Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L81 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014623 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Star formation triggered by the Galactic H II region RCW 120
First results from the Herschel
Space Observatory
A. Zavagno1 - D. Russeil1 - F. Motte2 - L. D. Anderson1 - L. Deharveng1 - J. A. Rodón1 - S. Bontemps2,3 - A. Abergel4 - J.-P. Baluteau1 - M. Sauvage2 - P. André2 - T. Hill2 - G. J. White5,6
1 - Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS &
Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille
Cedex 13, France
2 - Laboratoire AIM, CEA/IRFU CNRS/INSU Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
3 - Laboratoire d'Astrophysique de Bordeaux, CNRS/INSU Université de Bordeaux, BP 89, 33271 Floirac cedex, France
4 - Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud 11, 91405 Orsay, France
5 - Department of Physics & Astronomy, The Open University, UK
6 - Space Science Department, Rutherford Appleton Laboratory, Chilton, UK
Received 31 March 2010 / Accepted 12 April 2010
Abstract
Context. By means of different physical mechanisms, the expansion of H II regions can promote the formation of new stars of all masses. RCW 120 is a nearby Galactic H II region
where triggered star formation occurs. This region is well-studied -
there being a wealth of existing data - and is nearby. However, it is
surrounded by dense regions for which far infrared data is essential to
obtain an unbiased view of the star formation process and in particular
to establish whether very young protostars are present.
Aims. We attempt to identify all young stellar objects (YSOs),
especially those previously undetected at shorter wavelengths, to
derive their physical properties and obtain insight into the star
formation history in this region.
Methods. We use
-PACS
and -SPIRE images to determine the distribution of YSOs observed in the
field. We use a spectral energy distribution fitting tool to derive the
YSOs physical properties.
Results.
-PACS and -SPIRE images confirm the existence of a young source and allow us to determine its nature as a high-mass (8-10
)
Class 0 object (whose emission is dominated by a massive envelope
)
towards the massive condensation 1 observed at (sub)-millimeter wavelengths. This source was not detected at 24
m and only barely seen in the MISPGAL 70
m data. Several other red sources are detected at
wavelengths and coincide with the peaks of the millimeter condensations. SED fitting results for the brightest
sources indicate that, apart from the massive Class 0 that forms in condensation 1, low mass (0.8-4
)
stars are forming around RCW 120 with ages younger than
years.
This indicates that YSOs observed on the borders of RCW 120 are
younger than its ionizing star, which has an age of about 2.5 Myr.
Conclusions.
images allow us to detect new YSOs that are too young and embedded to be detected at shorter wavelengths (25 of the 49
sources are new detections). This offers a new and more complete view
of the star formation in this region. PACS and SPIRE fluxes were
obtained for the brightest YSOs and allow us to strongly constrain both
their spectral energy distribution and their physical properties
through SED fitting. A more accurate determination of their properties
allows us, for the first time, to discuss the star formation history in
this region by comparing
sources at
evolutionary stages.
Key words: stars: formation - H II regions - infrared: general
1 Introduction
![]() |
Figure 1:
A multiwavelength view of RCW 120. Left: the 870 |
Open with DEXTER |













Here we present the first
-PACS
and -SPIRE images of RCW 120. We use these images to discuss the
distribution and physical properties of the YSOs observed in the field.
Anderson et al. (2010) discuss the dust properties in this region.
2 Observations
RCW 120 was observed by the
telescope on October 9th, 2009 as part of the
science demonstration phase with PACS (Poglitsch et al. 2010) for the HOBYS (Motte et al. 2010) guaranteed time key program (GTKP)
and SPIRE (Griffin et al. 2010, Swinyard et al. 2010) for the ISM GTKP (Abergel et al. 2010). Data were taken
in five wavelength bands: 100 and 160
m for PACS (at resolutions 10
and 14
), and 250, 350, and 500
m for SPIRE (at resolutions of 18
,
25
,
and 36
). Two cross-scanned 30
(22
for SPIRE) maps at angles of 45
and 135
were obtained with the PACS and SPIRE instruments at a medium scan speed (30
/s).
We reduced the data using HIPE 2.0. The SPIRE images are the level 2
products of the SPIRE photometer pipeline. The PACS images were reduced
using a script kindly provided by M. Sauvage. The most recent
calibration changes for the PACS data were taken into account (see the
scan map release note of February 23, 2010). Aperture photometry was
performed on 49 sources in the field. All the images were convolved to
match the 500
m
image resolution. We selected a circular aperture on the sources and
subtracted a local background measured within an annular aperture.
![]() |
Figure 2:
Left: colour-composite image of RCW 120 of MIPSGAL 24 |
Open with DEXTER |
3 Herschel results
Figure 2 presents a composite image of RCW 120 at 24 m (green) and 100
m (red). The 24
m image shows the presence of bright sources, most of which are identified as YSOs by DE09. The PACS 100
m data detects bright sources corresponding to the sub-millimeter peaks, traced by the 870
m emission (see Fig. 1). Sources observed towards condensation 1 are shown in Fig. 2. A bright object (source 0 in Fig. 2) is clearly detected at 100 and 160
m with PACS and is also seen in the SPIRE data. This object is barely detected in the MIPSGAL 70
m data. Owing to its non-detection at shorter wavelengths, DE09 suggested that this source may be of Class 0.
data allow us to discuss the nature of this source by constraining its
SED in the far infrared range. The SED fitting for the reddest sources
detected in the field is presented in Sect. 3.1 and discussed in
Sect. 4. Results of the best-fit models are given in Table 1.
Using the numbering in DE09 (see their Figs. 10 and 2), we see that sources 36 and 38 are also bright at 100
m. Another source is detected, in-between these two, that was not detected before.
The PACS 100
m
image has sufficient resolution to determine the distribution of new
red objects, previously undetected at shorter wavelengths. These red
sources are mainly found towards the peaks of the millimeter
condensations. Source 50 seen towards the peak of
condensation 2, sources 69 and 76 towards condensation 4,
source 105 towards condensation 5, sources 106, 108, and 110
towards condensation 6, and source 58 towards condensation 7
are bright at 100
m.
Eight new highly-embedded sources are detected towards
condensation 5 and two new sources are detected towards
condensation 8 at 100
m.
Of the 49 sources detected and measured in PACS and SPIRE data, 25
(more than a half) were previously not (or marginally) detected at
24
m. Since all these new sources are probably YSOs in an early evolutionary stage, this illustrates how
changes our view of the star formation in this region.
Figure 1 shows a multiwavelength view of RCW 120. The
-GLIMPSE 8
m, SPIRE (250 and 500
m), and the APEX-LABOCA 870
m images are shown. The (sub)-millimeter condensations discussed in ZA07 and DE09 are identified and LABOCA 870
m contours are superimposed. The 8
m
image shows the emission from polycyclic aromatic hydrocarbons that
delineates the photodissociation region surrounding RCW 120.
Absorption patches are clearly seen corresponding to condensations 5,
6, and 9.
Extended emission is also seen in the PACS and SPIRE images. PACS 100 and 160 m emision delineate the photodissociation region. The shape of the 100
m extended emission outlining the PDR is surprisingly similar to that of the 8
m emission. This is also found for other Galactic H II regions (see, for example, the case of N49, Zavagno et al. 2010).
The dust responsible for these emissions are, however, different. The
similarity observed between the two types of emission apperas to
indicate that there is a temperature gradient in the form of different
layers that coexist in the PDR.
Emission at 100 m
is observed towards the interior of the ionized region. This is
probably caused by projection effect, material being emitted from the
front and/or back of the bubble. The longer wavelength SPIRE images
clearly trace the colder material, observed away from the ionization
front (Fig. 1). YSOs are
detected in there, far from the ionization front. The distribution of
the extended emission can be seen in Fig. 1 of Motte
et al. (2010) and Anderson et al. (2010).
3.1 SED fitting of YSOs
We performed aperture photometry on 49 sources in the field. We then used the SED-fitting tool of Robitaille et al. (2007) to derive the properties of the brightest objects seen on the PACS 100 m image. We performed the fit from 24
m to 870
m. The parameters of the best models are given in Table 1. Figure 3 shows the result of the fit for source 0 observed towards condensation 1.
![]() |
Figure 3:
Spectral energy distribution obtained for source 0, the massive YSO
observed towards condensation 1. The best-fit (dark solid line)
function obtained using the Robitaille et al. (2007) model is
shown. This is indicative of a massive (8-10 |
Open with DEXTER |
Table 1:
Parameters obtained using the SED fitting tool of Robitaille et al. (2007) for the brightest PACS
100 m sources.
4 Discussion
PACS and SPIRE images confirm the existence of a young source observed
towards condensation 1. These far-IR data allow us, for the first
time, to determine its nature. Its emission is dominated by a massive
envelope (10
). At 100
m, the source has a size (beam-corrected FWHM) of 10.9
,
which is 0.07 pc for a distance of 1.3 kpc. We fitted its SED
with a grey body to characterize its temperature. For a dust emissivity
index
,
we derived a temperature of 18 K. We computed the
/
ratio and derived a value of 0.12, characteristic of Class 0 protostars (André et al. 2000). All these indicators show that this source is a massive (8-10
)
Class 0 protostar. This source lies in condensation 1,
elongated along the ionization front and probably formed with the
material collected during the expansion of the ionized region.
Source 0 is thus the first detection of a massive protostellar
object formed by the collect and collapse process.
Compared to the results given in DE09, the results given in Table 1
show that the constraints on the long wavelength part of the spectrum
tend to increase the value derived for the envelope accretion rate, and
infer younger sources. The stellar ages span a range of between 103 and
years
for all the sources, but are not well constrained by the model for a
given source and therefore are not included in Table 1. These values are well below the age of
2.5 Myr derived for the RCW 120 ionizing star by Martins et al. (2010), indicating that YSOs observed on the border of RCW 120 are indeed younger than the ionizing star.
Source 105, detected farther away from the ionization front, seems to
be in an earlier evolutionary stage than, for example, source 36. Both
objects have stellar masses of about 1 ,
but the higher accretion rate and higher envelope mass derived for
source 105 point to a younger source (the possibility that source 105
is forming a higher mass star does not change the conclusion: high mass
stars evolve rapidly, meaning that this source is indeed young). This
is interesting, as source 105 is observed farther away from the
ionization front and in a dense region that may indicate a delay in the
propagation of star formation around RCW 120. However, the massive
Class 0 object, source 0, is observed towards condensation 1,
adjacent to the ionization front. This indicates that star formation
takes more time there, even if it is close to the ionization front.
Different physical conditions in this region (higher density, lower
temperature, higher magnetic field) can decelerate the star formation
process. The mass of the source also complicates a direct comparison
between the different regions. High resolution observations using
chemical and/or dynamical clocks will be performed to further
investigate this point, now that we have more accurately constrained
the YSO properties and can compare objects with
masses at possibly
evolutionary stage. Understanding the star formation triggered by H II regions
are complicated by the difficulty in ascertaining the mass and
evolutionary stage of YSOs observed on the borders of these regions.
This problem is important when searching for age gradients in regions
where triggered star formation is believed to occur.
A question that remained unanswered by Martins et al. (2010) was whether, for objects with similar spectral properties, brighter 24 m sources are more massive or in a later evolutionary stage. Sources 69 and 106 have similar properties (see Table 1), and are both dominated by Br
emission in the near-IR (Martins et al. 2010), but source 106 is much brighter at 24
m than source 69. Given the new constraints derived from their SEDs, we suggest that the difference in 24
m emission
is indicative of different evolutionary stages. Many other sources studied by Martins et al. (2010) towards three Galactic H II regions observed by
will be compared in a forthcoming paper. Observations using age tracers will also be peformed to confirm this hypothesis.
5 Conclusions
We have presented the first images obtained with PACS and SPIRE for the Galactic H II region
RCW 120. These images detected an embedded population of YSOs that
had not been previously detected, or had been barely detected, at
wavelengths shorter than 100 m.
The broad wavelength coverage of PACS and SPIRE allows us to constrain
the SED of these sources and derive their physical properties. Our main
conclusions are:
- We detect a massive (8-10
) YSO towards condensation 1 and show that this source is a massive Class 0 object. This is the first detection of a massive Class 0 formed by the collect and collapse process on the borders of an H II region. This source was barely detected previously in the MIPSGAL 70
m data but its mass and exact evolutionary stage were unknown.
- Our stronger constraints on the source SEDs allow us to determine more accurately the physical properties of the observed YSOs. This provides additional insight into the star formation history of this region because we can now trace (and compare) objects with similar masses but in different evolutionary stages.
- YSOs revealed by the PACS and SPIRE are observed farther away from the ionization front, especially towards condensation 5 where dark filaments are observed in the near- and mid-IR. This suggests that triggered star formation may occur by means of the tunnelling of radiation from the ionizing star of RCW 120 into the ambient medium.
Part of this work was supported by the ANR (Agence Nationale pour la Recherche) project ``PROBeS'', number ANR-08-BLAN-0241 (LA). PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain). SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA).
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Footnotes
- ...Herschel
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
All Tables
Table 1:
Parameters obtained using the SED fitting tool of Robitaille et al. (2007) for the brightest PACS
100 m sources.
All Figures
![]() |
Figure 1:
A multiwavelength view of RCW 120. Left: the 870 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Left: colour-composite image of RCW 120 of MIPSGAL 24 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Spectral energy distribution obtained for source 0, the massive YSO
observed towards condensation 1. The best-fit (dark solid line)
function obtained using the Robitaille et al. (2007) model is
shown. This is indicative of a massive (8-10 |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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