Dr Philip J Carter

University of Bristol

Research: Polluted white dwarfs

White dwarfs – the compact remnants left by most main sequence stars – are expected to have pristine atmospheres due to their extreme gravity, but there are a growing number of observed white dwarfs whose atmospheres are polluted with metals. This pollution is now believed to be due to ongoing accretion from remnant planetary systems, asteroids and/or minor planets (exoplanetesimals) having been scattered into the vicinty of the white dwarf and tidally disrupted.

The exact mechanisms via which these bodies are scattered onto orbits that pass close to the white dwarf is unknown. The regions close to the star are expected to have been cleared during the red giant phase that marked the end of the star's life. Understanding the delivery of this exoplanetary material to their white dwarfs is key to understanding the pollution, and the fates of planetary systems.

Veras et al., 2017, MNRAS, 465, 1008.

One particularly interesting white dwarf, WD1145+017, shows a constantly evolving series of asymmetric transits which block tens of percent of the light from the white dwarf. Individual transit features can be tracked for several nights, revealing periods of about 4.5 hours, but generally after a few weeks will have disappeared and been replaced with new transits (see Vanderburg et al., 2015, Gänsicke et al., 2016). It is thought that this complex transit signature is produced by disrupted or actively disrupting planetesimals in orbit around the white dwarf. The transit depths are too large to be produced by a solid asteroid, and are likely due to associated clouds of dust passing in front of the white dwarf. One suggestion is that the transits are mainly due to fragments of a single disrupting parent body (see Rappaport et al., 2016). We carried out a set of N-body simulations to test this model, and constrain the properties of such an asteroid.

Model asteroids

Fig. 1: The model asteroids used for the disruption simulations. We used both hexagonally closest packed asteroids (A) and randomly packed asteroids (B). We tested both homogeneous density asteroids and differentiated asteroids with a denser core (B2). (From Veras et al., 2017).

Our results show that homogeneous asteroids either disrupt rapidly, or, if they are sufficiently dense, are stable over long time scales. This makes it difficult to explain the observations with a homogeneous asteroid model.

Fig. 2: Disruption of a homogeneous hexagonally closest packed rubble pile (ρ = 2.6 g/cm^3). The images are shown in the rotating frame, where left is radially towards the white dwarf and the direction of the orbit is towards the top of the page. The white numbers in the upper part of each panel indicate the number of orbits. (Veras et al., 2017).

Low density differentiated asteroids disrupt rapidly as for the homogeneous case. However, there is a range of bulk densities where the asteroids have their outer layers stripped, and then enter a prolonged phase of intermittent mass loss. This behaviour could plausibly generate the kind of transit features that have been observed for WD 1145, the differentiated asteroid representing a parent body, and the mantle particles stripped off producing relatively short lived clouds of dust.

Fig. 3: Asteroid mass vs time for differentiated asteroids. Above about 3.2 g/cm^3 the asteroids disrupt more slowly, losing mantle over a period of 100s of orbits or more.

Fig. 4: Disruption of a differentiated randomly packed rubble pile (ρ = 3.5 g/cm^3). The white particles are mantle particles, and the green core particles underneath remain hidden. After about half an orbit, mantle particles start streaming from the L1 and L2 Lagrange points. After about four orbits, the streaming became intermittent. (Veras et al. 2017).

More animations can be found here.