Identifying the collisions that make extreme debris disks

A subset of debris disks are bright and show evidence of variability on short timescales, flux increasing or decreasing over months/years and in some cases oscillations on timescales of days (e.g. Su et al., 2019). These extreme debris disk phenomena are thought to be the result of energetic collisions between growing planetary bodies. These collisions can produce ejecta with narrow size distributions (Benz et al., 2007). The ejecta from such collisions can therefore be processed, thereby changing the population of grains, on much shorter timescales than traditional debris disks (Meng et al., 2014).

Extreme debris disks thus provide a key window into planet formation, but connecting the observed properties to the collisions that caused them is challenging. Much progress has been made in understanding how impact ejecta produces the complex signals observed (e.g. Watt et al., 2021); but there is currently no reliable way to relate the observed dust to the size of the impacting bodies. Typically, the inferred mass of dust is converted to a body size assuming the entire mass of the body was turned into dust. A collision both vaporising and ejecting the entire mass of a small body is, however, unlikely. High velocity planetary impacts have sufficient energy to cause shock-induced vaporisation of both water ice and silicates (Carter & Stewart, 2020; Davies et al, 2020), but most previous studies of impacts have ignored the effects of vaporisation. The masses of vaporised ejecta produced by collisions are thus poorly constrained. Vaporisation during planetary impacts is key to determining the sizes of ejected material. Understanding these collisions is essential for relating the observed signals from extreme debris disks to the impacts that caused them.

We present new, high resolution hydrodynamic simulations of planetary impacts designed to explore and quantify vapour production and the mass and state of ejecta. Vaporised ejecta is produced across a wide range of colliding masses and impact parameters. We derive a relation between the impact energy and the mass of vaporised ejecta that could be observed in young exoplanetary systems. By assuming a reasonable maximum impact velocity, the kinetic energy can be related to the minimum mass of the colliding planetary bodies. The emission observed from an extreme debris disk can thus be linked directly to the likely planetary collision that caused it.