Warm exo-Zodi from cool exo-Kuiper belts: the significance of P–R drag and the inference of intervening planets

Abstract

Poynting-Robertson drag has been considered an ineffective mechanism for delivering dust to regions interior to the cool Kuiper belt analogues seen around other Sun-like stars. This conclusion is however based on the very large contrast in dust optical depth between the parent belt and the interior regions that results from the dominance of collisions over drag in systems with detectable cool belts. Here, we show that the levels of habitable zone dust arising from detectable Kuiper belt analogues can be tens to a few hundreds of times greater than the optical depth in the Solar Zodiacal cloud. Dust enhancements of more than a few tens of 'zodi' are expected to hinder future Earthimaging missions, but relatively few undetectable Kuiper belts result in such levels, particularly around stars older than a few Gyr. Thus, current mid to far-IR photometric surveys have already identified most of the 20-25% of nearby stars where P-R drag from outer belts could seriously impact Earth-imaging. The LBTI should easily detect such warm dust around many nearby stars with outer belts, and will provide insight into currently unclear details of the competition between P-R drag and collisions. Given sufficient confidence in future models, the inevitability of P-R drag means that the nondetection of warm dust where detectable levels were expected could be used to infer additional dust removal process, the most likely being the presence of intervening planets.

What Sets the Radial Locations of Warm Debris Disks?

“…While Wyatt (2005) argued that most disks we can detect are collisiondominated rather than drag-dominated (that is, grains are destroyed by mutual collisions faster than they can move inward), Kennedy & Piette (2015) noted that some inward transport is inevitable unless planets are present interior to the cold belt to remove the inflowing dust. Since these grains originate in the outer part of the system, they may contain a mixture of icy and refractory material.…”

“…The specific scenario of inward transport by drag forces predicts that the warm component of dragged in material should be much fainter than the cold reservoir from which it originates (Kennedy & Piette 2015). We looked for this in our sample by plotting the fractional luminosities of the warm 5 and cold components-and their ratio-against the residual warm dust location from the trend (Figure 8).…”

Exozodiacal clouds: hot and warm dust around main sequence stars

“…Dashed lines show detection limits for debris discs around nearby stars. The expected null excesses always increase with parent belt optical depth, so for each distance the contours show the minimum parent belt optical depth for warm dust to be detectable at that level [120].…”

“…However, the presence of a planet in the system may prevent dust from reaching the habitable zone. Thus, a non-detection of this warm dust for debris disc systems observed with the LBTI may well be the indirect evidence of a planet sitting in between the HZ and the cold belt [120].…”

“…The maximum optical depth reached in inner regions is always close to ∼10 −5 (within a factor 3), which is in agreement with the KIN observations showing a roughly constant mid-IR level for different systems. Kennedy and Piette [120] showed that the LBTI will be sensitive to this PR-drag produced dust. Indeed, as shown in Figure 13, the level of warm dust produced by PR-drag from debris discs that are detected today (blue dots) is detectable with the LBTI (solid lines).…”