Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L96 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014643 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Evolution of interstellar dust with Herschel. First results in the photodissociation regions of NGC 7023![[*]](https://cdn.statically.io/img/doi.org/icons/foot_motif.png)
A. Abergel1 - H. Arab1 - M. Compiègne14 - J. M. Kirk3 - P. Ade3 - L. D. Anderson2 - P. André4 - J.-P. Baluteau2 - J.-P. Bernard7 - K. Blagrave14 - S. Bontemps8 - F. Boulanger1 - M. Cohen9 - P. Cox10 - E. Dartois1 - G. Davis11 - R. Emery6 - T. Fulton12 - C. Gry2 - E. Habart1 - M. Huang11 - C. Joblin7 - S. C. Jones16 - G. Lagache1 - T. Lim6 - S. Madden4 - G. Makiwa16 - P. Martin14 - M.-A. Miville-Deschênes1 - S. Molinari15 - H. Moseley17 - F. Motte4 - D. Naylor16 - K. Okumura4 - D. Pinheiro Gonçalves14 - E. Polehampton16,6 - J. Rodon2 - D. Russeil2 - P. Saraceno15 - M. Sauvage4 - S. Sidher6 - L. Spencer16 - B. Swinyard6 - D. Ward-Thompson3 - G. J. White6,18 - A. Zavagno2
1 -
Institut d'Astrophysique Spatiale, UMR 8617, CNRS/Université Paris-Sud 11, 91405 Orsay, France
2 -
Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F.
Joliot-Curie, 13388 Marseille Cedex 13, 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 -
Dept. of Physics & Astronomy, University College London,
Gower Street, London WC1E 6BT, UK
6 -
The Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK
7 -
CESR, Université de Toulouse (UPS), CNRS, UMR 5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
8 -
CNRS/INSU, Laboratoire d'Astrophysique de Bordeaux, UMR 5804, BP 89, 33271 Floirac Cedex, France
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 (China)
12 -
Blue Sky Spectrosocpy Inc, Lethbridge, Canada
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 -
Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Canada
17 -
NASA - Goddard SFC, USA
18 -
Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
Received 31 March 2010 / Accepted 10 May 2010
Abstract
Context. In photodissociation regions (PDRs), the physical
conditions and the excitation evolve on short spatial scales as a
function of depth within the cloud, providing a unique opportunity to
study how the dust and gas populations evolve with the excitation and
physical conditions. The mapping of the PDRs in NGC 7023 performed
during the science demonstration phase of Herschel is part of
the ``Evolution of interstellar dust'' key program. The goal of this
project is to build a coherent database on interstellar dust emission
from diffuse clouds to the sites of star formation.
Aims. We study the far-infrared/submillimeter emission of the PDRs and their fainter surrounding regions. We combine the Herschel and Spitzer
maps to derive at each position the full emission spectrum of all dust
components, which we compare to dust and radiative transfer models in
order to learn about the spatial variations in both the excitation
conditions and the dust properties.
Methods. We adjust the emission spectra derived from PACS and
SPIRE maps using modified black bodies to derive the temperature and
the emissivity index
of the dust in thermal equilibrium with the radiation field. We present
a first modeling of the NGC 7023-E PDR with standard dust
properties and abundances.
Results. At the peak positions, a value of
equal to 2 is compatible with the data. The detected spectra and the
spatial structures are strongly influenced by radiative transfer
effects. We are able to reproduce the spectra at the peak positions
deduced from Herschel
maps and emitted by dust particles at thermal equilibrium, and also the
evolution of the spatial structures observed from the near infrared to
the submillimeter. On the other hand, the emission of the
stochastically heated smaller particles is overestimated by a factor
2.
Key words: dust, extinction - photon-dominated region (PDR) - evolution - submillimeter: ISM
1 Introduction
The motivation behind the ``Evolution of interstellar dust'' key program is to explore with Herschel (Pilbrat et al. 2010) the far-infrared (FIR) to submillimeter (submm) emission properties of dust particles in a wide range of regions within our Galaxy, from very diffuse clouds to sites of star formation and protostars. Photometric data taken with SPIRE (Griffin et al. 2010) and PACS (Poglitsch et al. 2010) are complemented with spectroscopy using the FTS of SPIRE and PACS to derive the physical conditions of the gas from the lines of [C I], the high-level rotational lines of CO, and the major cooling lines of [C II] and [O I]. This project is coordinated with the Gould Belt survey (André et al. 2010) and HOBYS (Motte et al. 2010).
Around one third of the observing time of our project is dedicated to photodissociation regions (PDRs) to study how their dust populations and gas content evolve with the excitation and physical conditions. Our sample of PDRs covers a variety of geometries and spans a wide range of both intensity and hardness of the radiation field.
This paper presents SPIRE and PACS mapping of the reflection nebula
NGC 7023, which contains three PDRs illuminated by the Herbig B3
star HD 200775 (Rogers et al. 1995) located at 430 pc (van den Ancker 1997, at this distance 1'=0.125 pc).
The three PDRs (NW, E, and S) lies at 40'' northwest,
70'' south and
170'' east of the star, respectively.
As discussed by Gérin et al. (1998),
NGC 7023 consists of a sheet of dense material in which the star
was born, blowing away much of the surrounding gas. The three PDRs at
the edges of the remaining material are viewed approximately edge-on.
NGC 7023 has been observed extensively in the radio (e.g., Gerin
et al. 1998), in H2 lines (Lemaire et al. 1999) and in the visible (e.g., Berné et al. 2008; Witt et al. 2006). Several infrared (IR) features were discovered with ISO and Spitzer (Cesarsky et al. 1996; Werner et al. 2004), in addition to strong variations in the 5-35
m spectra explained by photo-chemical processing of the very small particles (Abergel et al. 2002; Rapacioli et al. 2006; Berné et al. 2007). We can now study with Herschel the big grain component, which contains most of the dust mass.
2 Interstellar dust
Interstellar dust comprises several components. The smallest grains are
carbonaceous particles (polycyclic aromatic hydrocarbons PAHs, and
``very small grains'' VSGs) containing 10-1000 carbon atoms. They are stochastically heated, and emit most of their thermal energy below
60
m. The big grains (BGs) have sizes of
100 nm
and consist of amorphous silicates and carbon.
They are in thermal equilibrium with the interstellar radiation field
(ISRF). Their emission spectrum peaks in the sub-mm, and can be modeled
as a modified black body
,
where
is the emissivity at
,
the spectral index,
the Planck function, and T the temperature.
Interstellar dust can be described by various models. In the post-Spitzer/pre-Herschel era, the silicate-graphite-PAH model of Draine & Li (2007)
and the silicate-amorphous carbon-PAH model of Compiègne et al.
(in prep.) account consistently for the observations (extinction,
scattering, emission, depletions). Both models consider separate
silicate and carbonaceous particles, which inferred to be present
because the 9.7 m band is polarized but the 3.4
m C-H bond stretch is not (e.g., Chiar et al. 2006). Silicate-core/carbonaceous-mantle models (e.g., Désert et al. 1990) and composite models with aggregates (e.g., Zubko et al. 2004) have also been proposed.
Up to now, the sub-mm emission has been measured by very few experiments. DIRBE and FIRAS on board COBE
produced all-sky maps of resolutions 40' and ,
respectively. The dust temperature is found to be on average
17.5 K (with
)
in the diffuse atomic medium (Boulanger et al. 1996) and to be lower in molecular clouds with no embedded bright stars (Lagache et al. 1998). Small patches of bright molecular clouds have been observed
in more detail from the ground by the JCMT (Johnstone et al. 2006), and from the balloon borne experiments PRONAOS (Ristorcelli et al. 2006) and Archeops (Désert et al. 2008). At low
temperatures (T<30 K), the dust optical properties appear to change
significantly in terms of absolute value of the emissivity
and the
spectral index
.
As also seen in laboratory measurements (e.g., Agladze et al. 1996; Mennella et al. 1998; Boudet et al. 2005), the physical processes responsible for these effects probably involve ice mantle formation, grain coagulation, and
low-energy structural transformations (e.g., Meny et al. 2007).
![]() |
Figure 1:
NGC 7023 maps obtained with Spitzer (from Kirk et al. 2009) and Herschel. Units of the color bars are MJy/sr. The position of the illuminating star is seen on the 3.6 |
Open with DEXTER |
3 Observations
NGC 7023 was mapped during the science demonstration phase on
September 9 and November 11 2009 by SPIRE and PACS, respectively.
For SPIRE, two perpendicular
large maps were performed with the nominal scan speed (30''/s), and a
repetition of 4 (for a total observing time of 1675 s). We
use the Level-2 naive maps delivered by the the HSC, with standard
corrections for instrumental effects and glitches. The overall absolute
flux accuracy is 15% (Griffin et al. 2010; Swinyard et al. 2010).
For PACS, two perpendicular
scan maps were performed with the medium scan speed (20''/s), a scan
length of 10', a cross-scan step of 15'', and a number of scan
legs of 41 (total observing time 5166 s). For the blue channel,
the 70
m
filter was selected. The data were processed with HIPE
(version 2.3.1). We performed simple projection with second level
deglitching. The 1/f noise components were removed using high pass
filtering, with a window size equal to the scan length. Data taken in
the two scanning directions were processed independently before
averaging. For the bright parts of detected structures, the differences
between the two computed maps are below the absolute flux
uncertainties, within 10% and 20% in the blue and red bands,
respectively (Poglitsch et al. 2010; Swinyard et al. 2010). On the other hand, the faint regions around bright structures appear to exhibit some artifacts.
The processed maps are shown with Spitzer maps from Kirk et al. (2009) in Fig. 1.
For the quantitative analysis, we subtracted a constant background taken around the position
.
We also degraded all maps to match the SPIRE resolution at 500
m (with the preliminary assumption of Gaussian beams).
The PDRs correspond to bright filaments at long wavelengths because of a combination of a steep increase in the column density and the extinction of incident radiation. The distance from the illuminating star to the peak of the brightness profiles across the PDRs increases with increasing wavelength, as illustrated in Fig. 2 for NGC 7023-E. This is also observed in Fig. 1: the size of the detected images increases with increasing wavelengths (this is not due to an increase in the beam size).
4 Spectrum of the BG component
The PACS and SPIRE data allows for the first time the measurement of
the spectrum of the BG component on both sides of the spectral peak.
The spectra obtained at different positions can be adjusted with a
modified black body to derive T and .
In this first result paper, we focus on the brightest positions (at 250
m) of the three PDRs (Fig. 3). Within the error bars, the three spectra are reasonably adjusted with a fixed value of
,
as for the average spectrum of the diffuse ISM. A free value of
provides slightly superior fits to the data in NGC 7023 NW and -S, with
and 2.6, respectively.
5 Interpretation of the emission spectrum
At each position in the maps, the combination of Spitzer and Herschel data provides an emission spectrum for all dust components, as illustrated in Fig. 5. We used the ``DUSTEM'' model of Compiègne et al. (in prep.) to compute the emission spectrum at the 250 m
peak position of NGC 7023-E using the reference dust population,
which allowed the reproduction of the observed extinction and emission
for the diffuse high Galactic latitude ISM. We followed Gerin
et al. (1998) in estimating the UV radiation field to be
250 times the strength of the radiation field in the solar neighborhood of Mathis et al. (1983). In Fig. 5, we see that the computed spectrum for the BG component (normalized to the data at 70
m) fails to reproduce the observed spectrum, peaking at shorter wavelengths (black line in Fig. 5).
![]() |
Figure 2:
Normalized brightness profiles across NGC 7023-E along the cut shown in Fig. 1. Black: data at the SPIRE resolution at 500 |
Open with DEXTER |
![]() |
Figure 3:
Spectra at the 250 |
Open with DEXTER |
To quantify the radiative transfer effects, we used the model described in Compiègne et al. (2008) coupled with DUSTEM. The PDR was represented by a plane-parallel slab that is assumed to have an arbitrary density profile nH(z) illuminated by the IRSF (Mathis et al. 1983) added to the stellar radiation field. This transfer model accounts for the effect of scattering by separating forward (i.e., transmitted) from backward scattering. The dust heating by IR photons emitted by dust is also considered.
![]() |
Figure 4: Density profile taken to model the NGC 7023-E PDR. |
Open with DEXTER |
![]() |
Figure 5:
Crosses: Spitzer and Herschel data points at the 250 |
Open with DEXTER |
The illuminated side and the rear side of the density profile were
constrained by the observations. We used the symmetric density profile
shown in Fig. 4.
The three parameters were adjusted (
z0 = 0.18 pc,
cm-3, and
)
to reproduce the brightness profiles at all wavelengths (Fig. 2). We also adjusted the length of the PDR along the line of sight (
pc) to match the observed brightness at 70
m.
The computed spectrum (Fig. 5) reproduces the PACS and SPIRE data for the BGs, but overestimates by a factor 2 the emission at 3.6 to 24
m
which is produced by PAHs and VSGs. This result may indicate that the
relative abundance of the small dust particles is lower, or the
absolute emissivity of the BGs is higher, than in the diffuse high
Galactic latitude ISM, as already observed in dense molecular clouds
(e.g., Stepnik et al. 2003).
We also derived the length of the PDR along the line of sight to be
0.65 pc, which is relatively large compared to the width of the
filament (
0.15 pc). A model with a higher BG emissivity infers a length that is comparable to the width.
We conclude that radiative transfer effects can explain the spatial shifts between the brightness profiles (Fig. 2), and the increase in the size of the detected images with increasing wavelength (Fig. 1). We can also explain why the 70 m map is more comparable to the Spitzer than the Herschel maps. The 70
m PACS bandpass is on the left-hand side of the spectral peak of the BGs (Fig. 5),
with a minor contribution from the VSGs, so the measured emission
strongly depends on the incident radiation field, as for the emission
at shorter wavelength produced by stochastically heated dust particles.
The 70
m map therefore resembles the Spitzer maps detecting the PAH and VSG emission, while the 160
m map is more comparable to the SPIRE maps studying the Rayleigh-Jean part of the spectrum.
6 Conclusions
For the first time, we have detected the FIR/submm filaments at the
PDRs of NGC 7023. We have demonstrated that the emissionspectra
derived from SPIRE and PACS maps can be adjusted with modified black
bodies. Our first results at the peak positions have indicated that the
value of
equal to 2 found in the diffuse atomic medium is compatible with the data. Spatial variations in
have also been found, but the data processing must be stabilized before any quantitative analysis is possible.
The combination of SPIRE, PACS, and Spitzer
maps has provided full emission spectra of all dust particles that have
been compared to dust and radiative transfer models. Our first results
for NGC 7023-E illustrate the dramatic influence of radiative
transfer on the spatial structures observed at long wavelengths. The
next step is to combine imaging and spectroscopic Herschel data to achieve deeper insight into both the dust and gas components.
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). PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (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). We thank Lori Allen for providing improved IRAC and MIPS maps of NGC 7023 taken from the ``Spitzer Gould Belt legacy program'', and Karin Dassas for her help in the PACS data processing.
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Footnotes
- ... NGC 7023
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
All Figures
![]() |
Figure 1:
NGC 7023 maps obtained with Spitzer (from Kirk et al. 2009) and Herschel. Units of the color bars are MJy/sr. The position of the illuminating star is seen on the 3.6 |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Normalized brightness profiles across NGC 7023-E along the cut shown in Fig. 1. Black: data at the SPIRE resolution at 500 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Spectra at the 250 |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Density profile taken to model the NGC 7023-E PDR. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Crosses: Spitzer and Herschel data points at the 250 |
Open with DEXTER | |
In the text |
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