Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L80 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014609 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Physical properties of the Sh2-104 H II region as seen by
Herschel![[*]](/icons/foot_motif.png)
J. A. Rodón1 - A. Zavagno1 - J.-P. Baluteau1 - L. D. Anderson1 - E. Polehampton2,3 - A. Abergel4 - F. Motte5 - S. Bontemps5,6 - P. Ade16 - P. André5 - H. Arab4 - C. Beichman8 - J.-P. Bernard7 - K. Blagrave14 - F. Boulanger4 - M. Cohen9 - M. Compiegne14 - P. Cox10 - E. Dartois4 - G. Davis11 - R. Emery16 - T. Fulton19 - C. Gry1 - E. Habart4 - M. Halpern13 - M. Huang11 - C. Joblin7 - S. C. Jones2 - J. Kirk16 - G. Lagache4 - T. Lin3 - S. Madden5 - G. Makiwa2 - P. Martin14 - M.-A. Miville-Deschênes4 - S. Molinari15 - H. Moseley18 - D. Naylor2 - K. Okumura5 - F. Orieux12 - D. Pinheiro Gonçalves14 - T. Rodet12 - D. Russeil1 - P. Saraceno15 - S. Sidher3 - L. Spencer2 - B. Swinyard3 - D. Ward-Thompson16 - G. White17,20
1 - Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS and
Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille
Cedex 13, France
2 -
Institute for Space Imaging Science, University of Lethbridge, Lethbridge,
Canada
3 -
Space Science Department, Rutherford Appleton Laboratory, Chilton, UK
4 -
Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud 11, 91405
Orsay, France
5 -
CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191
Gif-sur-Yvette, France
6 -
CNRS/INSU, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, BP 89, 33271
Floirac Cedex, France
7 -
Universié de Toulouse; UPS; CESR; and CNRS; UMR5187,
9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
8 -
Infrared Processing & Analysis Center, California Institute of
Technology, Mail Code 100-22, 770 South Wilson Av, Pasadena, CA 91125, USA
9 -
University of California, Radio Astronomy Laboratory, Berkeley, 601 Campbell
Hall, US Berkeley CA 94720-3411, USA
10 -
Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine,
38406 Saint-Martin-d'Hères, France
11 -
National Astronomical Observatories, PR China
12 -
Laboratoire des Signaux et Systèmes (CNRS & Supélec & Université
Paris-Sud 11), Moulon, 91192 Gif-sur-Yvette, France
13 -
Department of Physics and Astronomy, University of British Columbia, Vancouver,
Canada
14 -
Canadian Institute for Theoretical Astrophysics, Toronto, Ontario, M5S 3H8,
Canada
15 -
Istituto di Fisica dello Spazio Interplanetario, INAF, via del Fosso
del Cavaliere 100, 00133 Roma, Italy
16 -
Department of Physics and Astronomy, Cardiff University, UK
17 -
Centre for Astrophysics and Planetary Science, School of Physical Sciences,
University of Kent, Kent, UK
18 -
NASA - Goddard SFC, USA
19 -
Blue Sky Spectrosocpy Inc, Lethbridge, Canada
20 -
Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA,
UK
Received 31 March 2010 / Accepted 7 May 2010
Abstract
Context. Sh2-104 is a Galactic H II
region with a bubble morphology, detected at optical and radio
wavelengths. It is considered the first observational confirmation of
the collect-and-collapse model of triggered star-formation.
Aims. We aim to analyze the dust and gas properties of the
Sh2-104 region to better constrain its effect on local future
generations of stars. In addition, we investigate the relationship
between the dust emissivity index
and the dust temperature,
.
Methods. Using Herschel PACS and SPIRE images at 100, 160, 250, 350 and m we determine
and
throughout Sh2-104, fitting the spectral energy distributions (SEDs)
obtained from aperture photometry. With the SPIRE Fourier-transform
spectrometer (FTS) we obtained spectra at different positions in the
Sh2-104 region. We detect J-ladders of 12CO and 13CO, with which we derive the gas temperature and column density. We also detect proxies of ionizing flux as the [N II
and [C I
transitions.
Results. We find an average value of
throughout Sh2-104, as well as a
difference between the photodissociation region (PDR,
25 K) and the interior (
40 K) of the bubble. We recover the anti-correlation between
and dust temperature reported numerous times in the literature. The
relative isotopologue abundances of CO appear to be enhanced above the
standard ISM values, but the obtained value is very preliminary and is
still affected by large uncertainties.
Key words: stars: formation - ISM: bubbles - H II regions - dust, extinction - infrared: ISM - ISM: individual objects: Sh2-104
1 Introduction
Sharpless 104 (Sh2-104, Sharpless 1959) is an optically
visible Galactic H II region with a bubble morphology, excited by an
O6V star (Crampton et al. 1978; Lahulla 1985). It is located 4 kpc from
the Sun (Deharveng et al. 2003), with galactic coordinates
74.7620;
+0.60 (J2000).
Deharveng et al. (2003) proposed Sh2-104 as a strong candidate for massive triggered star formation through the collect-and-collapse process (Elmegreen & Lada 1977). The ionized region is also visible at radio wavelengths (Fich 1993), and an ultracompact (UC) H II region, coincidant with the IRAS 20160+3636 source, lies at its eastern border (Condon et al. 1998).
We present new submm images and spectra taken towards Sh2-104 with the Herschel Space Observatory (Pilbratt et al. 2010). These observations allow us to map a wavelength range not easily accessed before, providing new insights into the dust and gas properties of Sh2-104.
2 Observations
The Herschel observations were taken on 2009 December 17
simultaneously with
PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010), as
part of the guaranteed-time key-projects ``Evolution of Interstellar Dust'' of
SPIRE (Abergel et al. 2010), and
HOBYS of PACS (Motte et al. 2010). A
region was imaged with
PACS at 100 and
m (at resolutions of 10'' and 14''), and with
SPIRE at 250, 350 and
m (at resolutions of 18'', 25'' and
36''). Spectra were taken with the SPIRE-FTS long and short wavelength
receivers (SLW and SSW, respectively) at seven different positions with sparse
sampling, covering the
m range. The resolution at the receivers'
central pixels varies between 17-19'' for SSW and 29-42'' for SLW.
The data were reduced with the HIPE software version 2.0 with the
latest standard calibration (Swinyard et al. 2010).
![]() |
Figure 1:
Composite image of Sh2-104. The field is |
Open with DEXTER |
Figure 1 shows a color-composite image of Sh2-104 with PACS
m (blue), SPIRE
m (green) and SPIRE
m (red).
Different regions of interest, addressed in the following sections, are
superimposed.
We can see that the interior of the bubble is brighter in the PACS band, showing
the hotter temperatures of the material in this region. On the other hand,
outside the bubble the material is colder and emits stronger in the SPIRE
bands.
3 Dust properties
We assume the dust emission in Sh2-104 can be modeled by
an (optically thin) gray-body and that the emissivity of the dust
grains can be fitted with a power law
![]() |
(1) |
where












Table 1: Results from aperture photometry.
![]() |
Figure 2:
Example of the SEDs obtained for regions UCHII (filled red
squares) and PDR 1 (open blue squares). A dashed line is the fit with
|
Open with DEXTER |
To determine the dust temperature structure of Sh2-104, we performed aperture
photometry measurements on selected areas in the field.
The apertures are shown in Fig. 1 and sample the interior
of the bubble and the photodissociation region (PDR), including the UCH II region associated with IRAS 20160+3636. We used a single aperture (not shown) to
account for the background emission.
We fitted the PACS+SPIRE emission for all regions. These data represent the cold
emission component, therefore we fitted a single temperature. We did this first
allowing
to vary, and later fixing
.
The resulting
and
values are shown in Table 1, their
uncertainties are the formal
values from the fitting procedure.
Figure 2 shows examples of the fitted spectral energy distributions
(SEDs) for the regions UCHII and PDR 1. Apart from the clear
difference in
,
it can be seen that for UCHII the PACS+SPIRE
emission does not allow to sample the peak of the SED, which is reflected in a
larger error in the fit.
This is also seen in the four Interior # regions, suggesting the
presence of a warmer emission component that contributes to the
shorter-
emission. Observations in the mid-IR can provide a constraint
on this component, and a 2-temperatures fitting would be more appropriate to
determine the
and
values.
The temperatures throughout the PDR are between 20 and
30 K, and
the UCH II region is marginally warmer. The region outside the
bubble (region Outer) is the coldest, while the regions mapping the
interior of the bubble are hotter, with an average temperature of
40 K.
The average
obtained is
1.5, therefore the temperatures obtained
from the fit with a fixed
do not significantly differ from those
obtained with a free
(see Table 1).
Figure 3 shows the distribution of the spectral indices vs. dust temperature, along with the relationships found by Dupac et al. (2003)
and Désert et al. (2008). Within the uncertainties, our results agree with both
relationships. We can also identify two groups, one with temperatures
between 35 and 47 K and an emissivity index between 1.0 and 1.3, and
the other with
T = 17-29 K and
.
These two groups show
that on average higher
values are preferentially associated with colder
material.
![]() |
Figure 3:
Distribution of the emissivity index |
Open with DEXTER |
Although anti-correlation between
and
is
reported in the literature (e.g., Yang & Phillips 2007; Désert et al. 2008; Dupac et al. 2003), it
is yet not clear which physical processes are behind it.
Other authors studying this relationship examine the emission from
different regions scattered in the sky. In contrast, we are finding this
anti-correlation in the analysis of one contiguous complex object at
a specific location in the sky. A
relationship may
indicate a change of the dust properties (Stepnik et al. 2003). Grain fluffiness
in particular increases the emissivity index while keeping a relatively low
temperature (e.g., Stognienko et al. 1995; Fogel & Leung 1998).
Fluffy grains result from grain coagulation and growth. The grain coagulation timescale and feasibility depend on factors such as the existence of ice mantles, grain size, and relative grain velocities.
We are finding the highest
values and lowest
values toward the PDR of Sh2-104, which would imply then that the fluffiest and
largest grains are located in the PDR. The question remains whether the PDR
dust coagulated after the creation of the H II region, or if the birth of the
H II region has destroyed the already coagulated dust located in the ionized
cavity.
4 Gas properties
The seven pointings of the central pixels observed with SPIRE-FTS are marked with red (SLW) and blue (SSW) circles in Fig. 1. In total we detected the transitions described in Table 2. The richest spectrum (Fig. 4) was obtained towards pointing E, which targets the UCH II region associated with IRAS 20160+3636. The most prominent features are the CO J-ladder and the [N II![$]\;^3P_1{-}^3P_0$](/articles/aa/full_html/2010/10/aa14609-10/img11.png)

In the simple hypothesis of optically thin emission, we plotted 12CO
and 13CO excitation diagrams, following the formulation of
Johansson et al. (1984). An example is shown in Fig. 5 for pointing
E. The slope of the linear fit for 12CO results in a temperature
of
K, and
K for 13CO. The total
column densities derived are
and
,
respectively.
The fits in Fig. 5 do not include all the measurements. The
downturn seen in the lower levels of 12CO is interpreted
as the optically thick/thin regime turnover, and is most likely a real physical
feature and not an instrumental or calibration effect, because it is only seen
for
that species and not, for example, for 13CO. If the lines were emitted from
an optically thick medium, their intensity would be underestimated, thus their
respective column densities would also be a lower limit.
Therefore, the optically thin assumption would hold for 12CO only for
transitions higher than
,
and those are the points included in
the fit.
Table 2: Lines detected with SPIRE-FTS in the different pointings.
![]() |
Figure 4: SPIRE-FTS spectrum corresponding to the central pixels of SLW (pixel C3, red) and SSW (pixel D4, blue), towards the UCH II region associated with IRAS 20160+3636 (pointing E). The lines detected are labeled. The jump between continuum levels is a calibration effect. |
Open with DEXTER |
For 13CO on the other hand only the lines with
wavelengths in the SPIRE-FTS SLW range are detected. These correspond to the
to
transitions. The 13CO lines in
the SPIRE-FTS SSW wavelenght range (
to
)
are detected as upper limits, because the line positions are
found displaced from their expected positions, indicating that we are probably
seeing some ``outlier'' noise features rather than the lines themselves.
Therefore, we fit and show in Fig. 5 only the five
13CO transitions detected with SPIRE-FTS SLW.
Following the reasoning of the previous paragraph, the 13CO transitions are
most likely optically thin.
![]() |
Figure 5: 12CO (red squares) and 13CO (blue diamonds) rotational diagram for pointing E towards the UCH II region. The solid lines are the best linear fit. Points included in the fit are shown as filled, points excluded as open. The turnover into the optically thick regime is noted in the lower-level transitions of 12CO. |
Open with DEXTER |
The two distinct gas temperatures obtained with 12CO and 13CO suggest two different gas components or a stratification of the emitting region. The colder component is traced by the optically-thin 13CO transitions at energy levels for which 12CO is optically thick, while the hotter component is traced by the more energetic, optically-thin 12CO transitions. Therefore it is likely that the colder gas is located at grater depths in the PDR than the hotter gas.
Assuming similar emitting volumes and beam filling factors as well
as optically thin emission, our CO and
13CO column density values imply an abundance ratio
,
which is several times lower than the reported elemental
value of
(Wilson 1999). This would imply
an enhancement of the 13CO isotopologue abundance.
However, several factors can contribute to the low
abundance ratio we find. Perhaps the most important one would be the assumption
of optically thin emission for 12CO. We used the high-energy
transitions to derive its column density, and although in a first analysis they
appear to be optically thin, it might not be the case (see e.g.,
Habart et al. 2010). To address this issue we will
present and analyse PDR models of the Sh2-104 region in a forthcoming paper.
5 Summary
With Herschel PACS and SPIRE data we have analysed dust and gas properties of the bubble-shaped H II region Sh2-104. Aperture photometry of PACS+SPIRE images allowed us to derive the dust emissivity index






SPIRE-FTS spectra at different pointings throughout Sh2-104 have unveiled the CO
chemistry of the region in more detail. We detect the 12CO and 13CO
J-ladders up to the
and
transitions
respectively, revealing the warm gas component in the region.
Rotational diagrams towards the UCH II region in the PDR of Sh2-104 show that the
13CO emission is optically thin and also the 12CO for transitions
above the J=8 level. The emission shows
two different gas components, a colder one with a temperature of
170 K
and a hotter one at a temperature of
250 K.
The CO column densities derived would suggest an enhancement of the
13CO
isotopologue abundance ratio with respect to the elemental value, but the
uncertainties of the different assumptions are still too large to confirm that
result.
In a follow-up paper we will show models of the PDR of Sh2-104, which will provide
better constraints on the gas temperature, density and column density
structure.
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); Stockholm Observatory (Sweden); STFC (UK); and NASA (USA). 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). Part of this work was supported by the ANR (Agence Nationale pour la Recherche) project ``PROBeS'', number ANR-08-BLAN-0241.
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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: Results from aperture photometry.
Table 2: Lines detected with SPIRE-FTS in the different pointings.
All Figures
![]() |
Figure 1:
Composite image of Sh2-104. The field is |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Example of the SEDs obtained for regions UCHII (filled red
squares) and PDR 1 (open blue squares). A dashed line is the fit with
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Distribution of the emissivity index |
Open with DEXTER | |
In the text |
![]() |
Figure 4: SPIRE-FTS spectrum corresponding to the central pixels of SLW (pixel C3, red) and SSW (pixel D4, blue), towards the UCH II region associated with IRAS 20160+3636 (pointing E). The lines detected are labeled. The jump between continuum levels is a calibration effect. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: 12CO (red squares) and 13CO (blue diamonds) rotational diagram for pointing E towards the UCH II region. The solid lines are the best linear fit. Points included in the fit are shown as filled, points excluded as open. The turnover into the optically thick regime is noted in the lower-level transitions of 12CO. |
Open with DEXTER | |
In the text |
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