Jena 2013 – scientific programme

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EP: Fachverband Extraterrestrische Physik

EP 1: Astrophysik

EP 1.5: Talk

Monday, February 25, 2013, 15:15–15:30, HS 9

Herschel's "Cold Debris Disks'' — •Alexander Krivov¹ and Herschel DUNES consortium² — ¹Astrophysikalisches Institut und Universitäts-Sternwarte, FSU, Schillergäßchen 2-3, 07745 Jena — ²Everywhere in the world

Infrared excesses associated with debris disk host stars detected so far, peak at wavelengths of ~100μm or shorter. However, six out of 31 excess sources in the Herschel OTKP DUNES have been seen to show little or no excesses at 100μm, but instead, significant - and in some cases extended - excess emission at 160μm. This excess emission has been suggested to stem from debris disks colder than those known previously. We re-consider whether some or even all of the candidates may be associated with unrelated galactic or extragalactic emission 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 ~100μm, regardless of their material composition. To explain the dearth of small grains, we suggest that the cold disks are composed of unstirred primordial macroscopic grains. We show that such disks can survive for gigayears, largely preserving the primordial size distribution. They should be composed of solids larger than millimeters, but smaller than kilometers in size. Thus planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached.