Planetary Collisions in a Binary Star System

By Maggie Thompson, Ralph Shuping, and Joan Schmelz

Paper: Studying the Evolution of Warm Dust Encircling BD +20 307 Using SOFIA
Thompson, Maggie A., et al., 2019, ApJ, 875, 45.

Recent observations from SOFIA of a binary star system designated BD +20 307 indicate that there may have been a catastrophic collision between two planets within the last 10 years.

Many stars are brighter in the infrared than expected if the light were due to the star alone. This "infrared excess" results from dust in a circumstellar disk heated by visible and UV radiation from the central star. These dusty "debris disks" are thought to evolve via collisions and evaporation of solid bodies, ranging from small planetesimals to larger planet-sized objects. Dust in these debris disk is typically low-temperature (≲100 K) and orbits far from the host star, similar to the Kuiper Belt in our own Solar System, which is located beyond the orbit of Neptune.

An unusually warm, dusty debris disk surrounds two mature stars in BD +20 307. Because it is small (< 1 AU) and extremely dusty, this disk is predicted to have a short collisional timescale, making it an ideal target to search for changes in dust quantity and composition over time. However, given the old age (> 1 billion years) and extreme dust flux of this system, collisional cascade alone cannot explain the large amount of observed dust. This result strongly suggests that the dust must be transient.

Studying warm debris disks gives astronomers a rare opportunity to examine compositional changes of circumstellar material that vary on extremely short timescales and to better understand catastrophic collisions that occur late in the formation history of a planetary system.

Collisional cascade, a process in which large planetesimals in a disk collide and continually break down into smaller objects, can explain the infrared emission from most debris disks. However, there is a small set of known stars with much warmer debris disks due to dust orbiting much closer to the host star. These smaller debris disks do not last long because the dust will either be blown out of the system or dragged into the star. Therefore, observations of these warm debris disks can be used to test collisional models developed to understand the final stages of planetary system formation.

Observations from 2015 of BD +20 307 using the Faint Object Infrared Camera (FORCAST) onboard SOFIA were compared with observations taken a decade earlier using Spitzer, Keck, and Gemini. Researchers found that the infrared emission from BD +20 307 increased significantly (~10%) in a ten-year timespan. There are several mechanisms that could give rise to such an increase. The dust could heat up, either by increasing the stellar luminosity or by moving the dust closer to the star, but both of these are difficult to implement on such short timescales. Increasing the amount of dust in the system in 10 years is relatively straightforward, but it does involve a catastrophic planetary collision.

SOFIA continues to monitor BD +20 307. Researchers are in the process of analyzing the 2018 dataset and are excited to see what changes they can detect since the original observations in 2015.

If the origin of the copious amount of warm dust orbiting BD +20 307 is due to an extreme collision between planetary-sized bodies, then this system provides a unique opportunity for astronomers to better understand planetary systems around binary stars and catastrophic collisions that occur long after planets form.

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