Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L133 | |
Number of page(s) | 7 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014626 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
The
Pictoris disk imaged by Herschel PACS and SPIRE
,![[*]](/icons/foot_motif.png)
B. Vandenbussche1 - B. Sibthorpe2 - B. Acke1 - E. Pantin3 - G. Olofsson4 - C. Waelkens1 - C. Dominik5,6 - M. J. Barlow7 - J. A. D. L. Blommaert1 - J. Bouwman8 - A. Brandeker4 - M. Cohen9 - W. De Meester1 - W. R. F. Dent10 - K. Exter1 - J. Di Francesco11 - M. Fridlund12 - W. K. Gear13 - A. M. Glauser14,2 - H. L. Gomez13 - J. S. Greaves15 - P. C. Hargrave13 - P. M. Harvey16,17 - Th. Henning8 - A. M. Heras12 - M. R. Hogerheijde18 - W. S. Holland2 - R. Huygen1 - R. J. Ivison2,19 - C. Jean1 - S. J. Leeks20 - T. L. Lim20 - R. Liseau21 - B. C. Matthews11 - D. A. Naylor22 - G. L. Pilbratt12 - E. T. Polehampton20,22 - S. Regibo1 - P. Royer1 - A. Sicilia-Aguilar8 - B. M. Swinyard20 - H. J. Walker20 - R. Wesson7
1 - Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
2 - UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, EH9 3HJ, UK
3 - Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot,
IRFU/Service d'Astrophysique, Bât.709, CEA-Saclay, 91191 Gif-sur-Yvette
Cedex, France
4 - Department of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden
5 - Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
6 - Afdeling Sterrenkunde, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The Netherlands
7 - Department of Physics and Astronomy, University College London, Gower St, London WC1E 6BT, UK
8 - Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
9 - Radio Astronomy Laboratory, University of California at Berkeley, CA 94720, USA
10 - ALMA JAO, Av. El Golf 40 - Piso 18, Las Condes, Santiago, Chile
11
- National Research Council of Canada, Herzberg Institute of
Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
12 - ESA Research and Science Support Department, ESTEC/SRE-S, Keplerlaan 1, 2201AZ, Noordwijk, The Netherlands
13 - School of Physics and Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
14 - Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
15 - School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
16 - Department of Astronomy, University of Texas, 1 University Station C1400, Austin, TX 78712, USA
17 - CASA, University of Colorado, 389-UCB, Boulder, CO 80309, USA
18 - Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
19 - Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
20 - Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
21 - Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
22 - Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada
Received 31 March 2010 / Accepted 18 May 2010
Abstract
We obtained Herschel PACS and SPIRE images of the thermal emission of the debris disk around the A5V star Pic. The disk
is well resolved in the PACS filters at 70, 100, and 160
m.
The surface brightness profiles
between 70 and 160
m show no significant asymmetries along
the disk, and are compatible with
90% of the emission between 70 and 160
m originating in a region closer than 200 AU
to the star. Although only marginally resolving the
debris disk, the maps obtained in the SPIRE 250-500
m filters
provide full-disk photometry, completing the SED
over a few octaves in wavelength that had been previously inaccessible. The small
far-infrared spectral index (
)
indicates that the grain size distribution in the inner disk
(<200 AU) is inconsistent with a local collisional equilibrium.
The size distribution is either modified by non-equilibrium effects, or
exhibits a wavy pattern, caused by an under-abundance of impactors
which have been removed by radiation pressure.
Key words: stars: early-type - planetary systems -
circumstellar matter - stars: individual: Pic
1 Introduction
The Pic disk, discovered by IRAS (Aumann et al. 1984),
was the first debris disk to be directly imaged in scattered
light (Smith & Terrile 1984). It is seen close to edge-on and extends in the optical out to 95
,
corresponding to 1800 AU (Larwood & Kalas 2001).
Pic (A5V) is one of the closest (
pc, van Leeuwen 2007) and youngest debris disks. The estimated age (12 Myr, Zuckerman et al. 2001)
significantly exceeds typical timescales for the survival of pristine
circumstellar dust grains (e.g., Fedele et al. 2010),
hence continuous replenishment of the dust, presumably through
collisions of planetesimals, is needed. The closeness of the object
ensures that it can also be spatially resolved at long wavelengths: Holland et al. (1998) resolved the disk at 850
m and Liseau et al. (2003) at 1200
m.
Optical and near-infrared observations of the inner part (<100 AU) of the disk yield evidence of asymmetries such as warps and density contrasts, which may relate to the presence of planetesimals (Pantin et al. 1997; Mouillet et al. 1997; Telesco et al. 2005; Kalas & Jewitt 1995; Heap et al. 2000). Lagrange et al. (2009) imaged a possible companion at a projected distance of 8 AU from the star.
Images of Pic
in scattered stellar light directly detect small grains and indirectly
larger grains that produce the smaller ones through collisions. The
grain-size distribution can be quantitatively constrained from the
spectral energy distribution (SED) of the disk, in the infrared
and (sub)mm domains. The spectral index of the SED at the longest
wavelengths (Nilsson et al. 2009; Liseau et al. 2003)
is inferred to be fairly low, which according to modeling
(Ricci et al. 2010; Natta et al. 2007; Draine 2006) can be interpreted as a deficit of small grains.
In this paper, we present far-infrared imaging of the Pic debris disk in six Herschel
photometric bands between 70 and 500
m.
These bands cover for the first time the long-wavelength side of the
peak in the thermal emission of the disk, and the large aperture of the
telescope enables us to resolve the disk at far-IR wavelengths for the
first time. With these data, we measure the surface brightness profiles
of the disk and readdress the issue of the grain-size distribution in
the inner 200 AU.
![]() |
Figure 1:
Surface brightness maps of the |
Open with DEXTER |
2 Observations and data reduction
We obtained maps of Pic with the PACS and SPIRE
instruments of Herschel (Pilbratt et al. 2010).
The in-orbit performance, scientific capabilities,
calibration methods, and accuracy are outlined by
Poglitsch et al. (2010) for the PACS instrument and by
Griffin et al. (2010) and Swinyard et al. (2010)
for the
SPIRE instrument. The observations were carried out during the science
demonstration phase as proposed in the ``Stellar Disk Evolution''
guaranteed time proposal (PI G. Olofsson). Table 1 gives a summary of the observations.
The deep PACS observation at 70
m and 160
m is a standard PACS
photometer scan map, split into a scan and cross-scan on the sky. The sky
scan speed was 10
s-1. The homogeneously covered area of the deep map is
.
The observation at 100
m is much shallower, with a single
scan direction at a rate of 20
s-1, homogeneously covering an area of
.
The PACS beams at 70, 100, and 160
m are 5.6, 6.8, and 11.3
FWHM. In the SPIRE observation, the three bands are observed simultaneously
in a standard scan map. The map coverage is
.
The SPIRE FWHM beam sizes in the 250, 350, and 500
m channels are
18.1, 25.2, and 36.9
respectively.
The data processing is described in Appendix.
The absolute flux calibration accuracy of the resulting PACS maps is better than 10% at 70 and 100 m, and 20% at 160
m (Poglitsch et al. 2010).
The flux calibration accuracy of the SPIRE maps is better than 15%
(Swinyard et al. 2010).
The 1
noise levels of the maps are listed in
Table 2.
3 Analysis
In Fig. 1, we show the maps obtained in the three PACS
filters (70, 100, and 160 m) and the three SPIRE filters
(250, 350, and 500
m).
We also compare the point spread functions (PSFs) measured on the
asteroid Vesta using the same satellite scan speed, processed as the
Pic maps and rotated to align with the telescope pupil
orientation on the sky during the
Pic observations as listed
in online Table 1.
These images show a clearly resolved disk from 70-160 m.
Each map was fitted using a 2D Gaussian function.
Within the 2
Herschel pointing accuracy, the Gaussian center
matches the star's optical position.
The fitted position angles, listed in Table 2,
agree with the optical disk position angle of 30
8 reported by Kalas & Jewitt (1995).
Cross-sections orthogonal to the disk position angle in the NW to SE
direction show no significant broadening compared to the PSF. The disk
is not resolved in the vertical direction. The feature towards the NW,
visible in the 70-160
m images, is produced by the three-lobed PACS PSF.
Table 2: Overview of measured quantities.
In Fig. 3, we
present the surface brightness profiles along the disk position angle.
We compare them with the cross-sections aligned in the same
direction through the PSFs. At 250 and 350 m, the disk is marginally
resolved. At 500
m, the
Pic profile shows no significant departure from the PSF profile,
with the exception of a cold blob in the southwest.
As can be seen in Fig. 1,
the location of this feature in the 250-500
m maps coincides with
the flux peaks seen at 850 and 870
m by Holland et al. (1998)
and Nilsson et al. (2009), respectively.
However, the 100 arcmin2 region around
Pic (depicted
in online Fig. 2) shows
more than 50 background sources comparable to this feature in the
250
m map. The feature is therefore probably
a background source.
![]() |
Figure 3:
Surface brightness profiles along the disk in NE-SW direction, following the 30.8 |
Open with DEXTER |
Other asymmetries between the northeast and southwest profile are within the
errors induced by the asymmetry of the PSF.
No sharp disk edge is seen; in all filters, the surface brightness
declines gradually to the 1
detection limit of the maps.
Table 2 lists the extent of the
detected emission region in the NE-SW direction.
![]() |
Figure 4:
Normalised surface brightness profiles along the disk in NE direction
in the three PACS filters. The profiles were convolved with a Gaussian
to match the spatial resolution of the 160 |
Open with DEXTER |
The comparison of the surface brightness profiles in the three PACS filters
in Fig. 4 shows the same brightness profile along the
30.8
position angle in NE direction. The 70 and 100
m profiles
were convolved with a Gaussian to match the spatial resolution at 160
m.
The same convolution was applied to the 70 and 100
m PSF profiles.
The shape of these convolved PSF profiles defers significantly from that of the
160
m PSF profile. The
wiggles in the 160
m profile differ up to a factor of 3 from the convolved
70 and 100
m PSF profiles.
Within these uncertainties,
there is no evidence of a wavelength dependent surface brightness.
This indicates that the grains producing the emission at 70, 100, and 160
m
are confined to the same locus in the disk.
At 70
m, the broadening of the profile with respect to the PSF indicates
that 90% of the emission originates in a region within 11
or 200 AU of the star.
4 The far-infrared SED and grain size
We integrated the surface brightness maps over a 60
radius circular
aperture. Background subtraction was based on a rectangular region,
selected close enough to the object to be within the map region with
the same coverage as the center of the map. For the background outlier
rejection, the DAOphot
algorithm in the HIPE aperture photometry task was used. The aperture
photometry obtained provides a good
measure of the flux density of the integrated disk. The contribution of
the stellar photosphere at these wavelengths is negligible. The error
is dominated
by the present uncertainties in the absolute flux calibration of both
instruments. The full disk flux densities are listed in Table 2.
Figure 5 shows the new PACS and SPIRE photometry, and selected
infrared and (sub-)mm flux densities from the literature.
Because the disk is optically thin at these wavelengths, the
wavelength dependence of the emission directly probes the
dust grains, and, in particular, their size distribution.
We overplot two modified Rayleigh-Jeans laws (
),
normalized to the 160
m datum. The spectral index
indicates the
mean dust opacity
.
An index
corresponds to a black body with a
independent of
wavelength
,
indicating grains that are much larger than
.
Interstellar grains, which have a size distribution
f(a)
a-q with q=3.5 and an upper size limit of
m,
are characterized by
(Draine 2006).
In protoplanetary disks,
-values from 1.5 down to 0 are found,
depending on the disk geometry (Acke et al. 2004).
An error-weighted least squares fit of a Rayleigh-Jeans law to
the
Pic photometry at wavelengths beyond 160
m
yields
.
Nilsson et al. (2009)
obtained
from a
-corrected black-body fit to the full
disk SED, including mid-infrared photometry. The difference between both results
should not be over-interpreted since both approaches are sensitive in different ways
to simplifying assumptions about the temperature and size distribution within the disk. In any
case, both results consistently show a value below 0.7.
Ricci et al. (2010) demonstrate that such a low value cannot be explained with a q
= 3.5 power law.
This is a surprise insofar as the latter value is the expected result
for a population of bodies in a standard steady-state collisional
cascade (Dohnanyi 1969).
![]() |
Figure 5:
The infrared to mm SED of |
Open with DEXTER |
The grain size distribution in Pic must be flatter than the q = 3.5 power
law, meaning that the fraction of small particles must be lower.
Radiation pressure can push the smallest grains (with
)
onto hyperbolic orbits, hence reduce the
time these particles spend in the inner part of the disk, which can decrease their volume density by two orders of magnitude (Krivov et al. 2000).
The disk cannot be fully cleared of small particles, since it has been
seen in scattered light out to 1800 AU. The scattering grains are
probably the (sub-)
m grains that are blown out of the inner
disk, where the collisions take place.
However, this effect only reduces the densities for grains of size below
a few micrometers, and even fully removing these grains would not change
to the observed value.
The small value of
can be interpreted in a number of ways.
The grain size distribution can exhibit a wavy pattern, caused by
the absence of impactors small enough to be efficiently blown
out of the disk by radiation pressure. This causes an over-abundance
of the grains that are just bound, which means there are more impactors
for the next larger size population. The reduction of this population
causes an over-abundance of a following size population and so on
(Krivov et al. 2006).
The wavy size distribution can lead to small values of
when measured
in the FIR (Thébault & Augereau 2007). If the wavy structure were as strong
as found in this paper for normal and weak material properties, it would be
consistent with the small
value we have measured.
However, the strength and phase of the wavy pattern in the size distribution
depend on both the grain structure and the eccentricity of the dust orbits in the disk.
Alternative explanations of the small value of
cannot be excluded.
There are indications that the grains produced
in the deep impact experiment followed a flatter power law with
(Jorda et al. 2007).
Laboratory experiments illustrate that fragments produced in collisions of porous aggregates can follow much flatter slopes
(q=1.2, Güttler et al. 2010), demonstrating that the porosity of the colliding grains should not be disregarded.
Additional dynamical models should be developed to quantify the possible
contribution of these effects to the small
observed in
Pic.
5 Conclusions
We have presented images of the Pic debris disk
in six photometric bands between 70 and 500
m using the
PACS and SPIRE instruments.
We resolve the disk at 70, 100, 160, and 250
m.
The images at 70-160
m show no evidence of asymmetries in the far-infrared surface brightness along the disk of
Pic.
The observed profiles are compatible
with 90% of the emission originating in a region within a radius of 200 AU from the star.
The disk-integrated photometry in the six Herschel filters provides a far
infrared SED with small spectral index
,
which is indicative of
a grain size distribution that is inconsistent with a local collisional equilibrium.
The size distribution is modified by either non-equilibrium effects, or
exhibits a wavy pattern, caused by the under-abundance of
impactors that are small enough to be removed by radiation pressure.
PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, 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/INAF (Italy), and CICYT/MCYT (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). BV acknowledges the Belgian Federal Science Policy Office via the ESA-PRODEX office. The authors thank the referee for several helpful comments.
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Online Material
![]() |
Figure 2:
The 250 |
Open with DEXTER |
![]() |
Figure 6:
The 250, 350 and 500 |
Open with DEXTER |
Table 1: Observation log.
Appendix A: Data reduction
The PACS data were processed in the Herschel interactive
analysis
environment HIPE (v3.0), applying the standard pipeline steps. The
flux conversion was done using version 5 of the response
calibration.
Signal glitches due to cosmic ray impacts were masked out in two steps.
First the PACS photMMTDeglitching task in HIPE was applied on the
detector
timeline. Then a first coarse map was projected, which is then used
as a reference for the second level deglitching HIPE task
IIndLevelDeglitch. In the detector time series we masked the region
around the source
prior to applying a high-pass filter to remove the low frequency
drifts.
The scale of the high pass filter was taken to be half the length of an
individual scan leg on the sky, i.e. 3.7.
The detector time series signals were then summed up into a map using
the PACS photProject task. The pixel scale for the 70 and 100
m
maps was set to 1
,
while the scale for the 160
m map was 2
.
For the deep map in the 70 and 160
m filter we combined
the two detector time series and projected these together.
The SPIRE data were also reduced using HIPE and maps
were obtained via the default naiveMapper task. The SPIRE observation
consists of several repetitions of a map observation of the same
area. As a result it was possible to project the data with a pixel size of 4, 6, and 9
while still maintaining complete sampling across the source.
Footnotes
- ... SPIRE
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
- ...
- Figures 2, 6, Table 1 and Appendix are only available in electronic form at http://www.aanda.org
All Tables
Table 1: Observation log.
Table 2: Overview of measured quantities.
All Figures
![]() |
Figure 1:
Surface brightness maps of the |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Surface brightness profiles along the disk in NE-SW direction, following the 30.8 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Normalised surface brightness profiles along the disk in NE direction
in the three PACS filters. The profiles were convolved with a Gaussian
to match the spatial resolution of the 160 |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
The infrared to mm SED of |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The 250 |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
The 250, 350 and 500 |
Open with DEXTER | |
In the text |
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