Formation and evolution of extreme debris disks through giant impacts of planetary embryos

• Main Author: Watt, Lewis
• Format: Dissertation
• Language: English
• Published: University of Bristol 2023

In many formation scenarios, it is necessary for giant impacts between planetary embryos to form the terrestrial planets we find within our own Solar System. Giant impacts will eject a substantial amount of material to the surrounding stellar environment. The material will form a disk that potentially can be observed from outside the system. A subclass of debris disks known as extreme debris disks are candidates for recent evidence of a giant impact. Extreme debris disks are bright and warm which usually places them close to their host star within the terrestrial planet formation space. Typically they are observed around young stars with ages less than 200 Myr which places them in the expected time for late-stage terrestrial planet formation that involves giant impacts. Extreme debris disks are often observed with variability within their lightcurves on yearly and sub-yearly timescales which is much shorter than the variability expected from traditional debris disk on the scale of millions of years. Some variability within extreme debris disks has been observed to be periodic. The periodicity has been linked to giant impact produced dust forming an asymmetric disk with two pinch points: the collision point and anti-collision line. In chapter 3 of this thesis we show how a giant impact can form the periodic variability seen in some extreme debris disks. We also show how the behaviour of an extreme debris disk formed from giant impacts differs with varying collision parameters through many simulations. One main result we find is that all giant impacts produce dust anisotropically and the orientation of the giant impact within the stellar system will change the behaviour seen in the lightcurve of the disk. For the extreme debris disks to be observable soon after the giant impact, it is assumed that the vaporised escaping ejecta from the giant impact will form small grains with typical sizes of μm - mm. It is expected that the small vapour condensate grains will collisionally evolve very quickly to sizes below the blowout size of the star. The expected fast removal of dust does not line-up with the multi-year observations of extreme debris disks. Some extreme debris disks have exhibited increases in their lightcurves more steady than expected if caused by a giant impact. It has been proposed that a boulder population made of planetesimals formed from the ejecta of a giant impact could sustain the extreme debris disk for longer than expected if only made up of vapour condensate grains. We show in chapter 4 simulations and analysis of a collisionally evolved planetesimal population formed from giant impacts for varying disk and collision parameters. For disks placed close to the star that the collision rate between the planetesimals is substantial enough to sustain an extreme debris disk through extremely destructive collisions between planetesimals. The evolution of the collision rate is consistent across all varying parameters and it is the initial collision rate which sets how many collisions will take place in a boulder populated disk.