Disintegrating Planetary Bodies Around a White Dwarf

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Several months ago, the discovery of WD 1145+017 was announced. This white dwarf appears to be orbited by planetary bodies that are actively disintegrating due to the strong gravitational pull of their host. A follow-up study now reveals that this system has dramatically evolved since its discovery.

Signs of Disruption

Potential planetary bodies orbiting a white dwarf would be exposed to a particular risk: if their orbits were perturbed and they passed inside the white dwarf’s tidal radius, they would be torn apart. Their material could then form a debris disk around the white dwarf and eventually be accreted.

Interestingly, we have two pieces of evidence that this actually happens:

  1. We’ve observed warm, dusty debris disks around ~4% of white dwarfs, and
  2. The atmospheres of ~25-50% of white dwarfs are polluted by heavy elements that have likely accreted recently.

But in spite of this indirect evidence of planet disintegration, we’d never observed planetary bodies actively being disrupted around white dwarfs — until recently.

Unusual Transits

In April 2015, observations by Kepler’s K2 mission revealed a strange transit signal around WD 1145+017, a white dwarf 570 light-years from Earth that has both a dusty debris disk and a polluted atmosphere. This signal was interpreted as the transit of at least one, and possibly several, disintegrating planetesimals.

In a recent follow-up, a team of scientists led by Boris Gänsicke (University of Warwick) obtained high-speed photometry of WD 1145+017 using the ULTRASPEC camera on the 2.4m Thai National Telescope. These observations were taken in November and December of 2015 — roughly seven months after the initial photometric observations of the system. They reveal that dramatic changes have occurred in this short time.

Rapid Evolution

light curve

A sample light curve from TNT/ULTRASPEC, obtained in December 2015 over 3.9 hours. Many varied transits are evident (click for a better view!). Transits labeled in color appear across multiple nights. [Gänsicke et al. 2016]

Initial observations of WD 1145+017 showed a significant transit dip (>10%) only every ~3.6 hours, on average. In contrast, in the current observations, every light curve is riddled with numerous transit events that have durations of 3–12 minutes and depths of 10–60%. Many of the transit features overlap, so there are now only short segments of the light curve that don’t appear to be attenuated by debris.

Gänsicke and collaborators use the new data to analyze the transiting bodies. Though some transits are consistent from night to night, most evolve in shape and depth, appearing and disappearing over the course of the observing campaign. This rapid variability, along with the large size of the transiting bodies (several times the size of the white dwarf), support the conclusion that the transiting objects are not solid bodies. Instead, they are likely clouds of gas and dust flowing from smaller bodies that are being disrupted.

Because astronomical timescales are often extremely long, the observations of WD 1145+047 are especially exciting — this is a rare chance to watch a system evolve in real time! Given how rapidly it appears to be changing, continued observations are sure to soon reveal more about the planetary bodies orbiting this white dwarf.

Citation

B. T. Gänsicke et al 2016 ApJ 818 L7. doi:10.3847/2041-8205/818/1/L7

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