Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L99 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014657 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
The physical properties of the dust in the RCW 120 H II
region as seen by Herschel![[*]](/icons/foot_motif.png)
L. D. Anderson1 - A. Zavagno1 - J. A. Rodón1 - D. Russeil1 -
A. Abergel2 -
P. Ade3 -
P. André4 -
H. Arab2 -
J.-P. Baluteau1 -
J.-P. Bernard5 -
K. Blagrave6 -
S. Bontemps7 -
F. Boulanger2 -
M. Cohen8 -
M. Compiègne6 -
P. Cox9 -
E. Dartois2 -
G. Davis10 -
R. Emery11 -
T. Fulton12 -
C. Gry1 -
E. Habart2 -
M. Huang10 -
C. Joblin5 -
S. C. Jones13 -
J. M. Kirk3 -
G. Lagache2 -
T. Lim11 -
S. Madden4 -
G. Makiwa13 -
P. Martin6 -
M.-A. Miville-Deschênes2 -
S. Molinari14 -
H. Moseley15 -
F. Motte4 -
D. A. Naylor13 -
K. Okumura4 -
D. Pinheiro Gonçalves6 -
E. Polehampton11,13 -
P. Saraceno14 -
M. Sauvage4 -
S. Sidher11 -
L. Spencer13 -
B. Swinyard11 -
D. Ward-Thompson3 -
G. J. White11,16
1 - Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F.
Joliot-Curie, 13388 Marseille Cedex 13, France
2 -
Institut d'Astrophysique Spatiale, UMR 8617, CNRS/Université Paris-Sud 11, 91405 Orsay, France
3 -
Department of Physics and Astronomy, Cardiff University, Cardiff, UK
4 -
CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
5 -
CESR, Université de Toulouse (UPS), CNRS, UMR 5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
6 -
Canadian Institute for Theoretical Astrophysics, Toronto, Ontario M5S 3H8, Canada
7 -
CNRS/INSU, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, BP 89, 33271 Floirac cedex, France
8 -
University of California, Radio Astronomy Laboratory, Berkeley, 601 Campbell Hall, US Berkeley CA 94720-3411, USA
9 -
Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin d'Hères,
France
10 -
National Astronomical Observatories, PR China
11 -
The Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
12 -
Blue Sky Spectrosocpy Inc, Lethbridge, Canada
13 -
Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Canada
14 -
Istituto di Fisica dello Spazio Interplanetario, INAF, via del Fosso del Cavaliere 100, 00133 Roma, Italy
15 -
NASA - Goddard SFC, USA
16 -
Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
Received 31 March 2010 / Accepted 30 April 2010
Abstract
Context. RCW 120 is a well-studied, nearby Galactic H II region
with ongoing star formation in its surroundings. Previous work has
shown that it displays a bubble morphology at mid-infrared wavelengths,
and has a massive layer of collected neutral material seen at sub-mm
wavelengths. Given the well-defined photo-dissociation region (PDR)
boundary and collected layer, it is an excellent laboratory to study
the ``collect and collapse'' process of triggered star formation. Using
Space Observatory data at 100, 160, 250, 350, and 500
m, in combination with
and APEX-LABOCA data, we can for the first time map the entire spectral energy distribution of an H II region at high angular resolution.
Aims. We seek a better understanding of RCW 120 and its
local environment by analysing its dust temperature distribution.
Additionally, we wish to understand how the dust emissivity index, ,
is related to the dust temperature.
Methods. We determine dust temperatures in selected regions of
the RCW 120 field by fitting their spectral energy distribution
(SED), derived using aperture photometry. Additionally, we fit the SED
extracted from a grid of positions to create a temperature map.
Results. We find a gradient in dust temperature, ranging from 30 K in the interior of RCW 120, to
20 K for the material collected in the PDR, to
10 K toward local infrared dark clouds and cold filaments. There is an additional, hotter (
100 K)
component to the dust emission that we do not investigate here. Our
results suggest that RCW 120 is in the process of destroying the
PDR delineating its bubble morphology. The leaked radiation from its
interior may influence the creation of the next generation of stars. We
find support for an anti-correlation between the fitted temperature and
,
in rough agreement with what has been found previously. The extended wavelength coverage of the
data greatly increases the reliability of this result.
Key words: H II regions - ISM: individual objects: RCW120 - dust, extinction - photon-dominated region (PDR) - stars: formation - infrared: ISM
1 Introduction
RCW 120 (Rodgers et al. 1960) is a Galactic H II region that displays a ring morphology at mid-infrared and sub-mm wavelengths, and is presumably a bubble viewed in projection. It has recently been studied by Zavagno et al. (2007, hereafter ZA07) and Deharveng et al. (2009, hereafter DE09) in the context of triggered star formation. It is only 1.3 kpc from the Sun (see ZA07, and references therein), and is thus one of the closest Galactic H II regions.
ZA07 analysed the 1.2 mm emission of RCW 120 and found a fragmented
layer of neutral material adjacent to the photo-dissociation region
(PDR). They identified eight millimeter condensations, five of which
lie in the collected layer of material, but found no massive young
stellar objects (YSOs) within the condensations. They did, however,
locate numerous YSOs surrounding RCW 120, indicating that star
formation is active in the region. Using APEX-LABOCA observations at
870 m, and Spitzer-MIPS observations at 24
m,
DE09
extended this work and calculated column densities and masses for the
sub-mm condensations (plus an additional condensation, #9). They
found additional YSOs in the field, including a chain
of 11 evenly-spaced YSOs inside the most massive condensation
and a very
dense core harboring a (possibly class 0) YSO. This work
highlights
the impact RCW 120 is having on star formation far from the
ionizing
source. An analysis of the YSOs in the RCW 120 field using the
Herschel data presented here is given in a companion paper (Zavagno et al. 2010).
The ring morphology shown by RCW 120 is a common feature of Galactic H II regions; Churchwell et al. (2006,2007) identified almost 600 such objects in the Spitzer-GLIMPSE data (Benjamin et al. 2003). Deharveng et al. (2010) have shown that over 85% of infrared (IR) bubbles enclose H II regions. Because of their morphology, it is easy to locate the PDRs of such bubbles and one can more easily identify the swept-up material that is necessary for the collect and collapse process (Elmegreen & Lada 1977). These objects present an opportunity to assess the efficiency of triggered star formation throughout the entire Galaxy. Data from the Herschel telescope (Pilbratt et al. 2010) allow us for the first time to map the dust temperature variations over an entire H II region at high resolution. We can thus better determine the effect H II regions have on the creation of the next generation of stars.
2 Data
RCW 120 was observed by the Herschel telescope on 9 October 2009
with the PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010)
as part of, respectively, the HOBYS (Motte et al. 2010) and
``Evolution of Interstellar Dust'' (Abergel et al. 2010) guaranteed
time key programs. Data were taken in five wavelength bands: 100 and 160 m for PACS (at resolutions of
and
and a final map size of
), and 250, 350, and 500
m for SPIRE (at resolutions of
,
,
and
and a final map size of
). We
reduced these data using HIPE version 2.0. The SPIRE images used here
are level 2 products, produced by the SPIRE photometer pipeline. We
reduced the PACS data using a script provided by M. Sauvage. Recent
calibration changes for the PACS and SPIRE data were taken into
account (Swinyard et al. 2010); throughout we use calibration
uncertainties of 10% and 20% respectively for the PACS
100
m and 160
m bands, and 15% for all three SPIRE
bands (Swinyard et al. 2010).
We also utilize Spitzer-MIPSGAL data (Carey et al. 2009) at
24 m and 70
m (at resolutions of
and
), and APEX-LABOCA data at 870
m from D09 (at a
resolution of
). The MIPSGAL 70
m maps have strong
striping that we mediate with median filtering of the scan-legs
(see Gordon et al. 2007). In Fig. 1 we show a
three-color image with SPIRE
250
m (red), PACS 100
m (green), and
MIPSGAL 24
m data (blue). Regions of interest are
superimposed on the data; the condensation numbers are from ZA07 and
DE09.
3 Dust properties
We assume the dust emission in RCW 120 can be modeled by an optically
thin grey-body, and that the emissivity of the dust grains can be fitted
with a power law (Hildebrand 1983):
where







There has been much discussion about the value of the dust emissivity
index, ,
and whether its value is anti-correlated with the dust
temperature, T. Generally,
is thought to range from 1 to 2, but
may also vary with wavelength (Meny et al. 2007).
Dupac et al. (2003) found that
and T are inversely related.
Désert et al. (2008) found a similar inverse-relation
to exclude a constant value of
at the 99.9% confidence level.
Shetty et al. (2009a,b), however, suggested that this
relationship arises due to the influence of noise in a least-squares
fit of Eq. (1), and from the combination of multiple
emission components along the line of sight.
A correlation between
and T may indicate a change in
dust properties at high density (Stepnik et al. 2003).
![]() |
Figure 1:
Three-color image of RCW 120 composed of
250 |
Open with DEXTER |
3.1 Aperture photometry
To determine the dust temperature structure of RCW 120, we performed
aperture photometry measurements on selected areas in the RCW 120
field. We resampled all images to the resolution of the MIPSGAL
24 m data to avoid pixel edge effects. There are three
different areas of interest in the field of RCW 120: the interior of
the bubble, the PDR (including
condensations), and local infrared dark clouds (IRDCs) that appear in
emission at the PACS and SPIRE wavelengths. The apertures we used are
shown in Fig. 1. For each aperture, we selected a
nearby background aperture that best characterizes the local
background emission.
For all apertures, we fit a single temperature to the coldest emission
component of the spectral energy distribution (SED) using a
non-linear least squares routine in two trials: once with
,
and once with
allowed to vary. For all apertures,
the 24
m emission is significantly greater than what would be
predicted by a single, cold temperature. This hotter component,
likely caused in part by transiently heated small grains, is not
well-constrained by the available data and therefore we ignore it in
the present work. We also exclude the 70
m data point
(and sometimes the 100
m data point) if it is inconsistent
with the emission from a single, cold temperature component. We leave the column density as a free parameter.
Table 1: Dust properties derived from aperture photometry.
![]() |
Figure 2:
Example SED fits for the ``Condensation 1'' (red),
``Interior 1'' (blue) and ``Condensation 5'' (green) apertures.
Points included in the SED fits are shown filled; points excluded
from the fits are shown open. The fit with |
Open with DEXTER |
The results of our aperture photometry are shown in
Table 1, and examples of the SED fits are shown in
Fig. 2. Uncertainties listed in
Table 1 are the formal values, taking into
account uncertainties in the photometry measurements and calibration
uncertainties. The direction of the interior of RCW 120
has dust temperatures of
30 K. This result is rather
uncertain, however, because of the lack of emission above the background level in the interior of RCW 120 at wavelengths
250
m.
The lack of emission at long wavelengths is evidence for a flattened,
two dimensional ring instead of a bubble, as suggested by
Beaumont & Williams (2010).
The condensations and PDR have characteristic temperatures of
20 K and the IRDCs have temperatures of
10 K, on the
low end of the temperature distribution for IRDCs found by Peretto et al. (2010). The temperatures change little in the two trials.
Uncertainties in temperature are small (often
), and errors in
are
generally 10%.
The
and T values, together with the empirically derived
curves of Dupac et al. (2003) and Désert et al. (2008), are shown in
Fig. 3. Figure 3 clearly shows
two
groups, one for the PDR (and the condensations along
the PDR) at
,
and one for dark clouds and
cold filaments at
.
We find an
anti-correlation between
and the dust temperature. Fitting a
regression line of the same functional form as that of
Désert et al. (2008) reveals:
![]() |
(2) |
The functional form of the relationship given in Dupac et al. (2003) does not fit our data. While these data do support the



![]() |
Figure 3:
The relationship between the fitted temperature and dust
emissivity index |
Open with DEXTER |
3.2 Temperature map
While aperture photometry is well-suited for investigating the average
dust properties of small areas in the field of RCW 120, a higher
resolution map of dust temperature is useful for determining the
small-scale variations in dust properties. To construct such a
temperature map, we fit the SED extracted at each pixel with a
single-temperature grey-body model (Eq. (1)), again
leaving the column density as a free parameter.
Schnee et al. (2007) showed that when attempting to simultaneously
constrain
and T, uncertainties in the derived parameters can
be very large due to the effect of noise in the data. We find that
this is indeed the case and
therefore in the following we fix
to a value of two,
which is valid for most areas of the map (see Table 1).
Fitting the SED of each pixel requires matching the resolution and
pixel locations of all wavelengths to those of the lowest resolution
image. The SPIRE 500 m data point has little impact on the
derived fit parameters because we have higher-resolution data at
870
m; we exclude the 500
m data point so we can rebin
to the higher resolution of the SPIRE 350
m data instead. We
smooth all images with a two-dimensional Gaussian representing the
SPIRE beam at 350
m, and rebin to the 350
m resolution
using Montage
. We subtract
an average background value from the data at all wavelengths,
estimated from a field nearly devoid of emission. For all but the
coldest dust, the Spitzer 70
m data point is dominated
by emission from the cold component; we use this data point in the SED
fits here. The inclusion of these data, however, causes the fitted
temperature to be overestimated in very cold regions (see fits in
Fig. 2).
![]() |
Figure 4:
The temperature map derived from the SED fits of each pixel,
over the same area as Fig. 1. When deriving these
maps, we fixed |
Open with DEXTER |





The action of the ionized material appears to be creating openings in
the PDR in several locations, shown with green lines in
Fig. 4. These openings can also be seen in
Fig. 1. There appears to be dust that is hotter by
5 K outside the PDR at these locations, heated by radiation
leaking through the holes in the PDR. This higher temperature
dust may lead to the collapse of future generations of stars as it
interacts with local molecular material outside the PDR. For the
opening due south, this effect may have aided the creation of
the sources inside Condensation #8 seen in Fig. 1;
the directions of the other openings show no indication of
increased star-formation activity.
There are a number of cold filaments in the field of RCW 120 that have
dust temperatures of 10 K. All such cold filaments appear
bright at wavelengths >250
m and therefore appear red in
Fig. 1. These filaments are associated with IRDCs
seen in Spitzer 8.0
m data. The most obvious cold
patches in the field are from the IRDCs to the north, but there are
numerous, thin filaments throughout the field. A number of cold
filaments are oriented radially away from RCW 120, the coldest of
which are towards the south-west. We suggest that these southern
filaments are shaped by radiation from RCW 120 leaving the ionized
region. We suggest that the boundary of RCW 120 does not entrap all
the emission; where the radiation leaks out, the filaments
are compressed into the radial segments we see. In the direction of
large condensations, the radiation is confined to the bubble area.
4 Conclusions
We have analysed the dust properties of the nearby bubble H II region
RCW 120 using aperture photometry and temperature maps of Herschel, Spitzer, and APEX-LABOCA data. We have found a
gradient of dust temperature, from 10 K for local infrared
dark clouds, to
20 K for the PDR (including sub-mm)
condensations, to
30 K for the interior of the bubble.
Our results show support for a power-law form of the anti-correlation
between the dust temperature T and the dust emissivity index
,
in good agreement with Désert et al. (2008). Because of the
range of temperatures found in a small spatial area, presumably all at
the same distance only 1.3 kpc from the Sun, RCW 120 is an ideal
location to test for the
relationship. With the wavelength
coverage of Herschel, we are able to simultaneously constrain
and T for regions of cold dust.
The temperature map of the RCW 120 field reveals numerous locations in the PDR where radiation appears to be leaking into the surrounding interstellar medium. This implies that RCW 120 is in the process of fragmenting its nearly complete PDR layer. As the radiation leaves the confined environment, the radiation pressure shapes local cold filaments and may aid in collapsing local condensations outside the ionization front.
AcknowledgementsPart of this work was supported by the ANR (Agence Nationale pour la Recherche) project ``PROBeS'', number ANR-08-BLAN-0241. 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.
- ... Montage
- http://montage.ipac.caltech.edu/
All Tables
Table 1: Dust properties derived from aperture photometry.
All Figures
![]() |
Figure 1:
Three-color image of RCW 120 composed of
250 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Example SED fits for the ``Condensation 1'' (red),
``Interior 1'' (blue) and ``Condensation 5'' (green) apertures.
Points included in the SED fits are shown filled; points excluded
from the fits are shown open. The fit with |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The relationship between the fitted temperature and dust
emissivity index |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
The temperature map derived from the SED fits of each pixel,
over the same area as Fig. 1. When deriving these
maps, we fixed |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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