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.
protoplanetary disks
HIP 92043
circumstellar matter
HIP 49908
galaxies: statistics
stars: individual (HIP 29271
HIP 171
planets and satellites: formation
HIP 109378
HIP 73100)
Author
A. Krivov
Friedrich Schiller Universitat Jena
C. Eiroa
Universidad Autonoma de Madrid
T. Lohne
Friedrich Schiller Universitat Jena
J. P. Marshall
Universidad Autonoma de Madrid
B. Montesinos
CSIC-INTA - Centro de Astrobiologia (CAB)
C. del Burgo
Instituto Nacional de Astrofisica Optica y Electronica
O. Absil
Universite de Liege
D. R. Ardila
California Institute of Technology
J. C. Augereau
Verimag
A. Bayo
European Southern Observatory Santiago
Max Planck Institut fur Astronomie
G. Bryden
Jet Propulsion Laboratory, California Institute of Technology
W. Danchi
NASA Goddard Space Flight Center
S. Ertel
Verimag
J. Lebreton
Verimag
René Liseau
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
A. Mora
European Space Astronomy Centre
A. J. Mustill
Universidad Autonoma de Madrid
H. Mutschke
Friedrich Schiller Universitat Jena
R. Neuhauser
Friedrich Schiller Universitat Jena
G.L. Pilbratt
ESTEC/SRE-SA
A. Roberge
NASA Goddard Space Flight Center
T. O. B. Schmidt
Friedrich Schiller Universitat Jena
K. R. Stapelfeldt
NASA Goddard Space Flight Center
P. Thebault
Observatoire de Paris-Meudon
C. Vitense
Friedrich Schiller Universitat Jena
G. J. White
Open University
Rutherford Appleton Laboratory
S. Wolf
Christian-Albrechts-Universitat zu 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