The stellar population and complex structure of the bright-rimmed cloud IC 1396N

Abstract

Context. IC 1396N is a bright-rimmed cloud associated with an intermediate-mass star-forming region, where a number of Herbig-Haro objects, H2 jet-like features, CO molecular outflows, and millimeter compact sources have been observed. Aims. To study in detail the complex structure of the IC 1396N core and the molecular outflows detected in the region and to reveal the presence of additional YSOs inside this globule. Methods. We carried out a deep survey of the IC 1396N region in the J, H, K' broadband filters and deep high-angular resolution observations in the H2 narrowband filter with NICS at the TNG telescope. The completeness limits in the 2MASS standard are Ks~17.5, H~18.5 and J~19.5. Results. A total of 736 sources have been detected in all three bands within the area where the JHK' images overlap. There are 128 sources detected only in HK', 67 detected only in K', and 79 detected only in H. We found only few objects exhibiting a Near-Infrared excess and no clear signs of clustering of sources towards the southern rim. In case of triggered star formation in the southern rim of the globule, this could be very recent, because it is not evidenced through Near-Infrared imaging alone. The H2 emission is complex and knotty and shows a large number of molecular hydrogen features spread over the region, testifying a recent star-formation activity throughout the whole globule. This emission is resolved into several chains or groups of knots that sometimes show a jet-like morphology. The shocked cloudlet model scenario previously proposed to explain the V-shaped morphology of the CO molecular outflow powered by the intermediate-mass YSO BIMA 2 seems to be confirmed by the presence of H2 emission at the position of the deflecting western clump. New possible flows have been discovered in the globule,

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arXiv:0902.4543v1 [astro-ph.GA] 26 Feb 2009
Astronomy & Astrophysics manuscript no. beltran
February 26, 2009
c ? ESO 2009
The stellar population and complex structure of the bright-rimmed
cloud IC 1396N
M. T. Beltr´ an1, F. Massi2, R. L´ opez3, J. M. Girart4, and R. Estalella3
1Universitat de Barcelona, Departament d’Astronomia i Meteorologia, Unitat Associada a CSIC, Mart´ ı i Franqu` es 1, 08028
Barcelona, Catalunya, Spain
2INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Mart´ ı i Franqu` es 1, 08028 Barcelona, Catalunya, Spain
4Institut de Ci` encies de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ci` encies, Torre C-5, 08193, Bellaterra, Catalunya, Spain
Received date; accepted date
ABSTRACT
Context. IC 1396N is a bright-rimmed cloud associated with an intermediate-mass star-forming region, where a number of Herbig-
Haro objects, H2jet-like features, CO molecular outflows, and millimeter compact sources have been observed.
Aims. To study in detail the complex structure of the IC 1396N core and the molecular outflows detected in the region and to reveal
the presence of additional YSOs inside this globule.
Methods. We carried out a deep survey of the IC 1396N region in the J,H,K′broadband filters and deep high-angular resolution
observations in the H2narrowband filter with NICS at the TNG telescope. The completeness limits in the 2MASS standard are Ks∼
17.5, H ∼ 18.5 and J ∼ 19.5.
Results. A total of 736 sources have been detected in all three bands within the area where the JHK′images overlap. There are
128 sources detected only in HK′, 67 detected only in K′, and 79 detected only in JH. We found only few objects exhibiting a
Near-Infrared excess and no clear signs of clustering of sources towards the southern rim. In case of triggered star formation in the
southern rim of the globule, this could be very recent, because it is not evidenced through Near-Infrared imaging alone. The H2
emission is complex and knotty and shows a large number of molecular hydrogen features spread over the region, testifying a recent
star-formation activity throughout the whole globule. This emission is resolved into several chains or groups of knots that sometimes
show a jet-like morphology. The shocked cloudlet model scenario previously proposed to explain the V-shaped morphology of the
CO molecular outflow powered by the intermediate-mass YSO BIMA 2 seems to be confirmed by the presence of H2emission at the
position of the deflecting western clump. New possible flows have been discovered in the globule, and some of them could be very
long. In particular, the YSO BIMA 3 could be powering an old and poor collimated outflow.
Key words. ISM: individual objects: IC 1396N, IRAS 21391+5802– ISM: jets and outflows – ISM: lines and bands – infrared: ISM
– stars: formation
1. Introduction
Bright-rimmed clouds (BRCs) found in Hii regions are potential
sites of triggered star formation due to compression by ioniza-
tion/shock fronts. Many of them are associated with IRAS point
sources with cold color indices (low dust temperature), which
are most probably Young Stellar Objects (YSOs) or protostars.
Such clouds are of deep interest from the point of view of on-
going star formation. They frequently contain a small cluster of
Near-Infrared(NIR) stars that is elongatedtowardthe bright-rim
tip or the ionizingstar(s) of the Hii regionwith the IRAS sources
situated near the other end. There is a tendency for bluer (i.e.,
older) stars to be located closer to the ionizing star(s), and for
redder(i.e.,younger)stars to be closer to the IRAS sources. This
asymmetric distribution of the cluster members strongly sug-
gests small-scale sequential star formationor propagationof star
formation from the side of the ionizing star(s) to the IRAS posi-
tion in a few times 105yr, as a result of the advance of the shock
caused by the UV radiation from the ionizing star(s) (Sugitani
et al. 1995). Thus, BRCs represent one of the best laboratories
for studying the star-formation process at different evolutionary
stages.
Send offprint requests to: M. T. Beltr´ an, e-mail: mbeltran@am.ub.es
A good example of BRC with ongoing star-formation ac-
tivity is IC 1396N (BRC38; Sugitani et al. 1991), located in
the Cep OB2 association at a distance of 750 pc (Matthews
1979), and exposed to UV radiation from the O6.5 star HD
206267.Theregionis associatedwithIRAS 21391+5802,avery
young intermediate-mass object with a luminosity of 235 L⊙
(Saraceno et al. 1996), which is powering an extended CO bipo-
lar outflow (Sugitani et al. 1989). Beltr´ an et al. (2002) have
resolved the millimeter emission towards IRAS 21391+5802
into an intermediate-mass source named BIMA 2 surrounded
by two less massive and smaller objects, BIMA 1 and BIMA 3.
Recent higher angular resolution millimeter interferometric ob-
servations have revealed that the intermediate-mass protostar
BIMA 2 consists in fact of multiple compact sources (Neri et al.
2007). The gas emission surrounding IRAS 21391+5802 traces
different molecular outflows (Codella et al. 2001; Beltr´ an et al.
2002, 2004), some of them possibly being powered by yet unde-
tected YSOs (Beltr´ an et al. 2004).Beltr´ an et al. (2002) havecon-
ducteda detailedstudy ofthe bipolaroutflowassociated with the
intermediate-mass protostar BIMA 2, and shown that its com-
plex morphology and kinematics are possibly the result of the
interaction between the outflow and the dense cores surround-
ing the protostar. NIR images of the region have also revealed

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2 Beltr´ an et al.: IC 1396N
the presence of a number of small scale molecular hydrogen
and Herbig-Haro (HH) flows (Nisini et al. 2001; Sugitani et al.
2002a; Reipurth et al. 2003; Caratti o Garatti et al. 2006). This
evidence for ongoing star-formation activity at the head of the
cometary globule together with the relatively proximity of the
region make IC 1396N one of the best candidates to study po-
tential sequential star formation.
To do a complete and uniform census of the young stellar
population in the globule and reveal the presence of additional
young sources inside the globule, deep NIR observations at J,H
and K′have been carried out. In addition, deep high angular res-
olution observations in the S(1) v=1–0 line of H2at 2.12 µm
have also been performedto investigate the complexstructure of
this globule, and the morphology of the shocked gas that traces
the interaction between the outflows in the region and the dense
clumps surrounding the YSOs. The results of this NIR study are
presented here.
2. Observations and data reduction
The images were taken with NICS (Baffa et al. 2001) at the
3.58-m Telescopio Nazionale Galileo (TNG) telescope (ORM,
La Palma, Spain) through the standard J,H,K′broadbandfilters
and the H2narrowband filter centered at 2.12 µm, during the
nights between 16–17 July 2005. The plate scale is 0.25′′/pixel,
yielding a field of view of ∼ 4.2 × 4.2 arcmin2. Both in K′and
in H2, two positions roughly 100′′apart (east-west) were im-
aged, such as to have an overlappingfield, ∼ 150′′wide (in RA),
enclosing the globule. In H, the two imaged positions are sep-
arated by ∼ 50′′east-west, so the overlapping field is ∼ 200′′
wide. Due to shortage of time, only one field could be imaged
in J, centered on the globule. The seeing was ∼ 0.8′′in K′, H
and H2, and ∼ 0.9′′in J. In K′and H, groups of five on-source
integrations with a dithering of up to 10′′in RA and DEC were
interspersed between groups of five off-source integrations. The
off-source fields are located ∼ 6′from the target and had been
chosenthroughexaminationof2MASS images.Theditheringof
the off-source frames is up to 20′′in RA and DEC. In J, groups
of two on-source images were interspersed between groups of
two off-source images. Ditherings and off-source fields are the
same as above. Finally, in H2pairs of one on-source and one
off-source images were taken, with the same ditherings and off-
sourcefields as above.Each framewas integrated5 or10 s in K′,
depending on the background level; the total integration time is
600 s for each of the two positions. At H, each individual inte-
gration is 20 s and the total integration time is 600 s for each
of the two positions, as well. At J, each individual integration is
100 s and the total integrationtime is 600s. In H2, the individual
integrationtimes are 100 or 150 s, dependingon the background
level, and the total integration time is 2700 s for each of the two
positions.
Each frame was first corrected for cross-talk using the rou-
tine provided on the TNG web page (http://www.tng.iac.es).
Data were thenreducedin the standardway byusingIRAF1rou-
tines. Flat-field frames were acquired at sunset. Differential flat-
field images were constructed for K′and H2, whereas all avail-
able frameswith roughlythe same meanlevelof countswere av-
eragedtogetherfor H and J. Allon-sourceandoff-sourceframes
were then flat-field corrected. Sky frames were constructed by
1IRAF
Observatories, which are operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with the
National Science Foundation.
is distributed bythe NationalOptical Astronomy
median-averaging the six off-source frames closest to each on-
source frame (generally, three preceding and three following),
after removalof the imagedstars. The sky frames were then sub-
tracted from the correspondingon-source frames. At K′and H2,
the sky-subtracted images were multiplied by a factor when ob-
tained with different individual exposure times, such as to “con-
vert” the counts of all frames in a same band to a same exposure
time. After bad-pixel correction, all images in a same band were
registered and combined together by using a median filter. The
composite three color JHK′image of the area where the JHK′
frames overlap is shown in Fig. 1.
Photometry on all mosaiced images was performed by us-
ing DAOPHOT (in IRAF). The detected stars were retrieved
by running DAOFIND and, subsequently, by a visual check
in order to discard fake detections and add undetected faint
sources. Aperture photometry was carried out through PHOT,
by adopting an aperture ∼ 1FWHM in radius and an annulus ∼
2FWHM wide with an inner radius ∼ 2FWHM. The weather
was barely photometric, so the calibration was performed by
cross-correlating the sources found in the JHK′bands and the
2MASS point source catalog. A linear relation in the J–H or H–
K′instrumentalcolorswas fittedto thefoundpairsof instrumen-
tal magnitude and 2MASS magnitude and the correspondingin-
strumentalcolor.Hence,thegivenmagnitudesare inthe 2MASS
system (JHKs). The color coefficient is always less than 0.06 in
each band.As a checkfor consistency,we comparedour Kspho-
tometry and that of Nisini et al. (2001) for the isolated sources
outofthoselisted bythose authors.Our Ksvaluesareonaverage
0.34 ± 0.28 mag dimmer than those by Nisini et al. (2001). This
is very likely due to the much worse seeing (∼ 2–3′′) and the
much coarser sampling (∼ 1′′) of the PSF in the data reportedby
Nisini et al. (2001), given the highly variable background level
in the region.
Additional photometry of the detected H2features was per-
formed from the narrowbandH2image. The H2filter is centered
at the 2.12 µm line of molecular hydrogen.Continuum emission
falls within the bandpass,as well as line emission. Throughpho-
tometry in H2and K′, we estimated the fraction of stellar con-
tinuum affecting the total counts in the H2frame. First, based
on the characteristics of the two filters, two scale factors can be
derived by which we multiplied the H2and K′images, and then
wesubtractedthelatterfromtheformer.Theresultingsubtracted
image containsonly the line emission falling within the H2filter.
The calibration was performed by carrying out stellar photome-
try on the H2original image, the same way as for JHK′but also
including an aperture correction. The retrieved stars were cross-
correlated with those found in the HK′bands and their correct
flux was derived by interpolation with the correspondingones at
H and Ksin the 2MASS system. Through a fit, we determined
the conversion factor from counts to flux. The detection limit
(at a 3σ level) is ∼ 10−15erg cm−2s−1arcsec−2. The procedure
outlined above yielded an image where all stellar sources were
efficiently removed, therefore only containing H2line emission
knots. We classified each local emission peak as a knot and de-
fined a polygon around each knot such as to include emission
down to a ∼ 3σ limit. Polygon borders for close-by knots were
chosen by eye, based on morphologicalcriteria. Photometrywas
carried out by using POLYPHOT in IRAF.
The astrometric calibration was performed by deriving the
positions of 14 relativelybright, isolated stars spread all over the
K′frame, and correlating them with their 2MASS coordinates.
By afit (usingSTSDASroutinesinIRAF) weobtainedthetrans-
formation between frame and equatorial coordinates, allowing
an accuracy of ∼ 1′′.

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Beltr´ an et al.: IC 1396N3
Fig.1. Three color composite image of IC 1396N (J, blue, H, green, K′, red) taken with NICS at TNG. The black and white crosses
show the positions of the 3.1 mm sources, BIMA 1, 2, and 3 from Beltr´ an et al. (2002), while the white cross in the top shows the
position of the 1.3 mm continuum source C detected by Codella et al. (2001). Also labeled are two Class I sources discussed in the
text.
3. Results and discussion
3.1. The stellar population
Within the area where the JHK′images overlap (see Fig. 1), we
found 736 sources detected in all three bands, 128 sources de-
tected only in HK′, 67 sources with a K′detection only, and 79
sources detected only in JH. The sources with HK′or K′de-
tections only are preferentially located towards the globule (see,
e. g. Fig. 4), as expected for heavily extincted stars. Conversely,
the sources with JH detections tend to be located outside the
globule, indicating that these are just faint stars.
We obtained histograms of the number of sources as a func-
tion of magnitude by binning the number of sources detected
in all three bands in magnitude intervals. Then, we adopted as
completeness limit in each band the magnitude where the cor-
responding histogram peaks: Ks∼ 17.5, H∼ 18.5 and J∼ 19.5.
When adding also the sources with detections in two or one
bands only, the peak does not shift in any of the histograms but
Ks, where it appears to move towards Ks∼ 18. The derived com-
pleteness limits are roughly 1.5 mag below our detection limits
(at a 3σ level). An estimate of the minimum stellar mass de-
tectable all over the globule can be obtained by using pre-main
sequence (PMS) evolutionary tracks. However, one has to as-
sume an age and a maximum extinction for the stellar popula-
tion. As for the age, Lada & Lada (2003) noted that the embed-
ded phase of star cluster evolution lasts 2–3 Myrs and clusters
older than 5 Myrs are rarely associated with molecular gas. This
is in accord with the age of the open star cluster Trumpler 37,
surroundingthe globule (∼ 3×106yrs; Getman et a. 2007). The
age of the star exciting the PDR around the globule may also
give a hint of the age of the stellar population, since this star
either triggered star formation in the core, or began inhibiting
it by starting core destruction. If HD 206267 is an O6.5 V star
(Walborn & Panek 1984), then its lifetime in the main sequence
is ∼ 6 × 106yrs (e. g. Vanbeveren et al. 1998), that roughly
agreeswiththetimes givenabove.Ontheotherhand,theglobule
shows clear signature of much younger stars and protostars, and
the dynamical timescales estimated from the jets (see Sect. 3.3)
are even shorter, ∼ 103yrs. We can therefore assume an age of
106yrs as a sort of upper limit, since younger low-mass PMS
stars are brighter and, then, more easily detectable (lowering the
mass detection limit). As for the maximum extinction, Getman
et al. (2007) quote a few authors to conclude that the absorption
through the core is AV∼ 9–10 mag. Figures 2 and 3 clearly show
that this value is probably too low and most of the detected stars
exhibit AV≤ 20 mag. Nevertheless, there are a few sources with
AVup to ∼ 30 mag. More extincted sources could be not repre-
sented in the diagrams just because too faint, thus biasing any

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4Beltr´ an et al.: IC 1396N
Fig.2. Color-color diagram of the NIR sources found within
the area where JHK′images overlap. Full squares are sources
with detection in all bands, empty squares are sources with de-
tection in HKsonly (hence, the shown J–H is a lower limit).
The solid line marks the main sequence (in the 2MASS system),
the dashed lines follow the reddening law by Rieke & Lebofsky
(1985) with crosses at intervals of Av= 10 mag.
estimates based on the plots. In fact, towards BIMA 2, we can
derive an extinction much larger than 100 mag from the contin-
uummillimeterdata.However,thisis clearlya less evolved,very
young region amounting to a small fraction of the globule. We
can probably assume that for most of the globule the extinction
does not exceed a canonical AV∼ 30–40 mag.
We adopted the PMS evolutionary tracks by Palla & Stahler
(1999), along with the reddening law by Rieke & Lebofsky
(1985). Hence, assuming an age of 106yrs, PMS stars of ∼
0.1 M⊙are within the completeness limit at Ksfor AV = 30
mag and within the detection limit at Ksfor AV= 40 mag. The
same for PMS stars of ∼ 0.4 M⊙at H, whereas PMS stars of ∼
0.8 M⊙are within the detection limit at J for AV= 30 mag and
PMS stars of ∼ 2 M⊙are within the completeness limit at J for
AV= 30 mag. However,these magnitudesrefer to “naked”stars,
i. e., stars without a circumstellar disk.
The color-color diagram (CCD; J–H vs. H–Ks) of the NIR
sources found within the area where JHK′images overlap is
shown in Fig. 2. The main sequence locus is also drawn, by
using the colors of Koornneef (1983) after conversion to the
2MASS system throughthe relations givenby Carpenter(2001).
The CCD is consistent with that shown in Nisini et al. (2001), in
that most of the stars fall within the reddening band of the main
sequence and almost all those exhibiting a NIR excess lie only
slightly below the reddening band. The points spread around the
main sequencewith largerNIR excesses are mostlyfaint sources
found at the edge of the images, then affected by large errors.
However, our source # 331 (labeled in figure) actually exhibits a
largeNIR excess. Thissourcecoincideswith source#8 inNisini
etal.(2001)andHH777IRSinReipurthetal.(2003).According
to the latter authors,this sourcecould be binarybecause it seems
to be powering two flows, a major HH flow that expands to-
wards the southwest, labeled HH777 by Reipurth et al. (2003),
and a northwestern flow labeled G by Nisini et al. (2001). The
source is neatly elongated with respect to the PSF of a single
Fig.3. Color-magnitude diagram of the NIR sources found to-
wards the area where JHK′images overlap. Full squares are
sources with detections in all bands, empty squares are sources
with detections in HKsonly and crosses are sources detected
at Ksonly (hence, the shown H–Ksis a lower limit). The solid
line marks the zero age main sequence (from Allen (1976) and
Koornneef (1983) after conversion to the 2MASS system) at a
distance of 750 pc, the dashed lines indicate the completeness
limit and an arrow is drawn showing a reddening AV= 20 mag
according to the reddening law by Rieke & Lebofsky (1985). A
few spectral types on the ZAMS are labeled.
star, with the size of the major axis twice that of the minor axis,
which suggests the binarity of the source. The nearby stars do
not show such an elongation, therefore we discarded any possi-
ble focus effect. The elongation of source # 331 is very clear in
the H filter,inwhichit hasbeenpossibletodeconvolvetheemis-
sion into two stars # 331A and # 331B by PSF-fit photometry
with DAOPHOT in IRAF. In the K′filter the elongation is also
evident, but the PSF-fit photometry appears to be less precise.
Nevertheless, we cannot rule out the possibility that # 331B may
be just radiation from # 331A scattered by dust througha cavity.
This scenario would be consistent with the fact that the elonga-
tion of source # 331 roughly coincides with the direction of the
southwestern HH777 flow (see Fig. 2 of Reipurth et al. 2003).
However, as seen in Figs. 1 and 6, the other H2flow detected
nearby, the northwestern flow labeled G, also points right back
towards source # 331, which suggests that its poweringsource is
also located at that position. Hence, based on the fact that there
are two outflows associated with this position, we favor the sce-
nario of binarity to explain the elongation of source # 331. The
HKsmagnitudeof# 331Ais within0.4magofthose listedin the
2MASS catalog but, whereas its colors are consistent with those
given by Nisini et al. (2001), the Ksvalue we derive is almost 1
maglargerthan thatmeasuredbyNisini et al. (2001).This is still
consistent with the difference found between the two photome-
tries (see Sect. 2), also given that the source has been resolved
into two close-by companions that appear to be embedded in a
small diffuse nebulosity that probably could not be resolved by
Nisini et al. (2001). However, a degree of intrinsic variability
cannot be excluded, as well.
Thecolor-magnitudediagram(CMD; H–Ksvs. Ks) is shown
in Fig. 3 for the NIR sources in the same area as above. As seen

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Beltr´ an et al.: IC 1396N5
in this diagram, an upper limit for the spectral type of the ZAMS
stars in the cloud would be B1–B0, which corresponds to a stel-
lar mass of ∼ 17–20 M⊙(Vacca et al. 1996). Since such massive
stars evolve along the ZAMS from 8–10 M⊙on (e. g. Palla &
Stahler 1991) at the end of their accretionphase, this can be con-
sidered as a robust upper limit for the mass of the stars associ-
ated with IC1396N, irrespective of their age. Most of the points
lie within AV= 20 mag of the zero age main sequence (ZAMS).
However,source # 252 has similar Ksand H–Ksas # 331A, both
objects lying farther from the ZAMS than the remaining stellar
population. Source # 252 is also embedded in a patch of diffuse
nebulosity and is located near a cluster of H2-emission blobs al-
ready identified by Nisini et al. (2001) as knot A, and the cluster
of compact radio sources found by Beltr´ an et al. (2002). These
facts suggest that sources # 252 and # 331A may be in a similar
evolutionary stage, although this cannot be fully proved because
# 252 has not been detected at J. They are located towards the
center of the globule, ∼ 26′′apart (see Fig. 1). Source # 252 lies
close tothe IRAS uncertaintyellipse, north-westofit. Projecting
thembackontotheZAMS intheCMD, alongthereddeningvec-
tor, identify them as stars of spectral type B0 to B3. This has to
be considered as a “lower” limit for their actual spectral type (i.
e., they are of later spectral type), since # 331A exhibits a NIR
excess and probablyalso # 252does have it. Hence,based on the
location of # 331A in the CCD, they might be Class I sources of
intermediate mass (e.g. Sugitani et al. 2002b).
Getman et al. (2007) used CHANDRA X-ray observations
of IC 1396N, complementedwith Spitzer/IRAC photometryand
the available NIR photometry,identifying25 likely stellar mem-
bers of the globule. Although all are associated with IRAC MIR
sources, 6 of them do not have a NIR counterpart either in the
2MASS catalog or in the list of Nisini et al. (2001). We have
found three new matches, e. g. sources 70, 76, and 80 (see
Table 2 of Getman et al. (2007)), corresponding to our sources
224 (Ks∼ 16), 223 (Ks∼ 16) and 196 (Ks∼ 17.7), respectively.
They are all undetected both in J and in H, confirming their na-
ture of heavily extincted source. The remaining 3 X-ray sources
without a NIR counterpart lie in the area of the BIMA sources
and their X-ray spectra are heavily absorbed (NH>∼1023cm−1).
One of them (source 66) has been proposed as X-ray counter-
part of the protostar BIMA 2. However, note that our sources
# 331A (and 331B) and 252 do not have an X-ray counterpart
in the catalog of Getman et al. (2007). These authors quote a
completeness limit (in mass) of 0.4 M⊙, higher than our com-
pleteness limit in Ksbut similar to our completeness limit in H.
We suspect that their completeness limit may be even higher,
since the reddening towards the globule may be at least twice
larger than adopted by them. It is noteworthy that Getman et
al. (2007) found X-ray counterparts of possible Class I sources
with high absorption and no NIR counterparts, but fail to detect
our sources # 331A and 252. This would be consistent with #
331A and 252 being young intermediate-mass (proto-)stars. If #
331 is actually a double system, then its companion (possibly #
331B)mighteitherbeanotheryoungintermediate-massstar,ora
low-mass protostar (with a mass below the X-ray completeness
limit). Moreover, sensitive millimeter interferometric observa-
tions did not detect # 252 (Beltr´ an et al. 2002; Neri et al. 2007),
whichrules out the presenceof the expectedmassive enoughcir-
cumstellar disk. Hence, further data are needed to clarify its na-
ture and its identification as an intermediate-mass Class I source
remains highly speculative.
Fig.4. Stellar surface density map (in stars arcmin−2) of all
sources detected in the Ksband up to Ks= 20. Contours range
from 10 to 90 stars arcmin−2in steps of 10 stars arcmin−2. The
contours at 10 and 20 stars arcmin−2are thicker. Grey scale
ranges from −15 stars arcmin−2(black) to 69 stars arcmin−2
(white). The field shown has been imaged at all bands and the
offsets are in arcsec from the location of source # 331A (HH777
IRS). The positions of sources with detections in all bands are
markedbyfilledsquares,thoseofsourceswithdetectionsat HKs
onlybyemptysquaresandthoseofsourceswith onlya Ksdetec-
tion are marked by small crosses. The three large crosses around
(0′′,−30′′) are the compact embedded sources BIMA 1, 2 and
3 detected by Beltr´ an et al. (2002) at millimeter wavelengths,
and that at around (0′′,110′′) is the 1.3 mm continuum source C
detected by Codella et al. (2001).
3.2. Triggered star formation?
In the X-ray source population towards IC 1396N, Getman et
al. (2007) have found a clear clustering of sources at the south-
ern rim, with an elongated spatial distribution, and an evolution-
ary gradient (interpreted as an age gradient), oriented towards
the exciting star. These authors interpret this geometric and age
distribution in terms of triggered star formation by passage of
Hii region shocks into the molecular globule. We have searched
for evidence in the NIR of age gradients in the south-north di-
rection or clustering of stars towards the rim, but found none.
The number of sources with evidence of NIR excess towards
the globule is too low, so any analysis of the stellar population
in the NIR alone is bound to remain inconclusive with respect
to the identification of age gradients. Regarding the geometric
distribution of the sources in the NIR, there are no clear signs
of clustering towards the rim (even within the area where X-ray
sources cluster), as shown by the map of the star surface den-
sity (Fig. 4), which was obtained by counting all sources with a
detection at least in the Ksband (up to Ks = 20) in squares of
40′′× 40′′, displaced by 20′′both in RA and in DEC. The num-
ber of sources decreases in going from the southern edge of the
globule to the northern one; in the CCD, within the extinction
band of the main sequence the upper and lower limits of extinc-
tion initially increase, then decrease close to the northern edge,
as expected. What is more, the JHKssources lying below the
main sequence reddening band in the CCD tend to cluster out