Astronomy & Astrophysics manuscript no. 7297 c© ESO 2021
September 15, 2021
Letter to the Editor
Protostellar clusters in intermediate mass (IM) star forming regions
A. Fuente1, C. Ceccarelli2, R. Neri3, T. Alonso-Albi1, P. Caselli4,5, D. Johnstone6,7 , E.F. van Dishoeck8, and F.
Wyrowski9
1 Observatorio Astronómico Nacional (OAN), Apdo. 112, E-28803 Alcalá de Henares (Madrid), Spain
e-mail: a.fuente@oan.es
2 Laboratoire d’Astrophysique de l’Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
3 Institute de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St Martin d’Heres Cedex, France
4 INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
5 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 0213
6 Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
7 National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
8 Leiden Observatory, PO Box 9513, 2300 RA Leiden, Netherlands
9 Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany
Received February 14, 2007; accepted March 29, 2007
ABSTRACT
Context. The transition between the low density groups of T Tauri stars and the high density clusters around massive stars occurs
in the intermediate-mass (IM) range (M∗∼2–8 M⊙). High spatial resolution studies of IM young stellar objects (YSO) can provide
important clues to understand the clustering in massive star forming regions.
Aims. Our aim is to search for clustering in IM Class 0 protostars. The high spatial resolution and sensitivity provided by the new A
configuration of the Plateau de Bure Interferometer (PdBI) allow us to study the clustering in these nearby objects.
Methods. We have imaged three IM Class 0 protostars (Serpens-FIRS 1, IC 1396 N, CB 3) in the continuum at 3.3 and 1.3mm using
the PdBI. The sources have been selected with different luminosity to investigate the dependence of the clustering process on the
luminosity of the source.
Results. Only one millimeter (mm) source is detected towards the low luminosity source Serpens–FIRS 1. Towards CB 3 and
IC1396 N, we detect two compact sources separated by ∼0.05 pc. The 1.3mm image of IC 1396 N, which provides the highest
spatial resolution, reveal that one of these cores is splitted in, at least, three individual sources.
Key words. stars:formation–stars: individual (Serpens–FIRS 1, IC 1396 N, CB 3)
1. Introduction
Low and high mass stars (M∗>8 M⊙) are formed in different
regimes. While low mass stars can be formed isolated or in loose
associations, high mass stars are always found in tight clusters.
Intermediate-mass young stellar objects (IMs) (protostars and
Herbig Ae/Be [HAEBE] stars with M∗ ∼ 2 - 8 M⊙) constitute the
link between low- and high-mass stars. In particular the transi-
tion between the low density groups around T Tauri stars and the
dense clusters around massive stars occurs in these objects. Testi
et al. (1998,1999) studied the clustering around HAEBE stars
using optical and near-infrared (NIR) images and concluded that
transition occurs smoothly from Ae to Be stars. Thus, these stars
are key objects to study the onset of clustering.
Thus far, clustering has only been studied at infrared and op-
tical wavelengths because of the limited spatial resolution and
sensitivity of the mm telescopes. Thus, the earliest stages of
the cluster formation were hidden to the observers. The sub-
arcsecond angular resolution provided by the new A configu-
ration of the PdBI allows, for the first time, to study clustering
at mm wavelengths with a similar sensitivity and spatial resolu-
tion to the NIR studies. In this Letter, we present interferomet-
ric continuum observations of the IM protostars Serpens-FIRS 1
(precursor of a Ae star) and CB 3 (precursor of a Be star) aimed
Send offprint requests to: A. Fuente
to study the clustering phenomena in the early Class 0 phase. We
also use the data at highest spatial resolution towards IC 1396 N
reported in this special issue by Neri et al. (Paper II, hereafter).
1.1. Serpens-FIRS 1
Serpens-FIRS 1 is a 46 L⊙ Class 0 source located in a very active
star forming region. Previous mid-IR and NIR studies show that
the population of YSOs is strongly clustered, with the Class I
sources more clustered than the Class II ones (Kaas et al. 2004).
The sub-clusters of Class I sources are located in a NW-SE
oriented ridge following the distribution of dense cores in the
molecular cloud with a subclustering spatial scale of 0.12 pc (see
Fig. 1). The Class II stars are located surrounding the molecular
cores with a subclustering spatial scale of 0.25 pc. Adopting a
distance of 310 pc, the YSOs density in the sub-clusters ranges
from 360–780 pc−2. Several high angular resolution mm studies
have been made in the Serpens molecular cloud (Testi & Sargent
1998, William & Myers 1999, Hogerheijde et al. 1999, Testi et
al. 2000). We have imaged at higher spatial resolution a region
of 0.04 pc around the intense mm-source FIRS 1.
http://arxiv.org/abs/0704.1098v1
2 A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions
Table 1. Millimeter flux densities, sizes, spectral indexes and masses
Position Peak Gaussian width1 Int. Intensity Mass2 Size3 α4 Sensitivity5 Sampled area6
(mJy/beam) (”) (mJy) (M⊙) (AU) (M⊙) r(pc)
Serpens-FIRS 1
1.3mm 18:29:49.80 01:15:20.41 273(1) 0.50”×0.63” 357 0.1 65 1.57 0.01 0.02
3.3mm 18:29:49.80 01:15:20.41 63(0.5) 0.80”×1.73” 71 0.04
CB 3-1
1.3mm 00:28:42.60 56:42:01.11 20(1) 0.36”×0.48” 34 0.62 600 2.52 0.04 0.16
3.3mm 00:28:42.60 56:42:01.11 2.0(0.5) 0.88”×1.27” 2.9 0.32
CB 3-2
1.3mm 00:28:42.20 56:42:05.11 10(1) 0.31”×0.43” 13 0.24 330 1.87 0.04 0.16
3.3mm 00:28:42.20 56:42:05.11 2.1(0.5) 0.80”×1.00” 2.1 0.32
1 Half-power width of the fitted 2-D elliptical Gaussian
2 Mass estimated using the 1.3mm fluxes and assuming Td=100 K and κ1.3mm=0.01 g
−1 cm2
3 Deconvolved source size at 1.3mm
4 1.3mm/3.3mm spectral index
5 5×rms mass sensitivity derived from the 1.3mm image assuming Td=100 K and κ1.3mm=0.01 g
−1 cm2
6 Radius (HPBW/2) of the PdBI primary beam at the source distance
Fig. 1. Dust continuum mosaic (contours and grey scale) of the
Serpens main core as observed with the IRAM 30m telescope.
The location of the Class II (blue filled squares), flat (red crosses)
and Class I sources (red empty circles) is indicated (adapted
from Kaas et al. 2004). In the inset, we show the 3mm and
1.3mm (small inset) continuum images observed with the PdBI.
Note that only one compact core is detected in this region down
to a spatial scale of less than 100 AU. The dashed circle marks a
region of 0.2 pc radius around FIRS 1.
1.2. IC 1396 N
IC 1396 N is a ∼300 L⊙ source located at a distance of 750 pc
(Codella et al. 2001). A total population of ∼30 YSOs has been
found in this region (Getman et al. 2007, Nisini et al. 2001).
These YSOs present an elongated spatial distribution with an age
gradient towards the center of the Class I/0 system. The Class III
sources are located in the outer rim of the globule, the Class II
sources are congregated in the bright ionized rim and the Class
I/0 objects are located towards the dense molecular clump (see
Fig. 2). The average density of YSOs in the globule is ∼200 pc−2.
We have mapped a region of 0.1 pc around the Class 0/I system.
1.3. CB 3
CB 3 is a large globule (930 L⊙) located at 2.5 Kpc from the
Sun (Codella & Bachiller 1999). A strong submillimeter source
is observed in the central core (see Fig. 3 and Huard et al. 2000).
Deep NIR images of the region show ∼40 NIR sources, from
which at least 22 are very red, indicative of pre-main sequence
stars (Launhardt et al. 1998). Up to our knowledge, there are
no mid-IR and/or X-ray studies in this region. Then, the census
of YSOs is not complete in this IM source. We have mapped a
region of 0.32 pc around the submillimeter source.
2. Observations
The observations were made on January and February, 2006.
The spectral correlator was adjusted to cover the entire RF pass-
bands (580 MHz) for highest continuum sensitivity. The overall
flux scales for each epoch and for each frequency band were
set on 3C454.3 and MWC349 (for CB 3), and 1749+096 (for
Serpens–FIRS 1). The resulting continuum point source sensi-
tivities (5×rms) were estimated to 2.00 mJy at 237.571 GHz and
0.5 mJy at 90.250 GHz for CB 3 and 40.00 mJy at 237.571 GHz
and 7.0 mJy at 90.250 GHz for Serpens–FIRS 1. The corre-
sponding synthesized beams adopting uniform weighting were
0.4′′ × 0.3′′ at 237.571 GHz and 1.0′′ × 0.8′′ at 90.250 GHz
for CB 3 and 0.6′′ × 0.4′′ at 237.571 GHz and 1.7′′ × 0.7′′ at
90.250 GHz for Serpens–FIRS 1. (See Paper II for IC 1396 N.)
3. Results
In Table 1 we present the coordinates, sizes and mm fluxes of the
compact cores detected in Serpens–FIRS 1 and CB 3. The results
towards IC 1396 N are presented in Paper II. Only 1 mm-source
is detected in Serpens–FIRS 1 down to a separation of less than
100 AU. The other targets turned out to be multiple sources. We
have detected 2 mm-sources towards CB 3 and 4 mm-sources
towards IC 1396 N.
A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions 3
Fig. 2. On the left, we show the 5′×5′ Spitzer IRAC 3.6 µm image towards IC 1396 N (adapted from Getman et al. 2007). The
location of the globule is marked by the green contour and the Class III (yellow triangles), Class II (red circles) and blue squares
(Class 0/I) sources are indicated. On the right, we show the 3mm (up) and 1.3mm (down) continuum images observed with the
PdBI. In the 3mm image we also indicate the Class III (black triangles), Class II (red circles) and Class 0/I (filled blue squares)
sources.
The 4 compact sources towards IC 1396 N are grouped in
2 sub-clusters separated by 0.05 pc which are spatially coinci-
dent with the sources named BIMA 2 and BIMA 3 by Beltrán
et al. (2002). The projected distance between these sub-clusters
is similar to that found by Hunter et al. (2007) between the mm
sub-clusters in the massive star forming region NGC 6336 I. This
distance is also similar to the distance between the stars form-
ing the Trapezium in Orion (from 5000 to 10000 AU). Thus it
is a typical distance between the IM and massive stars in the
same cloud. Our high angular resolution observations reveal that
BIMA 2 is itself composed of 3 compact cores embedded in a
more extended component (see Fig. 2). These 3 compact cores
are new mm detections and constitute the first sub-cluster of
Class 0 IM sources detected thus far.
In CB 3 we have detected 2 mm-sources separated by 0.06 pc
(see Table 1 and Fig. 3). These compact cores are new detec-
tions and the separation between them is similar to that between
BIMA 2 and BIMA 3 in IC 1396 N. In fact, the structure of the
globule CB 3 resembles much that of IC 1396 N but the angu-
lar resolution of our observations prevent us from resolving any
possible sub-cluster of compact cores in this more distant source.
Note that the masses of CB 3-1 and CB 3-2 are similar to that of
the sub-cluster BIMA 2 (Paper II).
The number of detections is limited by the sensitivity of
our observations. In Table 1 we show the point source mass
sensitivity assuming a dust temperature of 100 K (typical for
hot cores and circumstellar disks around luminous Be stars)
and κ1.3mm=0.01 g
−1 cm2 for each target. It is possible that we
miss a population of weak Class 0/I sources in CB 3 where
the mass sensitivity is poor (0.04 M⊙). However, the sensitiv-
ity in Serpens–FIRS 1 (0.01 M⊙) and IC1396 N (0.007 M⊙) is
good enough to detect disks around early Be stars that usually
have masses of ∼0.01 M⊙ (see e.g Fuente et al. 2003, 2006).
We should have also detected massive disks (∼0.1 M⊙) around
Herbig Ae and T Tauri stars although the dust temperature is
lower, Td=15–56 K (Natta et al. 2000). But there is still the
possibility of the existence of HAEBE or T Tauri stars with
weak circumstellar disks that are not detected in our mm im-
ages. Another possibility is that we are missing a population
of hot corinos (we refer as “hot corino” to the warm material
(∼100 K) around a low mass Class 0 protostar regardless of its
chemical composition) with masses below the values reported in
Table 1. Our sensitivity is good enough to detect a hot corino
similar to IRAS 16293–2422 A and B (L∼10 L⊙) at the dis-
tance of our sources (see Bottinelli et al. 2004). Thus the possi-
ble “missed” hot corinos should correspond to lower luminosity
protostars. Finally, we can be missing a population of dense and
cold cores. Assuming a dust temperature of 10 K, these compact
cold cores should have masses of less than 0.17, 0.12 and 0.7
M⊙ in Serpens–FIRS 1, IC 1396 N and CB 3 respectively. These
masses are not large enough to form new IM stars.
4 A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions
4. Discussion
Testi et al. (1999) studied the clustering around a large sample of
HAEBE stars. In order to quantify the concept, they introduced
the parameter Nk, defined as the number of stars in a radius of
0.2 pc, the typical cluster radius. They showed that rich clus-
ters are only found around the most massive stars, although the
parameter Nk is highly variable. Some Be stars are born quite
isolated, while others have Nk >70. For our sources this number
is 22 (Launhardt et al. 1998, but the census is not complete), 29
(from Fig. 1) and 28 (Getman et al 2007; Nisini et al. 2001) in
CB3, Serpens and IC 1396 N respectively, where all previously
known YSOs (Class 0, I, II and III) in the regions are considered.
Our maps show 2 sources in CB 3 on a 0.3 pc scale, 1 source
in Serpens-FIRS 1 on a 0.04 pc scale, and 4 sources in IC 1396 N
on a 0.1 pc scale. Defining Nmm as the number of mm sources in
a radius of 0.2 pc, we can estimate Nmm from our observations
and provide a revised value for the total number of YSOs at this
scale. In Serpens our interferometric observations do not add any
new mm source to previous data. We have observed the most in-
tense mm clump in Fig, 1, the most likely to be a multiple source,
and only found 1 compact source. Based on the 30m map shown
in Fig. 1 and assuming that all the clumps host only one source
we estimate Nmm∼7 from a total of 29 YSOs. In CB 3, our data
add 2 new mm sources (Nmm=2) to the previous census of YSOs
based on NIR studies. In IC 1396 N, we estimate Nmm=4–16.
The upper limit has been calculated assuming a constant den-
sity of mm sources in the region. Usually, the Class 0/I stars
are not uniformly distributed in the clouds, but grouped in sub-
clusters that are coincident with the peak of dense cores. Thus
the value of Nmm is very likely close to 4 and we assumed this
number hereafter. Since BIMA 2 and BIMA 3 were previously
detected in the X-rays surveys by Getman et al. (2007), we only
add two new sources (due to the multiplicity of BIMA 2) to the
total number of YSOs in this region.
Summarizing, the total number of YSOs is now 29, 24 and
30 for Serpens–FIRS 1, CB 3 and IC 1396 N respectively. While
Serpens–FIRS 1 is an extraordinarily rich cluster compared with
the clusters around Ae stars reported by Testi et al. (1999), CB 3
and IC 1396 N do not seem to become one of the crowded
clusters (Nk∼70) detected by these authors around Be stars.
However, this conclusion might not be true. The interferometer
is only sensitive to dense and compact cores and provides a bi-
ased vision of the star forming regions. In fact our interferomet-
ric observations accounts for less than 1% of the total interstellar
mass in the studied globules, i.e., ∼ 10, 58 and 64 M⊙ are missed
in Serpens–FIRS 1, CB 3 and IC 1396 N respectively (Alonso-
Albi et al. 2007). One possibility is that this mass is in the form
of many weak hot corinos which could eventually become low
mass stars. The fate of these hot corinos is, however, linked to
the evolution of the IM protostar that is progressively dispers-
ing and warming the surrounding material (Fuente et al. 1998).
Another possibility is that the ”missed” mass is in the form of an
extended and massive envelope. This envelope (if not totally dis-
persed by the IM star) could produce new stars in a forthcoming
star formation event.
5. Summary
We have searched for clustering at mm wavelengths in 3 IM star
forming regions. We have detected 1, 2 and 4 compact cores in
Serpens–FIRS 1, CB 3 and IC 1396 N respectively. The compact
cores are not distributed uniformly but grouped in sub-clusters
separated by ∼0.05 pc. Such a separation is a typical distance
Fig. 3. Dust continuum emission at 850 µm as observed with
SCUBA towards CB 3. In the inset, we show the 3mm contin-
uum image observed with PdBI. Note that two compact cores
are detected towards the single-dish peak.
for both IM and massive stars within the same cloud. We have
used our mm observations to complete the census of YSOs in
these regions and compare them with the clusters found by Testi
et al. (1999) in the more evolved HAEBE stars. Serpens–FIRS 1
seems to belong to an extraordinarily rich cluster. The density
of YSOs in the high luminosity sources IC 1396 N and CB 3 is
consistent with the density found in the clusters around Be stars
although our sources are not found between the most crowded
regions. The large amount of interstellar gas and dust in the stud-
ied regions suggest that new star formation events are still pos-
sible.
Acknowledgements. We are grateful to Phil Myers for his careful reading of
the manuscript. A.F. is grateful for support from the Spanish MEC and FEDER
funds under grant ESP 2003-04957, and from SEPCT/MEC under grant AYA
2003-07584.
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	Introduction
	Serpens-FIRS 1
	IC 1396 N
	CB 3
	Observations
	Results
	Discussion
	Summary

  The transition between the low density groups of T Tauri stars and the high
density clusters around massive stars occurs in the intermediate-mass (IM)
range (M$_*$$\sim$2--8 M$_\odot$). High spatial resolution studies of IM young
stellar objects (YSO) can provide important clues to understand the clustering
in massive star forming regions.
  Aims: Our aim is to search for clustering in IM Class 0 protostars. The high
spatial resolution and sensitivity provided by the new A configuration of the
Plateau de Bure Interferometer (PdBI) allow us to study the clustering in these
nearby objects.
  Methods: We have imaged three IM Class 0 protostars (Serpens-FIRS 1, IC 1396
N, CB 3) in the continuum at 3.3 and 1.3mm using the PdBI. The sources have
been selected with different luminosity to investigate the dependence of the
clustering process on the luminosity of the source.
  Results: Only one millimeter (mm) source is detected towards the low
luminosity source Serpens--FIRS 1. Towards CB 3 and IC1396 N, we detect two
compact sources separated by $\sim$0.05 pc. The 1.3mm image of IC 1396 N, which
provides the highest spatial resolution, reveal that one of these cores is
splitted in, at least, three individual sources.

Introduction
Low and high mass stars (M∗>8 M⊙) are formed in different
regimes. While low mass stars can be formed isolated or in loose
associations, high mass stars are always found in tight clusters.
Intermediate-mass young stellar objects (IMs) (protostars and
Herbig Ae/Be [HAEBE] stars with M∗ ∼ 2 - 8 M⊙) constitute the
link between low- and high-mass stars. In particular the transi-
tion between the low density groups around T Tauri stars and the
dense clusters around massive stars occurs in these objects. Testi
et al. (1998,1999) studied the clustering around HAEBE stars
using optical and near-infrared (NIR) images and concluded that
transition occurs smoothly from Ae to Be stars. Thus, these stars
are key objects to study the onset of clustering.
Thus far, clustering has only been studied at infrared and op-
tical wavelengths because of the limited spatial resolution and
sensitivity of the mm telescopes. Thus, the earliest stages of
the cluster formation were hidden to the observers. The sub-
arcsecond angular resolution provided by the new A configu-
ration of the PdBI allows, for the first time, to study clustering
at mm wavelengths with a similar sensitivity and spatial resolu-
tion to the NIR studies. In this Letter, we present interferomet-
ric continuum observations of the IM protostars Serpens-FIRS 1
(precursor of a Ae star) and CB 3 (precursor of a Be star) aimed
Send offprint requests to: A. Fuente
to study the clustering phenomena in the early Class 0 phase. We
also use the data at highest spatial resolution towards IC 1396 N
reported in this special issue by Neri et al. (Paper II, hereafter).
1.1. Serpens-FIRS 1
Serpens-FIRS 1 is a 46 L⊙ Class 0 source located in a very active
star forming region. Previous mid-IR and NIR studies show that
the population of YSOs is strongly clustered, with the Class I
sources more clustered than the Class II ones (Kaas et al. 2004).
The sub-clusters of Class I sources are located in a NW-SE
oriented ridge following the distribution of dense cores in the
molecular cloud with a subclustering spatial scale of 0.12 pc (see
Fig. 1). The Class II stars are located surrounding the molecular
cores with a subclustering spatial scale of 0.25 pc. Adopting a
distance of 310 pc, the YSOs density in the sub-clusters ranges
from 360–780 pc−2. Several high angular resolution mm studies
have been made in the Serpens molecular cloud (Testi & Sargent
1998, William & Myers 1999, Hogerheijde et al. 1999, Testi et
al. 2000). We have imaged at higher spatial resolution a region
of 0.04 pc around the intense mm-source FIRS 1.
http://arxiv.org/abs/0704.1098v1
2 A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions
Table 1. Millimeter flux densities, sizes, spectral indexes and masses
Position Peak Gaussian width1 Int. Intensity Mass2 Size3 α4 Sensitivity5 Sampled area6
(mJy/beam) (”) (mJy) (M⊙) (AU) (M⊙) r(pc)
Serpens-FIRS 1
1.3mm 18:29:49.80 01:15:20.41 273(1) 0.50”×0.63” 357 0.1 65 1.57 0.01 0.02
3.3mm 18:29:49.80 01:15:20.41 63(0.5) 0.80”×1.73” 71 0.04
CB 3-1
1.3mm 00:28:42.60 56:42:01.11 20(1) 0.36”×0.48” 34 0.62 600 2.52 0.04 0.16
3.3mm 00:28:42.60 56:42:01.11 2.0(0.5) 0.88”×1.27” 2.9 0.32
CB 3-2
1.3mm 00:28:42.20 56:42:05.11 10(1) 0.31”×0.43” 13 0.24 330 1.87 0.04 0.16
3.3mm 00:28:42.20 56:42:05.11 2.1(0.5) 0.80”×1.00” 2.1 0.32
1 Half-power width of the fitted 2-D elliptical Gaussian
2 Mass estimated using the 1.3mm fluxes and assuming Td=100 K and κ1.3mm=0.01 g
−1 cm2
3 Deconvolved source size at 1.3mm
4 1.3mm/3.3mm spectral index
5 5×rms mass sensitivity derived from the 1.3mm image assuming Td=100 K and κ1.3mm=0.01 g
−1 cm2
6 Radius (HPBW/2) of the PdBI primary beam at the source distance
Fig. 1. Dust continuum mosaic (contours and grey scale) of the
Serpens main core as observed with the IRAM 30m telescope.
The location of the Class II (blue filled squares), flat (red crosses)
and Class I sources (red empty circles) is indicated (adapted
from Kaas et al. 2004). In the inset, we show the 3mm and
1.3mm (small inset) continuum images observed with the PdBI.
Note that only one compact core is detected in this region down
to a spatial scale of less than 100 AU. The dashed circle marks a
region of 0.2 pc radius around FIRS 1.
1.2. IC 1396 N
IC 1396 N is a ∼300 L⊙ source located at a distance of 750 pc
(Codella et al. 2001). A total population of ∼30 YSOs has been
found in this region (Getman et al. 2007, Nisini et al. 2001).
These YSOs present an elongated spatial distribution with an age
gradient towards the center of the Class I/0 system. The Class III
sources are located in the outer rim of the globule, the Class II
sources are congregated in the bright ionized rim and the Class
I/0 objects are located towards the dense molecular clump (see
Fig. 2). The average density of YSOs in the globule is ∼200 pc−2.
We have mapped a region of 0.1 pc around the Class 0/I system.
1.3. CB 3
CB 3 is a large globule (930 L⊙) located at 2.5 Kpc from the
Sun (Codella & Bachiller 1999). A strong submillimeter source
is observed in the central core (see Fig. 3 and Huard et al. 2000).
Deep NIR images of the region show ∼40 NIR sources, from
which at least 22 are very red, indicative of pre-main sequence
stars (Launhardt et al. 1998). Up to our knowledge, there are
no mid-IR and/or X-ray studies in this region. Then, the census
of YSOs is not complete in this IM source. We have mapped a
region of 0.32 pc around the submillimeter source.
2. Observations
The observations were made on January and February, 2006.
The spectral correlator was adjusted to cover the entire RF pass-
bands (580 MHz) for highest continuum sensitivity. The overall
flux scales for each epoch and for each frequency band were
set on 3C454.3 and MWC349 (for CB 3), and 1749+096 (for
Serpens–FIRS 1). The resulting continuum point source sensi-
tivities (5×rms) were estimated to 2.00 mJy at 237.571 GHz and
0.5 mJy at 90.250 GHz for CB 3 and 40.00 mJy at 237.571 GHz
and 7.0 mJy at 90.250 GHz for Serpens–FIRS 1. The corre-
sponding synthesized beams adopting uniform weighting were
0.4′′ × 0.3′′ at 237.571 GHz and 1.0′′ × 0.8′′ at 90.250 GHz
for CB 3 and 0.6′′ × 0.4′′ at 237.571 GHz and 1.7′′ × 0.7′′ at
90.250 GHz for Serpens–FIRS 1. (See Paper II for IC 1396 N.)
3. Results
In Table 1 we present the coordinates, sizes and mm fluxes of the
compact cores detected in Serpens–FIRS 1 and CB 3. The results
towards IC 1396 N are presented in Paper II. Only 1 mm-source
is detected in Serpens–FIRS 1 down to a separation of less than
100 AU. The other targets turned out to be multiple sources. We
have detected 2 mm-sources towards CB 3 and 4 mm-sources
towards IC 1396 N.
A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions 3
Fig. 2. On the left, we show the 5′×5′ Spitzer IRAC 3.6 µm image towards IC 1396 N (adapted from Getman et al. 2007). The
location of the globule is marked by the green contour and the Class III (yellow triangles), Class II (red circles) and blue squares
(Class 0/I) sources are indicated. On the right, we show the 3mm (up) and 1.3mm (down) continuum images observed with the
PdBI. In the 3mm image we also indicate the Class III (black triangles), Class II (red circles) and Class 0/I (filled blue squares)
sources.
The 4 compact sources towards IC 1396 N are grouped in
2 sub-clusters separated by 0.05 pc which are spatially coinci-
dent with the sources named BIMA 2 and BIMA 3 by Beltrán
et al. (2002). The projected distance between these sub-clusters
is similar to that found by Hunter et al. (2007) between the mm
sub-clusters in the massive star forming region NGC 6336 I. This
distance is also similar to the distance between the stars form-
ing the Trapezium in Orion (from 5000 to 10000 AU). Thus it
is a typical distance between the IM and massive stars in the
same cloud. Our high angular resolution observations reveal that
BIMA 2 is itself composed of 3 compact cores embedded in a
more extended component (see Fig. 2). These 3 compact cores
are new mm detections and constitute the first sub-cluster of
Class 0 IM sources detected thus far.
In CB 3 we have detected 2 mm-sources separated by 0.06 pc
(see Table 1 and Fig. 3). These compact cores are new detec-
tions and the separation between them is similar to that between
BIMA 2 and BIMA 3 in IC 1396 N. In fact, the structure of the
globule CB 3 resembles much that of IC 1396 N but the angu-
lar resolution of our observations prevent us from resolving any
possible sub-cluster of compact cores in this more distant source.
Note that the masses of CB 3-1 and CB 3-2 are similar to that of
the sub-cluster BIMA 2 (Paper II).
The number of detections is limited by the sensitivity of
our observations. In Table 1 we show the point source mass
sensitivity assuming a dust temperature of 100 K (typical for
hot cores and circumstellar disks around luminous Be stars)
and κ1.3mm=0.01 g
−1 cm2 for each target. It is possible that we
miss a population of weak Class 0/I sources in CB 3 where
the mass sensitivity is poor (0.04 M⊙). However, the sensitiv-
ity in Serpens–FIRS 1 (0.01 M⊙) and IC1396 N (0.007 M⊙) is
good enough to detect disks around early Be stars that usually
have masses of ∼0.01 M⊙ (see e.g Fuente et al. 2003, 2006).
We should have also detected massive disks (∼0.1 M⊙) around
Herbig Ae and T Tauri stars although the dust temperature is
lower, Td=15–56 K (Natta et al. 2000). But there is still the
possibility of the existence of HAEBE or T Tauri stars with
weak circumstellar disks that are not detected in our mm im-
ages. Another possibility is that we are missing a population
of hot corinos (we refer as “hot corino” to the warm material
(∼100 K) around a low mass Class 0 protostar regardless of its
chemical composition) with masses below the values reported in
Table 1. Our sensitivity is good enough to detect a hot corino
similar to IRAS 16293–2422 A and B (L∼10 L⊙) at the dis-
tance of our sources (see Bottinelli et al. 2004). Thus the possi-
ble “missed” hot corinos should correspond to lower luminosity
protostars. Finally, we can be missing a population of dense and
cold cores. Assuming a dust temperature of 10 K, these compact
cold cores should have masses of less than 0.17, 0.12 and 0.7
M⊙ in Serpens–FIRS 1, IC 1396 N and CB 3 respectively. These
masses are not large enough to form new IM stars.
4 A. Fuente et al.: Protostellar clusters in intermediate mass (IM) star forming regions
4. Discussion
Testi et al. (1999) studied the clustering around a large sample of
HAEBE stars. In order to quantify the concept, they introduced
the parameter Nk, defined as the number of stars in a radius of
0.2 pc, the typical cluster radius. They showed that rich clus-
ters are only found around the most massive stars, although the
parameter Nk is highly variable. Some Be stars are born quite
isolated, while others have Nk >70. For our sources this number
is 22 (Launhardt et al. 1998, but the census is not complete), 29
(from Fig. 1) and 28 (Getman et al 2007; Nisini et al. 2001) in
CB3, Serpens and IC 1396 N respectively, where all previously
known YSOs (Class 0, I, II and III) in the regions are considered.
Our maps show 2 sources in CB 3 on a 0.3 pc scale, 1 source
in Serpens-FIRS 1 on a 0.04 pc scale, and 4 sources in IC 1396 N
on a 0.1 pc scale. Defining Nmm as the number of mm sources in
a radius of 0.2 pc, we can estimate Nmm from our observations
and provide a revised value for the total number of YSOs at this
scale. In Serpens our interferometric observations do not add any
new mm source to previous data. We have observed the most in-
tense mm clump in Fig, 1, the most likely to be a multiple source,
and only found 1 compact source. Based on the 30m map shown
in Fig. 1 and assuming that all the clumps host only one source
we estimate Nmm∼7 from a total of 29 YSOs. In CB 3, our data
add 2 new mm sources (Nmm=2) to the previous census of YSOs
based on NIR studies. In IC 1396 N, we estimate Nmm=4–16.
The upper limit has been calculated assuming a constant den-
sity of mm sources in the region. Usually, the Class 0/I stars
are not uniformly distributed in the clouds, but grouped in sub-
clusters that are coincident with the peak of dense cores. Thus
the value of Nmm is very likely close to 4 and we assumed this
number hereafter. Since BIMA 2 and BIMA 3 were previously
detected in the X-rays surveys by Getman et al. (2007), we only
add two new sources (due to the multiplicity of BIMA 2) to the
total number of YSOs in this region.
Summarizing, the total number of YSOs is now 29, 24 and
30 for Serpens–FIRS 1, CB 3 and IC 1396 N respectively. While
Serpens–FIRS 1 is an extraordinarily rich cluster compared with
the clusters around Ae stars reported by Testi et al. (1999), CB 3
and IC 1396 N do not seem to become one of the crowded
clusters (Nk∼70) detected by these authors around Be stars.
However, this conclusion might not be true. The interferometer
is only sensitive to dense and compact cores and provides a bi-
ased vision of the star forming regions. In fact our interferomet-
ric observations accounts for less than 1% of the total interstellar
mass in the studied globules, i.e., ∼ 10, 58 and 64 M⊙ are missed
in Serpens–FIRS 1, CB 3 and IC 1396 N respectively (Alonso-
Albi et al. 2007). One possibility is that this mass is in the form
of many weak hot corinos which could eventually become low
mass stars. The fate of these hot corinos is, however, linked to
the evolution of the IM protostar that is progressively dispers-
ing and warming the surrounding material (Fuente et al. 1998).
Another possibility is that the ”missed” mass is in the form of an
extended and massive envelope. This envelope (if not totally dis-
persed by the IM star) could produce new stars in a forthcoming
star formation event.
5. Summary
We have searched for clustering at mm wavelengths in 3 IM star
forming regions. We have detected 1, 2 and 4 compact cores in
Serpens–FIRS 1, CB 3 and IC 1396 N respectively. The compact
cores are not distributed uniformly but grouped in sub-clusters
separated by ∼0.05 pc. Such a separation is a typical distance
Fig. 3. Dust continuum emission at 850 µm as observed with
SCUBA towards CB 3. In the inset, we show the 3mm contin-
uum image observed with PdBI. Note that two compact cores
are detected towards the single-dish peak.
for both IM and massive stars within the same cloud. We have
used our mm observations to complete the census of YSOs in
these regions and compare them with the clusters found by Testi
et al. (1999) in the more evolved HAEBE stars. Serpens–FIRS 1
seems to belong to an extraordinarily rich cluster. The density
of YSOs in the high luminosity sources IC 1396 N and CB 3 is
consistent with the density found in the clusters around Be stars
although our sources are not found between the most crowded
regions. The large amount of interstellar gas and dust in the stud-
ied regions suggest that new star formation events are still pos-
sible.
Acknowledgements. We are grateful to Phil Myers for his careful reading of
the manuscript. A.F. is grateful for support from the Spanish MEC and FEDER
funds under grant ESP 2003-04957, and from SEPCT/MEC under grant AYA
2003-07584.
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	Introduction
	Serpens-FIRS 1
	IC 1396 N
	CB 3
	Observations
	Results
	Discussion
	Summary
