Are debris discs self-stirred?

Abstract

This paper aims to consider the evidence that debris discs are self‐stirred by the formation of Pluto‐size objects. A semi‐analytical model for the dust produced during self‐stirring is developed and applied to the statistics for A‐stars. We show that there is no significant statistical difference between fractional excesses of A stars 50 Myr old, and therefore focus on reproducing the broad trends, the ‘rise and fall’ of the fraction of stars with excesses that the pre‐stirred model of Wyatt et al. does not predict. Using a population model, we find that the statistics and trends can be reproduced with a self‐stirring model of planetesimal belts with radius distribution between 15–120 au, with width dr=r/2. Discs must have this 15 au minimum radius in order to show a peak in disc fraction, rather than a monotonic decline. However, the marginal significance of the peak in the observations means that models with smaller minimum radii also formally fit the data. Populations of extended discs with fixed inner and/or outer radii fail to fit the statistics, due mainly to the slow 70 m evolution as stirring moves further out in the disc. This conclusion, that debris discs are narrow belts rather than extended discs, is independent of the significance of 24 m trends for young A‐stars. Although the rise and fall is naturally explained by self‐stirring, we show that the statistics can also be reproduced with a model in which discs are stirred by secular perturbations from a nearby eccentric planet. Detailed imaging, which can reveal warps, sharp edges and offsets in individual systems, is the best way to characterize the stirring mechanism. From a more detailed look at Pictoris Moving Group and TW Hydrae Association A‐stars, we find that the disc around Pictoris is likely the result of secular stirring by the proposed planet at 10 au; the structure of the HR 4796A disc also points to sculpting by a planet. The two other stars with discs, HR 7012 and Tel, possess transient hot dust, though the outer Tel disc is consistent with a self‐stirred origin. We suggest that planet formation provides a natural explanation for the belt‐like nature of debris discs, with inner regions cleared by planets that may also stir the disc, and the outer edges set by where planetesimals can form.

Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti

“…Whatever its origin, the decreasing surface density profile in the outer regions of the 49 Ceti dust disk stands in sharp contrast to the increasing surface density profiles of AU Mic and HD107146, which are consistent with theoretical predictions for self-stirred debris disks (e.g., Kenyon & Bromley 2002;Kennedy & Wyatt 2010). While only three debris disks have yet been observed at millimeter wavelengths with sufficient sensitivity and angular resolution to provide reliable constraints on the radial surface brightness profile of continuum emission, the results so far are suggestive that 49 Ceti's gas-bearing dust disk may be undergoing a fundamentally different physical process than that of the gas-poor debris disks around AU Mic and HD 107146, resulting in the qualitatively different surface density profiles.…”

“…Millimeter-wavelength observations with the SMA by Wilner et al (2012) and with ALMA by MacGregor et al (2013) show it to be consistent with a dust surface density increasing steeply with radius as S µ r 2.8 between radii of 10-40 au before being abruptly truncated. This surface density profile is consistent with predictions for a self-stirred disk with ongoing planet formation (Kennedy & Wyatt 2010), but the timescale required to initiate the collisional cascade is much longer than its age (see Kennedy & Kenyon 2008). Strubbe & Chiang (2006) suggest that a birth ring exists at a radius of ∼40 au, in which grains of all sizes are created through collisions between parent planetesimal bodies.…”

The First 40 Million Years of Circumstellar Disk Evolution: The Signature of Terrestrial Planet Formation

“…Currie et al (2008a) found some evidence in support of this suggestion. The self-stirring models of Kennedy & Wyatt (2010) also produce broad maxima in the excess emission roughly in this time regime. The recent detailed smooth particle hydrodynamics (SPH) simulations of this process by Genda et al (2015) substantially improve on these results.…”

Dust spreading in debris discs: do small grains cling on to their birth environment?

“…We generate parent belts at three different distances for each host star: 30, 100 and 200 AU assuming a flat radial distribution with a radial distribution index of 0.0, a free initial eccentricity of 0.1 and a relative width of ±10%. The narrowness of these belts is justified since the majority of debris discs are found to be confined into narrow rings (e.g., Kennedy & Wyatt 2010). However, in recent studies discs were also found to possess extended planetesimal belts (e.g., Matrà et al 2018), which makes the additional investigation of a broad ring model a logical consequence.…”