Planets around stellar remnants IV

Thursday’s sessions were largely devoted to theory and modelling rather than observations, and began with my new supervisor, Eva Villaver, reviewing various aspects of the effects of giant stars on any planets they host. Particularly during the Asymptotic Giant Branch, stellar radii attain very large values, of around 1AU for a star like the Sun. Tidal torques acting on the planet’s orbit, which are heavily dependent on the stellar radius, are therefore very strong, and any Jovian planet within about 3AU of a Solar-mass star will be engulfed. What happens then is a difficult question. Although we heard talks earlier in the week about the potential of planets to unbind stellar envelopes to form hot subdwarfs, this may not be possible for planets less than 10 Jovian masses or so. Planets may also be evaporated by the high energy flux, losing about 1 Jovian mass of matter every million years. The prospects for planets surviving from the main sequence to the white dwarf phase seem bleak.

Harold Yorke then discussed planet formation around massive O stars. The discs surrounding these stars when young are very massive but very short-lived, so planet formation by core accretion–colliding dust grains and rocks to slowly build up planets–is not likely, but planet formation by gravitational instabilities in the gas disc is possible. However, the final mass of these `planets’ is uncertain, as they would accrete a great deal of material in the massive gas disc, and may well end up as small stars.

Kaitlin Kratter then discussed the stability of planets in orbits around one component of a binary system, as the stars evolve and lose mass. The regions where orbits are stable on the main sequence change when a star in a binary system loses mass, and previously stable planets can be captured into orbits around the other star, or collide with one of the stars. This may provide another way of delivering material to polluted white dwarfs, although few polluted WDs are known to have binary companions.

Dimitri Veras next described how the orbits of circumbinary planets–in which the planet orbits both stars of a binary system–change under stellar evolution. Here the orbit of the planet expands as mass is lost from the binary system, and if mass loss is very rapid the planet can be expelled from the system entirely. Planets around binaries of stars of one to two Solar masses are however usually safe, unless they orbit at very large distances.

Stein Sigurdsson then talked about the planet orbiting the binary pulsar PSR 1620-26, in the globular cluster M4. Due to the high stellar densities in globular clusters, capture of a planet from another star is a possible explanation for the planet’s origin. However, planets are not found around main sequence stars in globular clusters, so the origin may still be a puzzle.

Eduardo Martín presented a novel mechanism for creating Hot Jupiter planets, on orbits very close to their host stars. Rather than forming in the primordial circumstellar disc, he proposed that they arise from a merger of a contact binary (W UMa star), during which a fresh disc of material is thrown out. This may offer an explanation for the large radii of the so-called inflated hot Jupiters.

Roman Rafikov discussed models for the evolution of discs around WDs. Following the disruption of an asteroid, the material must be brought in from a disc at around a Solar radius to the surface of the White Dwarf, in order to cause observed metal pollution. The nature of a dust disc is similar to Saturn’s rings, and the timescale for such rings to spread is very long: Saturn has not accreted its ring material despite having several Gyr in which to do so. However, around WDs, two effects enhance the movement of dust towards the star. One is the Poynting–Robertson effect, a drag force caused by the starlight. The other is gas drag from dust that gets close to the WD and sublimates. This latter effect can trigger rapid bursts of accretion which can move significant quantities of material onto the WD quickly.

Noam Soker discussed transient events arising from the destruction of planets and planetesimals. Such events can be very violent, with the merger of a Brwon Dwarf and a Jovian planet for example casuing the BD to brighten by up to 8 magnitudes, and the destruction of an asteroid by a neutron star offering an explanation for an unusual gamma ray burst in 2010.

Shane Frewen then presented work on the dynamics of planetesimals perturbed by eccentric planets, attempting to explain how to scatter asteroids close to a White Dwarf in order to provide metal pollution. The most effective planets for sending bodies close to the star are highly eccentric and of mass somewhat less than Jupiter’s. However, many planetesimals orbiting close to planets are destabilised on the main sequence, and after the star loses mass on its way to beoming a white dwarf not many more planetesimals are destabilised. A larger source population could however be provided if the planetesimals experience gas drag in the planetary nebula, and migrate in to the previously depopulated region.

Brad Hansen described models of the formation of rocky planets around pulsars. The idea is to integrate the orbits of a collection of large protoplanets, assumed to have formed from a disc, in the same way as is done for terrestrial planet formation around Solar type stars. A variety of protoplanet configurations, corresponding to the discs expected from different formation mechanisms, such as supernova fallback and WD merger, were tried. The configuration yielding the configuration most like the planets of PSR B1257+12 corresponded to a supernova fallback disc, although since these may not be rare this raises the question of why there are not many more pulsar planets.

To end Thursday’s talks, Cole Miller gave a grimmer assessment of the prospects for forming planets around pulsars, since the environment in which they form is so harsh: heating from the pulsar’s radiation and ablation by the wind could easily destroy planetesimals of up to a kilometer in size. This suggests that planets must have formed quickly, and the planet formation process would be all-or-nothing: there could be no asteroid belts surviving as in main sequence planetary systems such as our own.

After Thursday’s talks, the conference dinner took place, in a seaside restaurant with views of the old Arecibo lighthouse:

Late 19th Century Arecibo lighthouse. The rocks in the water around here looked pretty vicious.

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