During the last two decades, numerous circumstellar dust disks have been detected, both in thermal emission and in scattered light. These systems can schematically be divided into two categories: massive and gas rich protoplanetary disks, observed around young stellar objects and corresponding to the first stages of planetary formation, and more tenuous and gas depleted debris disks, around more evolved systems where the bulk of planetary accretion is probably already over and where the observed dust is believed to be produced by collisions among leftovers of the formation process (for more details, see for example the excellent series of lectures by Mark Wyatt: http://www.ast.cam.ac.uk/ wyatt/ ).
Figure 1: Numerical simulation of a collisional avalanche propagating in a debris disk: a 40 km object is initially shattered at 20 AU, releasing 1020 g of dust. The colour index shows the dust surface density. The total timescale is 1000 years.
One striking result derived from these observations is that almost no debris disk is nicely smooth and symmetric: spiral arms, narrow and broad annuli, "clumps", brightness asymmetries, etc. are commonly observed (for an impressive family portrait of debris disk, see http://astro.berkeley.edu/ kalas/disksite/pages/gallery.html). These morphological features can provide hints on important ongoing processes in the disks and improve and understanding of planetary formation scenarios. The usual explanations proposed for most of these asymmetries is the perturbing influence of an embedded planet or bound stellar companion. Alternative explanations, not directly relying on the presence of an undetected planet, are however possible.
A joint team of the Stockholm and Paris observatories has recently studied an alternative scenario in which the source of these asymmetries is the violent break-up of a large planetesimal or cometary object releasing vast quantities of dust particles. The main point is here that the consequences of such a localized event might strongly exceed the initial excess of dust released by the break-up: the smallest initially released particles are indeed ejected outward by the star’s radiation pressure and might impact other grains in the disks, thus producing a new generation of dusty debris which will also be ejected outward and impact other grains on their way out, etc., possibly creating an outward propagating collisional "avalanche". To study this complex chain reaction mechanism, a new innovative numerical model has been developed, which follows both the dynamical and size distribution evolutions of a dusty disk. The obtained results show that collisional avalanches are a very efficient mechanism, creating outward propagating spiral structures (Figure 1), whose amplitude strongly depends (almost exponentially) on the dustyness of the disk in which it propagates. For systems having dust masses slightly above that of the archetypal debris disk of Beta-Pictoris, avalanche-created asymmetries might be bright enough, and avalanche creating events frequent enough to become observable. The observable signatures of an avalanche could be single-armed spirals or lumpy structures in disks viewed head-on or two-side brightness asymmetries in edge-on viewed systems.