Planets around stellar remnants III

Wednesday saw a crammed schedule of talks. We first moved away from planets to dusty discs around stellar remnants, with Geoff Bryden opening with a review of dust discs around Main Sequence stars, and then a description of the different physical processes influencing dust discs around Neutron Stars. Notably, pulsar winds can ablate dust grains and significantly alter the size distribution of particles. There is at least one neutron star with a dust disc: 4U 2259+586, an anomalous X-ray pulsar (these pulsars are not accreting gas; the X-rays come from magnetospheric processes) which has mid IR emission from dust but no sign of gas.

Next, Ben Zuckerman reviewed the study of White Dwarf pollution. Due to the very strong gravitational fields of WDs, any metals in their atmospheres sink on a timescale of years to megayears depending  on atomic mass, so any metals present in their atmosphere must have been delivered at astronomically recent times. The best candidate for the pollution is asteroids which are tidally disrupted when they pass close to the WD, and then are accreted onto it. Indeed, for the polluted WDs whose metal content is known in detail, the composition of the pollution is very similar to the composition of rocky bodies in the inner Solar System. At least 25% of WDs show pollution, suggesting that bombardment of them by asteroids is common. Ben closed with an intriguing peek at an upcoming result showing that a dust disc around a Main Sequence star appears to have disappeared in 2009/10…

Next John Debes described the sizes of these WD dust discs in more vivid terms: roughly, they are similar in size to Saturn’s rings, and vary from thin belts to very wide discs. John then described how asteroids in a Solar-System type asteroid belt can be destabilised by a Jovian planet as the star loses mass just before becoming a WD–the Kirkwood gaps in the asteroid belt where orbits are unstable grow and more asteroids are encompassed by the unstable region. A few per cent of the unstable asteroids get hurled onto orbits that take them sufficicently close to the WD to be tidally disrupted and form a disc and provide a pollution source.

Jay Farihi next gave more physical details about the dust in these discs. They typically show evidence for silicate rocks in their infrared spectra, suggesting an origin from terrestrial planets or asteroids. Estimated disc masses are around the size of large asteroids in the Solar System, as are the estimated masses of accreted material providing the atmospheric pollution. Together, these talks gave a very strong case for the idea that WD pollution and dust discs are caused by asteroids passing close to the WD.

Next, Boris Gaensicke described how some WDs have gaseous discs. These can be hydrogen dominated, but several are only composed of gaseous metals. The gas emission lines are split, allowing the Keplerian velocity of the gas to be determined. Several show time variability, hinting at non-circular discs. Furthermore, the emission lines can be highly asymmetric, which is naturally explained as us seeing a tidal stream from a recent disruption event.

Stephan Hartmann then described explicitly the modelling of asymmetric gaseous emission lines towards the WD SDSS J1228+1040, where a simple viscous disc model can reproduce the observations. Such a model is however unrealistic as it ignores illumination from the WD, actually the dominant source of heating.

Patrick Dufour then described another particular WD, SDSS J0738+1835, which has accreted a body at least as big as Ceres, the largest asteroid, and also hosts a disc with gas and dust components. The elemental composition of the accreted matter is rather deficient in refractories, in comparison to most other polluted WDs, so the composition of extra-Solar asteroids and planets is clearly somewhat variable between systems.

Kate Su and Jana Bilikova then teamed up to talk about dust discs around hot, young WDs stil surrounded by planetary nebulae. Spitzer has found discs around 9 such WDs, well inside the large planetary nebula, a typical example being the WD and disc at the centre of the Helix Nebula. 6 of the discs are similar to the Helix disc, explicable as Kuiper Belt type discs that have survived the star’s giant stages and mass loss. The other three are more complicated, as there appears to be a close binary companion to the WD in each case, as well as the disc.

We then moved back to planets, Andrzej Niedzielski talking about planets around sub-giant and giant stars. Several dozen of these are now known, with none having been found within about 0.5 AU of the host star, in contrast to planets around Main Sequence stars where many are on close orbits. It is not clear whether this is the result of tidal engulfment of the close-in planets, or a signpost of the formation of the planets, since the masses of the giants targetted tend to be higher than the masses of the main sequence stars. He also pointed out that some giant stars have far more Lithium than they should. Lithium is quickly destroyed inside stars and should not persist until the giant stages, so perhaps these stars have had their lithium replenished by swallowing a planet or two.

Johny Setiawan then described some giant stars and their planets in some detail. Many of these giants are of very low metalicity, somewhat challenging for conventional theories of planet formation. He also showed an RV curve for a 20 Solar mass O-type star, hinting at a substellar companion. If confirmed, this will be the most massive host of an exoplanet or low-mass brown dwarf known.

David Spiegel next talked about the survival of planets to tidal forces as stars expand, and what happens if they get engulfed by the expanding stellar envelope. The huge uncertainties in tidal theory make it very hard to predict a planet at a radius of a few AU will survive its host’s expansion or not. It is also hard to produce planets that end up on intermediate orbits of a few tenths of an AU, or low mass planets on any orbit less than about 1AU, with our current understanding of what happens to planets that enter the envelope. Yet, these objects are seen.

Frederic Hessman finished the day with a description of the three circumbinary planets discovered by Kepler (two in this week’s Nature). He then described the previously reported circumbinary planets detected by timing the eclipses of the binary, drawing attention to many problems in the analyses. These include varying eclipse durations as well as times (not expected from planetary perturbations), inconsistency in the dyanical models (typically the influence of the binary on the putative planet’s orbit is neglected, so the solution is not self-consistent), to trivial dynamical instability (a notorious example, to which already at least two refutation papers have been published, actually claims two planets whose orbits overlap…). He made an explicit call for “stricter referees”. Of the 10 or so timing-based circumbinary planet claims, only NN Serpentis looks robust.