 kpc; Hilditch et al. 2005).
We retrieved the dataset of these observations, including the science frames,
twilight flats, dark images, and standard stars from the ESO Science Archive
Facility
.
2.2 Data reduction
![]() |
Figure 2:
Typical photometric uncertainties in the three J,
H, and |
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To reduce the data, we used
ESO's ISAAC pipeline, which is based entirely on the libraries of the
data reduction package ECLIPSE.
The package consists of the main eclipse-library and the additional instrument
specific pipeline packages. Another algorithm
of ECLIPSE is the jitter routine, which implements efficient
filtering and processing methods for infrared data reduction. The detector
pixels have intrinsically different sensitivities because of the quantum efficiency
variations from pixel to pixel. There is also a readout gradient of the array,
which itself is not homogeneously illuminated by the telescope.
These effects, as well as artifacts caused by dust on the optical surface,
are corrected within the data reduction process.
The dark current of the ISAAC Hawaii array is low, so the detector bias, also called the ``zero level offset'', is the dominant feature of the dark frames. A master dark, constructed with ECLIPSE, was subtracted from the science frames. The effects of dust, the inhomogeneity of the illumination, and pixel to pixel variations were then removed using a master flat-field frame. Since no flat-field screen exists at UT1, for ISAAC only twilight flats are available. For the subtraction of both sky and bias, the darkest twilight flat frame was used. After this subtraction, the remaining 15 twilight flats were combined to produce a master flat-field with ECLIPSE, which also creates a bad pixel map output file. The jitter routine in ECLIPSE was then used to reduce the science data for the frames observed with acceptable seeing, by filtering out low-frequency sky variations from the set of jittered images, a method called sky combination. Two sets of jittered images were combined for the whole area of NGC 346. The corresponding final frames are shown for each filter in Fig. 1. The association NGC 346, the numerous young compact star clusters located in the bright HII-region N 66, and the intermediate-age star cluster BS 90 are easily identified in these images. The photometry, discussed in the following section, was performed on the single jittered frames for each filter. The derived catalogs were then matched to produce the complete photometric catalog of the observed regions.
3 Photometry
Our photometry was performed within the Image Reduction and Analysis
Facility (IRAF) system, with the
package DAOPHOT
(Stetson 1987). The effect of both the quality of the imaging system and
the seeing conditions on the obtained magnitudes of the stars is
quantified by the point spread function (PSF), which represents
the image of a typical point source in the observations.
DAOPHOT provides accurate PSF photometry of crowded stellar
fields. The magnitudes of all stars detected in the observed jittered images are
defined relative to the brightness of Vega.
We first have to specify the input parameters of DAOPHOT to optimize
the detection and photometry of stars, before using the routine daofind to
detect all stellar sources in the science frames. The routine
phot is then used to perform
aperture photometry of the detected stars and determine their
instrumental magnitudes. The radius of the aperture is selected
to be between 1 and 2 times that of the FWHM, depending on the effective
seeing. For individual stars, the aperture radius can be up to 5 or more of
that of the FWHM.
Table 1:
Sample of the photometric catalog of stars found in this study
in the region of NGC 346/N66 and a nearby control field in all three
J, H, and
filters with VLT ISAAC imaging.
The typical PSF of our images was constructed by interactively selecting
approximately 20 isolated bright stars per frame using DAOPHOT
pstselect. The PSF was modelled to be the
sum of an analytic bivariate Gaussian function and empirical corrections
from the best Gaussian of the true observed brightness values within
the average profile of several stars in the image. This process and the computation of
the PSF to be fitted was performed by the routine psf. PSF photometry was
performed with the routine allstar, which
after classifying the stars in groups, compiles a catalog of the
most likely candidate stars, based on
their PSF fitting and the physical conditions, and subtracts them from
the original image. The photometric process was repeated for the
subtracted frames and the magnitudes of its newly discovered stars were
determined. Approximately 30% more stars were found in the region of
NGC 346 from the second photometry run, but most of them have
large photometric uncertainties. The final numbers of identified stars in each
filter is 11 900 in J, 6 406 in H, and 5 837 in .
We match
these photometric catalogs with a procedure developed in IDL
to identify stars in common. After selecting the stars with the highest photometric
quality, i.e., stars with photometric uncertainties equal or smaller than 0.1 mag,
we identified 2 783 stars in both the J and H filters, 3 067 stars in J
and
,
2 550 stars in H and
,
and 2 506 stars in all three
filters collectively.
![]() |
Figure 3:
Completeness for all stars detected in the three
observed fields in all three J, H and |
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![]() |
Figure 4: Chart of all stars detected with good photometric accuracy in all three filters in all three fields observed with VLT/ISAAC. This map shows the relative positions of the observed fields: The northern field that includes the intermediate-age cluster BS 90, the central field, where the association NGC 346 and the main part of the H II region N66 are located, and the remote control-field, which covers the representative stellar populations of the general SMC field in the area. North is up and East is to the left of the map. |
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3.1 Completeness and photometric accuracy
Typical uncertainties in our photometry are shown in
Fig. 2. The completeness of our photometry is
evaluated by artificial star experiments, by adding artificial
stars into the original frames with the DAOPHOT
subroutine addstar. A few hundred artificial stars were
added to each observed frame for every filter. This process
was repeated several times, and the completeness of our photometry was
derived by applying the photometric process to each new frame and
comparing the final stellar catalogs with those of the original artificial stars based
on the coordinates of the stars. The derived completeness of our photometry
is shown in Fig. 3 for all three bands. Based on the
completeness measurements and the photometric uncertainties, the
limiting magnitudes of our photometry are J 22.0 mag,
H
20.5 mag, and
20.5 mag.
3.2 Photometric calibration
We calibrate the magnitudes of the stars with near-IR absolute photometry
of the same field from the Two Micron All Sky
Survey (2MASS). The 2MASS
filter system
agrees very well with that of ISAAC, and
the 2MASS Point Source Catalog (All-Sky 2003) is therefore particularly suitable for the calibration of our
photometry. The application ALADIN
was used
to identify all stars found with 2MASS in the same
field-of-view as ours and to compare the magnitudes
measured from both ISAAC and 2MASS data sets. To calculate the magnitude offsets
between the two photometries, we selected more than 20 bright stars with the highest quality
photometric data from our photometric catalog in each filter. The comparison of our photometry with that
of 2MASS is shown in Fig. 5. The offsets per filter
are calculated in terms of the median of the differences between the magnitudes of the
stars, which are common to both ISAAC and 2MASS samples. The apparent
magnitude offsets are 1.812 mag in J, 0.678 mag
in H, and 0.367 mag in
.
The uncertainties in this calibration can be quantified
by the standard deviations derived from the dispersion in 2MASS-ISAAC magnitudes
for the common stars (Fig. 5 - bottom). They are
mag,
mag, and
mag, respectively.
3.3 The photometric catalog
We derived celestial coordinates for all stars detected with high quality photometry
(
0.1 mag) from their pixel coordinates in the final
jitter FITS images and used the applications xy2sky and sky2xy
available from the World Coordinate Systems (WCS) Tools
(ver. 3.7.2). We
transformed the (X,Y) coordinates of the stars into celestial coordinates according
to the astrometric corrections provided in the FITS header
of the corresponding jittered images by applying xy2sky. We then
transformed the celestial coordinates of all stars with the use of sky2xy
into a common (X,Y) pixel coordinate system with respect to the central field, where NGC 346/N 66
is mainly observed, by using the astrometric corrections provided in the FITS header
of the
image of this field. A sample of the final compiled catalog of the stars
found in all three filters is given in Table 1. This table is available in its entirety
at the CDS. In Fig. 4, the map of all sources detected in all three wavebands is shown.
4 Observed stellar populations
In our subsequent analysis, we consider only sources detected with the highest
quality photometric parameters and with photometric uncertainties based on
the PSF fitting of
0.1 mag in every waveband. The region
NGC 346/N66 is known to host a mixture of stellar populations
(e.g., Gouliermis et al. 2006), including the evolved SMC field stars,
the young main-sequence (MS) and pre-main sequence (PMS) populations of
the association NGC 346 and its vicinity (e.g., Hennekemper et al. 2008),
and the
4.5 Gyr-old faint MS and bright RGB stars of the
cluster BS 90 (Rochau et al. 2007).
4.1 Color-magnitude diagrams
The color-magnitude diagrams (CMDs) were compiled from our photometry
for all stars detected in both the area of NGC 346/N66 (where BS 90
is also included) and the control field, specifically the J-H versus (vs.)
H,
vs.
,
and
vs.
CMDs, shown in Fig. 6. The variety of stellar
types in the observed region is illustrated by the superimposed
isochrone models. These evolutionary models, which are designed for
the ESO Imaging Survey WFI UBVRIZ and SOFI JHK VEGAmag
systems, were developed by Girardi et al. (2002). The superimposed isochrones,
corresponding to ages
4 Myr and
4.5 Gyr, were
selected to correspond to the metallicity of the SMC, namely Z=0.004 (Bouret et al. 2003; Haser et al. 1998). Both the bright MS of NGC 346 and the red giant branch (RGB) of
the field and BS 90 are clearly defined in the CMDs of Fig. 6,
these features being the most clearly distinguished in the
,
CMD. To determine the interstellar
reddening of the observed stellar populations, we assume a Galactic
interstellar extinction law
where RV is roughly equal to






4.2 Variations in the color-magnitude diagram
The stellar systems NGC 346 and BS 90 represent two quite different types of clusters both from morphological and evolutionary point-of-views. Their evolutionary difference is naturally based on the stellar member populations of each system, which should define the prominent features in the corresponding CMDs of the systems. This becomes clearer when the CMDs of selected areas centered on these systems, comprising the most representative stellar populations in the systems, are constructed. In Fig. 7, the




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Figure 5:
Comparison of our photometry with that derived by 2MASS
for more than 20 common stars, located in all three observed fields
for all three J, H, |
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![]() |
Figure 6:
CMDs of all stars found in all observed fields of the
region NGC 346/N66 and its nearby control field of the SMC. CMDs
in different combinations of the three bands are shown. Specifically,
a) J-H, H; b)
|
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The bright main sequence stars in NGC 346/N66 are
known to be O-, B-, or early A-type stars of masses between 3 and
60 M
,
while its low- and intermediate-mass stellar content is
characterized by a large sample of pre-main sequence (PMS) stars
(e.g., Hennekemper et al. 2008). The most massive of these stars,
such as Herbig Ae/Be stars, are possibly also located in the red part of the
main-sequence of Fig. 7a. However, our
photometry is not in general deep enough to detect significant numbers of low-mass PMS stars.
The part of the CMD between the MS and the RGB should also host classical
Be stars (see e.g., Bik et al. 2006). To perform a more accurate
identification of these sources and to utilize the near-IR three-band
color-color diagram of the detected sources in the young association NGC 346,
we focus our subsequent analysis on the central area of NGC 346/N66.
![]() |
Figure 7:
|
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5 Stellar sources under formation
There are no comparable studies to ours in the SMC, but there have been a few previous near-IR photometric investigations of resolved populations in star-forming regions of the low-metallicity environment of the Large Magellanic Cloud (LMC). A combination of ground-based near-IR data and space optical observations of the 30 Dor Nebula by Rubio et al. (1998) helped identify numerous stellar IR sources in or near the bright nebular filaments west and northeast of R136, suggesting that a new stellar generation is being produced by the energetic activity of the massive central cluster in the remanent interstellar material around its periphery. Near-IR photometry of the second largest HII region in the LMC, N11B, also highlighted several bright IR sources (Barbá et al. 2003). Several of these sources have IR colors with YSO characteristics, and they are prime candidates to be intermediate-mass Herbig Ae/Be (HAeBe) stars.
Deep near-IR imaging of the N159/N160 star-forming region in the LMC detected
candidate HAeBe stars down to 3 M
,
based on their near-IR
colors and magnitudes (Nakajima et al. 2005). Two embedded HAeBe clusters were also
discovered, one of them, and two neighboring H II regions, providing hints
of the beginning of sequential cluster formation in N159S. The spatial distributions of the
HAeBe and OB clusters indicate large-scale sequential cluster formation over the entire
observed region from N160 to N159S. Near-IR photometry obtained to study the stellar content of the
LMC star-forming region N4 is used to study the connection between the interstellar
medium and its stellar content (Contursi et al. 2007). This analysis found several
bright IR sources with characteristics of massive, early O-type stars. However,
according to these authors, IR spectroscopy of these sources would confirm
their very young and massive nature.
Chen et al. (2009) presented the most complete identification of
YSO candidates in the LMC H II complex N44. These authors combined
mid- and far-IR Spitzer Space Telescope imaging with complementary
ground-based imaging in UBVIJK to classify the YSOs into Types I, II, and
III, according to their spectral energy distributions (SEDs). In their sample, 65%
of these objects were resolved into multiple components or extended sources.
The distribution of the YSO candidates compared with those of the underlying
stellar population and interstellar gas illustrates a correlation between the current
formation of O-type stars and previous formation of massive stars, providing evidence
of triggered star formation in N44.
In connection to the aforementioned studies, our photometry, although not very deep,
provides original near-IR colors of candidate young sources in the vicinity
of N66. However, from near-IR photometry
alone it is impossible to accurately determine the nature of the
most IR-bright sources, unless this photometry is part of a multi-band
optical and IR investigation, as we discuss later. In the present study,
we provide only a first set of accurate near-IR colors for all young objects
in the region, and select the most interesting IR-bright objects from our
photometric catalog, based on their near-IR colors and their near-IR
excess inferred from the J-H,
color-color diagram (C-CD).
This selection is described in the following section.
5.1 Selection of young stellar sources
To identify the sources that represent the most recent star
formation in the region, we first consider the contribution of both
the evolved MS and RGB stellar populations of the SMC field,
the cluster BS 90, and the association NGC 346 to the complete
stellar sample. As shown in Figs. 6 and
7, the MS is well defined with its brighter members
belonging to the association, but it is its red part and the region between
the MS and RGB that host stellar sources at their earlier stages of
formation (see e.g., Bik et al. 2006). Therefore, and to eliminate
the significant contamination of this part of the CMD by the
general field and BS 90, which is very close to the central part of N66,
we constrain our analysis to the ISAAC FOV that covers the main body
of NGC 346/N66, defined as the central field with
in the map of Fig. 4.
Color-composite images of
this field, which covers the association NGC 346, the bar of N66, and
several subclusters and HII regions in its vicinity, are shown in
Fig. 8. The dust emission, seen in the 8
m band,
indicates the centers of the most recent star formation and demonstrates
the different information revealed by different wavelengths
for the same region.
The use of the near-IR C-CD is a reliable method for detecting
sources, which are characterized by near-IR excess emission,
from their positions in the diagram. In this diagram, the
evolved populations are easily identified by comparison with
models and considering interstellar extinction, PMS stars, namely
classical and weak-line T Tauri (CTT, WTT)
stars (e.g., Appenzeller & Mundt 1989), Herbig Ae/Be (HAeBe) stars
(e.g., Perez & Grady 1997; Waters & Waelkens 1998), and other YSOs (e.g., André 1994; Lada & Wilking 1984) are located in the reddest part. This is a signature of their
IR excess due to their circumstellar dust in the form of cocoons or
disks. Hence, when selecting candidate YSOs and
PMS stars, we consider only the sources that are detected in the
central ISAAC field in all three
wavebands. The
,
CMD and J-H,
C-CD of these sources are shown in Fig. 9.
Considering that BS 90 to be a large cluster with a tidal radius between
2
15 (Sabbi et al. 2007) and 3
15 (Rochau et al. 2007),
its spatial extent covers the central field of NGC 346/N66, and consequently
affects the stellar content of the region. This is clearly demonstrated
in the CMD of Fig. 9, where a prominent
RGB can be seen. While these stars are bright in the near-IR, they
do not exhibit any significant excess emission. Our
classification to identify young
stellar sources based on their near-IR excess, therefore allows us to effectively
discard most of the evolved stellar contaminants from our sample of selected
young sources in formation. However, a definite selection requires
a sophisticated multi-band study on an individual source basis, as we discuss in
Sect. 5.2.
![]() |
Figure 8:
Color-composite images of the main part of the region
NGC 346/N66. Left: Image constructed with the combination
of the jittered images in J (blue), H (green), and |
Open with DEXTER |
![]() |
Figure 9:
|
Open with DEXTER |
![]() |
Figure 10:
Selection of the sources identified to have a near-IR excess based on their
locations in the
|
Open with DEXTER |
In the
vs.
CMD of Fig. 9, the
areas occupied by HAeBe (Eiroa et al. 2002) and classical Be stars (Dougherty et al. 1994) are
delineated by two ellipses, and the positions of massive Galactic YSOs identified
by Hanson et al. (1997) and Bik et al. (2006) are indicated by green stellar symbols.
In the C-CD of Fig. 9, the typical positions of T Tauri (TTS),
HAeBe, and classical Be stars are drawn to indicate the loci,
where these sources in our sample should be expected in this diagram.
We note, however, that typical TTS are fainter than the detection limit of
our photometry and therefore are barely discernible in the
CMD of Fig. 9, and only a few of them being seen in the C-CD.
We refer, however, to these PMS stars for reasons of completeness.
The diagrams of Fig. 9 appear to comprise
a large variety of different stellar types at quite different evolutionary
stages. The regions in the CMD and C-CD, where classical Be stars
are typically expected, also include MS populations. Searching for Be stars,
Keller et al. (1999) found in six fields centered on young clusters of the
Magellanic Clouds (NGC 346 included), that the average
fraction of Be to normal B stars is similar to that found in the Galaxy
(
20%, see, e.g., Cox 2000, p. 414). No connection between
the Be star fraction and age or metallicity was found by these authors,
and the classical Be stars detected in NGC 346 do not
have any influence on the evolution of the region. According to their near-IR
colors, red giants and subgiants are expected, to
occupy the blue edges of the typical locations of HAeBe
stars. As a consequence, we should be careful to differentiate the sources that
are most probably at their earlier stages of evolution, based on their
near-IR excess, from the evolved red stars of the region.
Based on the discussion above, we make a first tentative selection
of the sources that most probably have near-IR excess using a diagram
of the color fraction
of all detected objects as a function of their brightness in
.
In this diagram, shown in Fig. 10
(left), one can see that most sources are located along a horizontal
sequence of stars with
,
corresponding
to the RGB and MS stars that exhibit no near-IR excess. Young stellar sources
with a near-IR excess should be located away from the horizontal sequence.
We first assume that all sources with
should not have any near-IR excess, as they are located
blueward of the MS in the C-CD of Fig. 9.
We then apply a first-order selection of the remaining sources based on the criterion that
an absolute offset from the horizontal sequence in the color fraction
of about 0.3 is a reasonable limit to separate the
stars with near-IR excess from those that show no excess. As a consequence,
we select as sources with near-IR excess those that have color fractions
or
in the
diagram of Fig. 10 (left).
However, an investigation of the positions of these near-IR bright sources in the
CMD of Fig. 9 shows that there is significant contamination of
these objects by classical Be and RGB stars, which should be eliminated.
Therefore, we place yet tighter constraints on our sample by selecting only the
sources that fall at the positions
expected for HAeBe stars as shown in Fig. 9 (left) and redder.
More precisely, we select the sources that fulfill the following criteria: (i) They are
located to the blue part of the MS in the C-CD with
.
(ii) They
have fractions of color indices
or
.
(iii) They are located to the red part of a diagonal line that
tangentially crosses the blue edge of the HAeBe area, specified in the
,
CMD of Fig. 9. These sources
represent our final sample of objects selected as the most likely star-forming candidates,
and the subject of our analysis here. The positions of these sources
are indicated by red points in the diagram
vs.
of Fig. 10.
Their positions in the C-CD are also shown in Fig. 10
(right panel). In this C-CD, it is indeed evident that our selected sample
probably corresponds to objects with strong near-IR excess emission.
![]() |
Figure 11:
Positions of stellar sources in our photometric catalog, with
strong near-IR excess emission, based on their positions
in the J-H,
|
Open with DEXTER |
5.2 The sample of candidate young stellar sources in NGC 346/N66
The selection scheme returned 263 candidate young stellar sources in the main
part of the region NGC 346/N66. The positions of these sources are
shown in a map of this area in Fig. 11 as circular blue points. They are
superimposed on
a color-composite image constructed by combining our ISAAC image
in the
filter (green component) with archived images
obtained with the Infrared Array Camera (IRAC; Fazio et al. 2004)
onboard the Spitzer Space Telescope and in particular channel
4 (8.0
m, red component), within the GTO science program 63
(PI: J. R. Houck). These IRAC data were used to detect candidate YSOs
in the general region of N66 by Simon et al. (2007).
The objects identified by these authors as ``definite'' YSOs are also
plotted in Fig. 11 with large red circular symbols.
In this map the positions of the detected
candidate YSOs in NGC 346/N66 clearly correspond to dusty structures
seen in the 8 m emission as red filaments, these objects being
located at
peaks of mid-IR emission in these filaments. These peaks can
also be seen - at lower spatial resolution - in the
24
m band of Multiband Imaging Photometer for
SIRTF (MIPS; e.g. Rieke et al. 2003; Heim et al. 1998) onboard
Spitzer, and they are also observed in the 2.14
m H
line and the ISOCAM LW2 band that covers
5-8
m (Contursi et al. 2000; Rubio et al. 2000). We note that
the spatial distribution of our candidate young stellar sources
follows the filamentary dust emission,
being concentrated along a few southern dusty arcs (at the middle
and bottom of the map) and one northern dusty arm (at the
top of the image). The latter, along with the south-eastern
filament, is understood to be the product of a more recent
triggered star formation event (Gouliermis et al. 2008).
Hennekemper et al. (2008) discuss the spatial
distribution of the low-mass PMS stars detected by
Gouliermis et al. (2006), and of sources with
excess H
emission that are the most likely candidates
to be intermediate-mass PMS stars (their Figs. 1 and 2).
The spatial distribution of all these objects follows
the same trend as our sources, and it is probable that
both catalogs include several objects in common. In addition, the positions
of a significant number of candidate young stellar sources found in
our near-IR photometry, shown in Fig. 11, also seem
to be quite clustered in several concentrations. We also note that almost all of these
concentrations coincide with subclusters of PMS stars observed
with HST/ACS identified by applying two cluster
analysis techniques (Schmeja et al. 2009).
We first compared a nominal search-box of 2
of the catalog of our sources with that of the YSOs found
with Spitzer by Simon et al. (2007) and PMS stars with H
excess found with Hubble by Hennekemper et al. (2008) in the
same region. This comparison returns more than
40% of
the H
excess stars and
60% of the YSOs also detected in
the near-IR by ourselves. There are cases where more than one objects
found with Hubble coincide with one of our sources, while
in other cases a few of our sources are found as components of one
of the YSOs found with Spitzer within the specified search-box.
This phenomenon is caused by differences in the
resolving efficiency of the three instruments, demonstrating
the importance of resolving multiple sources into their
true components and classifying their true nature. The resolution
achieved by VLT/ISAAC is at least 10 times higher than that of
Spitzer/IRAC, allowing a reasonable identification of any components
in multiple YSOs. However, 15% of our sources coinciding with
H
-excess objects identified by Hennekemper et al. (2008),
are resolved by HST/ACS to be multiple systems themselves.
Therefore, the differences in resolution between the
data sets obtained with HST, VLT, and Spitzer is a crucial issue
in identifying true single-objects, or the components
of multiple systems in our sample. As we discuss later, to perform
a more detailed study in the near-IR, observations of the highest
possible spatial resolution are essential.
It is almost certain that our sample of candidate PMS stars and YSOs is incomplete, since for example TTS are not included because of our detection limit. Other young stellar sources such as HAeBe stars may also be missing due to the strong constraints of our selection criteria. In addition, our sample may be contaminated by evolved stars, which in general do not exhibit any significant near-IR excess. While the positions of most of the sources in our catalog coincide with the dust filaments of N66, illustrating their youthfulness, there are candidate young sources in our sample that are located away from the dusty filaments of Fig. 11. These are usually assumed to be evolved stars, or even background galaxies, rather than stellar sources in formation. However, decontaminating our sample of old stars in the field and BS 90 located in the central region of NGC 346/N66 on a statistical basis for such a small sample, or based on their positions away from the dust filaments, will possibly compromise the catalog of sources with true near-IR excess and it produce selection effects.
A more sophisticated selection on a source-by-source basis is certainly required to identify the most prominent objects that represent the most recent star formation in the region. This, however, can only be achieved through the excessive use of imaging in many different wavebands and/or spectroscopy (e.g., Chen et al. 2009), so that complete SEDs of individual sources can be constructed and consequently their true nature accurately defined (e.g., Whitney et al. 2008). Observations of higher spatial resolution will allow the components of any unresolved bright sources in our sample to be recognized. Such a thorough analysis would certainly include the use of previous observations of the region NGC 346/N66 from various telescopes at different wavelengths including those of HST (Hennekemper et al. 2008; Gouliermis et al. 2006) and Spitzer (Bolatto et al. 2007; Simon et al. 2007), as well as new near-IR observations of higher spatial resolution and sensitivity. A preliminary study of the available data and the preparation of follow-up observations in the near-IR is currently being performed by ourselves.
6 Conclusions
We have presented a detailed near-IR photometric study with VLT/ISAAC
of the star-forming region NGC 346/N66 in the SMC. We have used
archival ISAAC imaging of the general area of N66, which
includes the stellar association NGC 346, the intermediate-age
cluster BS 90, and a southern empty control field of the SMC. We
have performed photometry on images obtained in the filters J (1.25 m),
H (1.65
m), and
(2.16
m) and derived a catalog
of more than 2 500 stars detected in all three wavebands. The
color-magnitude diagrams of these stars include a collection of
different stellar populations, comprising the young MS stars
of the association mixed with the RGB and old MS stars of BS 90
and the general field of the SMC, but also objects that are at their
earlier stages of their formation. We select the best PMS and YSO
candidates in our sample on the basis of their positions in the
near-IR color-color and color-magnitude diagrams.
We focus the selection of these sources on the central field observed with ISAAC, which covers only the main part of the nebula N66 and the association NGC 346 to avoid any severe contamination of our sample with the evolved red stars of BS 90 and the field. In this area, our photometry detected 1 174 stars in all three wavebands for which the near-IR CMD and C-CD are constructed. In these diagrams, the evolved stellar populations are mostly aligned along the sequences of RGB and MS stars as they are expected to be by the evolutionary models depending on the interstellar extinction, but certainly contaminate the sample of young stellar sources. The reason is that, while these sources, such as low-mass T Tauri stars, intermediate-mass Herbig Ae/Be stars, and massive YSOs, exhibit excess emission in the near-IR due to circumstellar dust, their positions in the CMD and C-CD do not necessarily cover the reddest part.
Bearing this in mind, we make a tentative selection of PMS and YSO
candidates with
(redder than MS and RGB)
in the diagram of the color fraction
as a function
of the
brightness. In this diagram, sources that have
an excess in their near-IR colors are located away from the horizontal
sequence of evolved stars. We select the sources that have an offset
from the horizontal sequence of
and we decontaminate the sample by selecting sources located in the
area of the
,
CMD that HAeBe stars are expected
to occupy and with redder colors. We consider
the selected objects as the most probable candidates
to be stars in formation. This selection delivers 263 candidate
young stellar sources, which are located along the dusty filamentary
structures of N66 seen in the 8
m emission from Spitzer
and the 2.14
m H
line.
Combining observations at several wavelengths to construct complete SEDs
of individual sources is the most accurate means of establishing their true
nature. Objects from our catalog of young stellar sources do indeed coincide
with candidate YSOs detected with Spitzer and sources with excess emission
in H
in the region observed with Hubble. However, since a large amount
of data per object is required for detailed SED studies, it is necessary to enhance
the available data sets with new data, preferably obtained with cameras of higher
resolving power, so that multiple objects can be resolved in their components.
We thank M. Rubio and R. H. Barbá for their comments and suggestions. D.A.G. kindly acknowledges the support by the Deutsche Forschungsgemeinschaft (DFG) through grant GO 1659/1-2. This research has made use of the SIMBAD database, operated at the CDS, Strasbourg, France, of NASA's Astrophysics Data System, and images obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. It is also based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 063.I-0329.
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Footnotes
- ... observations
- Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 063.I-0329.
- ...
- Table 1 is available in its entirety only in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A56
- ...
jittering
- http://www.eso.org/projects/dfs/papers/jitter99/
- ...
Facility
- http://archive.eso.org/cms/
- ... ECLIPSE
- http://www.eso.org/eclipse
- ... system
- http://iraf.noao.edu/
- ... DAOPHOT
- http://iraf.noao.edu/scripts/irafhelp?daophot
- ...
Survey
- http://irsa.ipac.caltech.edu
- ... ALADIN
- http://aladin.u-strasbg.fr/aladin.gml
- ... Tools
- Available at http://tdc-www.harvard.edu/wcstools/
All Tables
Table 1:
Sample of the photometric catalog of stars found in this study
in the region of NGC 346/N66 and a nearby control field in all three
J, H, and
filters with VLT ISAAC imaging.
All Figures
![]() |
Figure 1:
Mosaic images from the combination of the northern and center
jittered ISAAC frames of the area of NGC 346/N 66 in the a) J-;
b) H-; and c) |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Typical photometric uncertainties in the three J,
H, and |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Completeness for all stars detected in the three
observed fields in all three J, H and |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Chart of all stars detected with good photometric accuracy in all three filters in all three fields observed with VLT/ISAAC. This map shows the relative positions of the observed fields: The northern field that includes the intermediate-age cluster BS 90, the central field, where the association NGC 346 and the main part of the H II region N66 are located, and the remote control-field, which covers the representative stellar populations of the general SMC field in the area. North is up and East is to the left of the map. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Comparison of our photometry with that derived by 2MASS
for more than 20 common stars, located in all three observed fields
for all three J, H, |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
CMDs of all stars found in all observed fields of the
region NGC 346/N66 and its nearby control field of the SMC. CMDs
in different combinations of the three bands are shown. Specifically,
a) J-H, H; b)
|
Open with DEXTER | |
In the text |
![]() |
Figure 7:
|
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Color-composite images of the main part of the region
NGC 346/N66. Left: Image constructed with the combination
of the jittered images in J (blue), H (green), and |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
|
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Selection of the sources identified to have a near-IR excess based on their
locations in the
|
Open with DEXTER | |
In the text |
![]() |
Figure 11:
Positions of stellar sources in our photometric catalog, with
strong near-IR excess emission, based on their positions
in the J-H,
|
Open with DEXTER | |
In the text |
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