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

TL;DR

This study investigates the nature of cold debris disks detected at 160μm, exploring whether they are true circumstellar disks with large grains or unrelated background objects, and suggests they are likely primordial, large-grain disks that have halted planetesimal formation.

Contribution

The paper provides a detailed analysis and simulations indicating that some cold debris disks are primordial, large-grain disks that have stopped growing before forming smaller planetesimals.

Findings

Cold debris disks are likely composed of large, primordial grains.

Transport and low excitation scenarios are unlikely explanations.

Disks can survive for gigayears with primordial size distributions.

Abstract

(abridged) Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100{\mu}m or shorter. However, six out of 31 excess sources in the Herschel OTKP 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 suggested to stem from debris disks colder than those known previously. Using several methods, 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 SEDs 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{\mu}m, regardless of their material composition. To explain the dearth of small grains, 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 survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic 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.