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203
STAR FORMATION IN CLUSTERS: FROM ISO TO FIRST
P. Saraceno
1
, M. Benedettini
1
, C. Codella
1
, A.M. Giorgio
1
, S. Molinari
1
, S. Pezzuto
1
, L. Spinoglio
1
,and
L. Testi
2
1
Istituto di Fisica dello Spazio Interplanetario, CNR,Via del Fosso del Cavaliere 100, 00133 Roma, Italy
2
Osservatorio astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
Abstract
FIRST will give a great contribution to the study of
star formation. We think that at the beginning of the mis-
sion it will be crucial to carry out few selected key projects
able to give important indications for all the following ob-
servations. We suggest: the survey of large areas of sky (in
particular the galactic plane) and the systematic study of
a few clusters and protoclusters close to the Sun in or-
der to take the full advantage of the spatial resolution of
FIRST.
for the determination of the dust temperature, which is
necessary to estimate the dust masses.
ii) The formation of massive stars: the mass of the most
massive object of a cluster seems to increase with the stel-
lar density of the cluster (e.g. Zinnecker et al. 1993). As
an example, in Taurus, a region of low mass star forma-
tion, the average distance between stars is 0.3 pc (Gomez
et al. 1993), while the intermediate mass stars like Herbig
AeBe are inside clusters with a separation ranging from
0.2 pc, for the less massive objects, to 0.06 pc for the most
massive ones (Testi et al. 1999). Finally, in the high mass
stars region of the Trapezium cluster a stellar density in
excess of 2200 pc
−
3
has been found (Herbig & Terndrup
1986), with an average distance among stars of less than
15.000 AU. These small distances are of the order of the
stellar envelopes, making highly probable the occurring of
interactions among protostars during the star formation
process.
Key words: stars:formation; stars:clusters
1. Introduction
The formation of isolated stars is a process now fairly
well understood and several models have been suggested
during the last two decades (e.g. Mouskovias & Ciolek
1999, Shu et al. 1987, Palla & Stahler 1999, Bernasconi &
Mader 1996).
However, the observed large fraction of stars belong-
ing to multiple systems and the observational evidence
that most stars form in clusters (e.g. Nordh et al. 1996 for
Chamaleon; Lada et al. 1991 for L1630; Wilking & Lada
1985 for Taurus) show that interactions among the form-
ing objects play an important role during star formation.
This has also been highlighted by Palla & Stahler (2000),
who observationally found that star formation in clusters
is constant for million years, then it undergoes to a steep
acceleration. In particular the study of star formation in
clusters is important for:
i) The study of the origin of the initial mass function,
crucial to understand the evolution of the stellar popula-
tions in our and other galaxies. In fact, even if the mech-
anism that fixes the final mass of stars is not understood,
it is well agreed that the explanation has to be found in
protoclusters, which represent the first stages of cluster
formation (Williams et al. 1995, Andre & Motte 2000).
Millimeter surveys of protoclusters (e.g. Testi & Sargent
1998, Motte et al. 1998) are affected by large uncertainties
in the determination of the masses, because the spectral
energy distributions (SEDs) of the pre-stellar cores (e.g.
L1544 in Fig.1) peak at 100 -200
PACS
SPIRE
Figure 1. Spectral energy distribution of the pre-stellar core
L1544 and the protostar IRAS 16293, together with simple
gray body fits. PACS and SPIRE spectral windows are reported
(adapted from Andre et al. 2000).
2.Starformationinclusters:theroleofFIRST
, in the FIRST spec-
tral range, and observations around the peak are crucial
It is known that protostars are luminous: in particular
low mass stars are much more luminous during the proto-
stellar phase than during the Main Sequence phase (e.g.
Stahler 1988, D’Antona & Mazzitelli 1994). Therefore, if
Proc. Symposium
‘The Promise of the Herschel Space Observatory’
12–15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
µm
204
P. Saraceno et al.
stars are born in clusters, the factor that limits the de-
tections is more likely confusion (e.g. Franceschini et al.
1991) rather than sensitivity. Besides, because protostars
are evolving very fast, we can easily have in the same
beam objects in different evolutionary stages, with com-
pletely different physical characteristics that can not be
easily discriminated (Saraceno et al. 2000). It follows that
it is very important to have high spatial resolution in the
FIR and submillimeter, where the SEDs of protostars peak
(Fig.1).
two kinds of key projects for the study of star formation
with FIRST.
1) The survey of large areas of sky, as discussed by P.
Andre and by S. Molinari at this conference, using the fast
mapping capability of SPIRE. Molinari et al. show that all
the galactic plane can be surveyed with SPIRE in 70 days
at a limiting sensitivity of 100 mJy. This survey could be
complete down to
∼
0.1 M
for D
≤
500 pc and to
∼
10
5 kpc. The galactic plane survey should be one
of the highest priority projects of FIRST and should be
published in the first year to make possible the follow-up
observations similar to those discussed in the next point.
2) the study of known condensations using PACS and
SPIRE, to measure the SED of individual members of clus-
ters and protoclusters and using the spectrometers of both
instruments to study the ISM of clusters. Most of the time
of these programmes will be used by PACS, whose pho-
tometric capabilities are necessary both to measure the
SEDs around the peaks and to minimize source confusion
using its high spatial resolution.
Since the proposal 1) is discussed by other authors in
these proceedings, in the following sections we will discuss
the programs introduced in 2).
≤
Mass detectable
(460 pc, 5σ, 1hr)
10
1
Mo
10
0
10
-1
0.08 M
10
-2
3. Pointed observations of known clusters.
10
-3
4 pc found by Herbig & Tern-
drup (1986) in the Trapezium cluster is of the order of the
size of the stellar envelopes and of the disks observed by
HST in the same region. Therefore we tentatively assume
that this dimension is of the order of the limit given by
source confusion. This separation corresponds to a reso-
lution of
≤
0
.
10
-4
30 K
60
arcsec at the distance of Taurus, well above the FIRST
spatial resolution. Therefore we think that
all the known
clusters and protoclusters within 500 pc have to be stud-
ied by FIRST at high priority
because, within this dis-
tance, we will not be much limited by source confusion.
Within this distance we should easily detect all accreting
objects to the limit of H burning (e.g. Fig.9 of Stahler
1988), and all the pre-stellar condensations to the limit
of the Jupiter mass (
∼
18 arcsec at the distance of Orion and
∼
10
-5
20 K
10 K
10
-6
0
200
400
600
λ
[
µ
]
Figure 2. Minimum detectable mass, for different dust tempera-
tures. The solid lines represent the PACS and SPIRE detection
limits, while the dashed lines on the right give the limits for the
JCMT (15m) with the SCUBA camera. The horizontal dashed
line corresponds to the mass limit for Hydrogen ignition.
M
). This is shown in Fig.2,
where the minimum mass detectable by FIRST is plot-
ted for an object at the distance of Orion. The minimum
mass is computed for dust temperatures of 10, 20 and 30
K, as: M
dust
=S
ν
∼
10
−
3
kB
ν
(T) (Hildebrand 1983) where
S
ν
is the minimum detectable flux, D the distance of the
cloud, k the dust emissivity and B
ν
(T) the Planck function
at the temperature T. The figure shows that all the spa-
tially resolved proto-brown dwarfs should be detected by
FIRST. Moreover the FIR colors will be able to discrimi-
nate among the different evolutionary stages of protostars
(Pezzuto et al. 1999, Saraceno et al. 1999a).
Only two studies of protoclusters in the millimeter con-
tinuum have been published so far. One is the Serpens
core, mapped with the OVRO interferometer at 3mm with
D
2
/
The two imaging instruments of FIRST, PACS and
SPIRE, have both broad-band and line imaging capabil-
ities. PACS works in the range where the SEDs of pro-
tostars peak and, having a pixel resolution as small as 5
arcsec, it is the best instrument for the study of known
clusters, while SPIRE, having a lower spatial resolution
but a larger field of view (a factor
∼
M
for D
The mean separation of
5 larger than PACS)
is the best instrument to survey large areas of sky. These
fundamental capabilities of the two instruments suggest
Star Formation in Clusters: from ISO to FIRST
205
Table 1. Example of clusters and protoclusters within 500 pc to
be measured with FIRST in high priority.
Possible targets
Area PACS SPIRE
5
σ
50mJy
[hours]
ρ
-Oph main cloud
1
◦
x1
◦
50
10
Perseus NGC1333
1
◦
x2
◦
100
20
(& sourroundingcores)
Orion Complex
1
◦
x5
◦
250
50
(L1641, L1630,
BN-KL, ΛOri)
Chamaleon I
0.5
◦
x2
◦
50
10
Serpens protocluster 0.5
◦
x0.5
◦
12
2
Lupus 1-2-3
1
◦
x2
◦
40
8
Total
502 100
Figure 3. Bottom panel: in black the 5
×
5 arcmin map of the
Serpens cloud core observed at 3mm with the OVRO interfer-
ometer (Testi & Sargent 1998),the resolution is 5.5”
×
4.3”
and the limiting sensitivity 3 mJy. In color the simulation of
FIRST observation at a limiting sensitivity of 50 mJy ; Top
panel: the derive mass spectrum, bleak from observation, in
color the assumed one for the simulation.
sensitivity of
50 mJy in the three bands of PACS is of the order of
two hours, while only 0.4 hours are needed for SPIRE.
This very simple simulation shows that FIRST multi-band
imaging observations will detect many more sources than
those of the ground based millimeter observations, allow-
ing precise definition of temperatures and masses of the
individual cluster members, producing large samples of
high statistical significance.
In Table 1 we give a tentative list of the most relevant
clouds within 500 pc with a rough estimate of the area to
be mapped and of the integration times needed, given the
present sensitivities of PACS and SPIRE.
σ
3” (Testi & Sargent 1998) at a lim-
iting sensitivity of 3 mJy; the other one is the
5”
×
4
.
Ophiuchi
core, mapped with the IRAM 30m telescope at 1.3mm
with a resolution of 11” and a limiting sensitivity of
ρ
∼
8
mJy (Motte et al. 1998).
Fig. 3 (lower panel) shows in black the millimeter map
of the Serpens protocluster (Testi & Sargent 1998) with
the 26 condensations detected: in the upper panel the de-
rived mass function is reported in black. On the observed
millimeter map we superimposed, in color, a simulation
of what FIRST will be able to detect at 100
4. Imaging spectroscopy
µm
,ata5
The spectroscopic observations of the ISO satellite have
shown the great power of FIR lines to trace the warm gas
of the star forming regions (Saraceno et al. 1999b), where
it is possible to find the signature of the interactions go-
ing on among the members of a clusters. The spectroscopic
imaging capability of PACS and SPIRE will provide, for
each spatially resolved element, a full spectrum at interme-
diate spectral resolution tracing the physical and chemical
conditions of the gas and allowing the study of the pro-
cesses going on (outflows, shocks, stellar winds, ionizing
fields, etc.). Given the low extinction of the FIR lines, it
will be possible to trace the innermost parts of the cluster,
obscured in the near infrared.
M
).
The number of possible detections of FIRST has been es-
timated assuming a linear extrapolation of the observed
mass function (in color, Fig.3, upper panel) down to 0.08
M
; the number of objects in each mass bin has been esti-
mated and the mm flux has been computed assuming for
the dust T = 20 K, emissivity k=0.005 cm
2
/g at 1.3 mm
and a
sensitivity of 50 mJy, (roughly the flux of 0.08
= 1.5. These simulated objects, assumed point-
like, were smeared with a 100
β
µm
diffraction pattern (2d
gaussian) and distributed normally around the location
where most objects were detected in the observations at
3.4 mm.
Given the field of view of SPIRE and PACS the en-
tire Serpens core will be mapped with few exposures: the
time needed to survey this area at a 5
aresolutionof5
.
σ
206
IC 1396N
than the CO one (with very few exceptions). This seems
to be due to a water vapour abundance definitively lower
than the predicted one (Saraceno et al. 1999b, Nisini et al.
1999, van Dishoeck 1998a, van Dishoeck 1998b, Spino-
glio et al. this conference): a result recently confirmed by
the Submillimeter Wave Astronomy Satellite (SWAS) (see
the papers of Melnick, Bergin, Snell, Neufeld, Matthew
and collaborators in the special issue of the Astrophysical
Journal dedicated to SWAS), which in addition did not
detect any O
2
emission.
Both the ISO and SWAS results show that the cur-
rent models do not explain the abundance of the oxygen
bearing molecules, asking for a more detailed study of the
oxygen chemistry. The high spatial and spectral resolu-
tion observations of H
2
O, OH, O
2
, O and CO that will be
possible with FIRST will provide a much clearer picture
of the oxygen chemistry.
SPIRE
PACS
References
Figure 4. The CO lines detected by ISO-LWS toward the
IC1396N source (Saraceno et al., in preparation) plotted as a
function of the upper rotational quantum number (J
up
). The
model fit indicates the presence of two regions at different tem-
perature in the beam. The SPIRE and PACS wavelength ranges
are indicated.
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10
−
4
), has
a relatively high dissociation energy (11.09 eV) and has
transitions excited from the millimeter (in the cold gas of
molecular clouds) to the near infrared (in the atmospheres
of cool stars). In the FIR, the CO lines are generally op-
tically thin and the line emission can be fitted to derive
temperature and volume density. An example is given in
Fig.4, where the ISO-LWS observations of IC1396N are re-
ported with a model fit that clearly shows the presence in
the beam of two gas components at different temperatures.
FIRST will observe this source with a beam 100 times
smaller, allowing to better discriminate the different emit-
ting regions. Moreover, the SPIRE extended range in the
submillimeter will allow to measure lines at lower values of
the rotational quantum number (J
up
) giving a definitely
more accurate determination of temperature and density
of the lower temperature gas component. The higher sen-
sitivity of PACS, with respect to ISO-LWS, will allow to
explore the high J
up
part of the spectrum tracing the gas
at temperatures as high as T
∼
2000 K. Finally the high
10
7
) will allow to dynam-
ically discriminate the different emitting components.
Another important molecule used to trace the gas in-
side clusters is H
2
O. The ISO results have shown that, in
contrast with model predictions (e.g. Kaufman & Neufeld
1996), the gas cooling due to H
2
O is significantly lower
One of the best tools to trace the gas inside the clus-
ters is the CO molecule (Nisini et al. 1997, Saraceno et
al. 1999b). CO is very abundant ([CO]/[H
2
]
spectral resolution of HIFI (R
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