HERSCHEL's "COLD DEBRIS DISKS": BACKGROUND GALAXIES OR QUIESCENT RIMS OF PLANETARY SYSTEMS?
Journal article, 2013
Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around similar to 100 mu m or shorter. However, 6 out of 31 excess sources studied in the Herschel Open Time Key Programme, DUNES, have been seen to show significant-and in some cases extended-excess emission at 160 mu m, which is larger than the 100 mu m excess. This excess emission has been attributed to circumstellar dust and has been suggested to stem from debris disks colder than those known previously. Since the excess emission of the cold disk candidates is extremely weak, challenging even the unrivaled sensitivity of Herschel, it is prudent to carefully consider whether some or even all of them may represent unrelated galactic or extragalactic emission, or even instrumental noise. We re-address these issues using several distinct methods and conclude that it is highly unlikely that none of the candidates represents a true circumstellar disk. For true disks, both the dust temperatures inferred from the spectral energy distributions and the disk radii estimated from the images suggest that the dust is nearly as cold as a blackbody. This requires the grains to be larger than similar to 100 mu m, even if they are rich in ices or are composed of any other material with a low absorption in the visible. The dearth of small grains is puzzling, since collisional models of debris disks predict that grains of all sizes down to several times the radiation pressure blowout limit should be present. We explore several conceivable scenarios: transport-dominated disks, disks of low dynamical excitation, and disks of unstirred primordial macroscopic grains. Our qualitative analysis and collisional simulations rule out the first two of these scenarios, but show the feasibility of the third one. We show that such disks can indeed survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic solids larger than millimeters, but smaller than a few kilometers in size. If larger planetesimals were present, then they would stir the disk, triggering a collisional cascade and thus causing production of small debris, which is not seen. Thus, planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached.
HIP 92043
protoplanetary disks
circumstellar matter
stars: individual (HIP 29271
HIP 171
planets and satellites: formation
HIP 49908
HIP 109378
HIP 73100)
galaxies: statistics
Author
A. Krivov
Friedrich Schiller University Jena
C. Eiroa
Universidad Autonoma de Madrid (UAM)
T. Lohne
Friedrich Schiller University Jena
J. P. Marshall
Universidad Autonoma de Madrid (UAM)
B. Montesinos
Centro de Astrobiologia (CAB)
C. del Burgo
National Institute of Astrophysics, Optics and Electronics
O. Absil
University of Liège
D. R. Ardila
California Institute of Technology (Caltech)
J. C. Augereau
Grenoble Alpes University
A. Bayo
European Southern Observatory Santiago
Max Planck Society
G. Bryden
Jet Propulsion Laboratory, California Institute of Technology
W. Danchi
National Aeronautics and Space Administration (NASA)
S. Ertel
Grenoble Alpes University
J. Lebreton
Grenoble Alpes University
René Liseau
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
A. Mora
European Space Astronomy Centre (ESAC)
A. J. Mustill
Universidad Autonoma de Madrid (UAM)
H. Mutschke
Friedrich Schiller University Jena
R. Neuhauser
Friedrich Schiller University Jena
G.L. Pilbratt
European Space Research and Technology Centre (ESA ESTEC)
A. Roberge
National Aeronautics and Space Administration (NASA)
T. O. B. Schmidt
Friedrich Schiller University Jena
K. R. Stapelfeldt
National Aeronautics and Space Administration (NASA)
P. Thebault
Observatoire de Paris-Meudon
C. Vitense
Friedrich Schiller University Jena
G. J. White
Open University
STFC Rutherford Appleton Laboratory
S. Wolf
University of Kiel
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 772 1 32Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
DOI
10.1088/0004-637x/772/1/32