<i>HERSCHEL</i>'s “COLD DEBRIS DISKS”: BACKGROUND GALAXIES OR QUIESCENT RIMS OF PLANETARY SYSTEMS?

Abstract

(abridged) Infrared excesses associated with debris disk host stars detected\nso far peak at wavelengths around ~100{\\mu}m or shorter. However, six out of 31\nexcess sources in the Herschel OTKP DUNES have been seen to show significant -\nand in some cases extended - excess emission at 160{\\mu}m, which is larger than\nthe 100{\\mu}m excess. This excess emission has been suggested to stem from\ndebris disks colder than those known previously. Using several methods, we\nre-consider whether some or even all of the candidates may be associated with\nunrelated galactic or extragalactic emission and conclude that it is highly\nunlikely that none of the candidates represents a true circumstellar disk. For\ntrue disks, both the dust temperatures inferred from the SEDs and the disk\nradii estimated from the images suggest that the dust is nearly as cold as a\nblackbody. This requires the grains to be larger than ~100{\\mu}m, regardless of\ntheir material composition. To explain the dearth of small grains, we explore\nseveral conceivable scenarios: transport-dominated disks, disks of low\ndynamical excitation, and disks of unstirred primordial macroscopic grains. Our\nqualitative analysis and collisional simulations rule out the first two of\nthese scenarios, but show the feasibility of the third one. We show that such\ndisks can survive for gigayears, largely preserving the primordial size\ndistribution. They should be composed of macroscopic solids larger than\nmillimeters, but smaller than kilometers in size. Thus planetesimal formation,\nat least in the outer regions of the systems, has stopped before "cometary" or\n"asteroidal" sizes were reached.\n