Characterizing Terrestrial Planet Formation with Young Debris Disks

This work is focused on the formation of terrestrial planets. To determine the behavior of planetary debris disks in the era of terrestrial planet formation, first we study 15 young clusters and associations with ages up to ~40 Myr. By adopting a threshold in relative excess at the phenomenological boundary between protoplanetary and debris disks, we find that the incidence of qualifying disks decays in the first 10 Myr, remains nearly constant between 10 and 25 Myr, and then continues to decline. Next, from the archival Spitzer data, we find that two disks around solar-like stars, ID8 in NGC 2547 and HD 23514, in the age range of the giant impact era have their 24 micron excesses varied on timescales of a few years, even though the stars are not variable in the optical. In the follow-up Spitzer observations in 2012 and 2013, a debris-producing impact in the terrestrial planet zone around the 35-million year-old solar analog star ID8 was detected in real time. We observed a substantial brightening of the debris disk at 3-5 micron, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were ground into dust by collisions. At last, we extend the time-domain study to more extreme debris disks, which are likely indicative of recent and/or onging collisions of rocky objects. We use Spitzer 3.6 and 4.5 micron time-series observations in 2012 and 2013 (extended to 2014 in one case) to monitor 5 more extreme debris disks. This makes the first systematic time-domain investigation of planetary impacts outside the solar system. Significant variations with timescales shorter than a year are detected in five out of the six extreme debris disks we have monitored. However, different systems show diverse sets of characteristics in the time domain, including long-term trends, disk temperature variations, and possible periodicity.Release after 01-Sept-201