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Herschel's "Cold Debris Disks": Background Galaxies or Quiescent Rims of Planetary Systems?

Krivov, A. V. and Ardila, D. (2013) Herschel's "Cold Debris Disks": Background Galaxies or Quiescent Rims of Planetary Systems? Astrophysical Journal, 772 (1). Art. No. 32. ISSN 0004-637X. doi:10.1088/0004-637X/772/1/32.

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Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100 μm 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 μm, which is larger than the 100 μm 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 ~100 μm, 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.

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Additional Information:© 2013 American Astronomical Society. Received 2012 December 4; accepted 2013 May 23; published 2013 July 2. We are grateful to the referee for useful comments that greatly helped to improve the manuscript. A.V.K. and T.L. thank Jürgen Blum and Carsten Güttler for their explanations on collisional outcomes and Ludwig Trepl for discussions on the ROSAT data. The work of A.V.K., T.L., and S.W. was partly funded by the Deutsche Forschungsgemeinschaft (grants Kr 2164/10-1, Lo 1715/1-1, and Wo 857/7-1). C.E., J.P.M., and B.M. were partly supported by Spanish grant AYA 2011-26202. J.-C.A. and S.E. acknowledge financial support of the CNES-PNP. S.E. also thanks the French National Research Agency (ANR) for financial support through contract ANR-2010 BLAN-0505-01 (EXOZODI). A.B. was co-funded under the Marie Curie Actions of the European Commission (FP7-COFUND).
Funding AgencyGrant Number
Deutsche Forschungsgemeinschaft (DFG)Kr 2164/10-1
Deutsche Forschungsgemeinschaft (DFG)Lo 1715/1-1
Deutsche Forschungsgemeinschaft (DFG)Wo 857/7-1
Spanish grantAYA 2011-26202
French National Research Agency (ANR)ANR-2010 BLAN-0505-01 (EXOZODI)
European Commission Marie Curie ActionsFP7-COFUND
Subject Keywords:circumstellar matter; galaxies: statistics; planets and satellites: formation; protoplanetary disks; stars: individual (HIP 29271, HIP 49908, HIP 109378, HIP 92043, HIP 171, HIP 73100)
Issue or Number:1
Record Number:CaltechAUTHORS:20130816-144105342
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Official Citation:Herschel's "Cold Debris Disks": Background Galaxies or Quiescent Rims of Planetary Systems? A. V. Krivov et al. 2013 ApJ 772 32
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:40712
Deposited On:16 Aug 2013 23:12
Last Modified:10 Nov 2021 00:07

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