A significant fraction of white dwarfs are observed to be polluted with metals despite high surface gravities and short settling times. The current theoretical model for this pollution is accretion of rocky bodies, which are delivered to the white dwarf through perturbations by orbiting planets. Using N-body simulations, we examine the possibility of a single planet as the source of pollution. We determine the stability of test particles on circular orbits in systems with a single planet located at 4 au for a range of masses and eccentricities, comparing the fractions that are ejected and accreted by the star. In particular, we compare the instabilities that develop before and after the star loses mass to form a white dwarf, a process which causes the semi-major axes of orbiting bodies to expand adiabatically. We determine that a planet must be eccentric (e > 0.02) to deliver significant (> 0.5 per cent) amounts of material to the central body, and that the amount increases with the planetary eccentricity. This result is robust with respect to the initial eccentricities of the scattered particles in the case of planetary eccentricity above ∼ 0.4 and the case of randomly-distributed particle longitude of pericentre. We also find that the efficiency of the pollution is enhanced as planetary mass is reduced. We demonstrate that a 0.03 M Jup planet with substantial eccentricity (e > 0.4) can account for the observed levels of pollution for initial disc masses of order 1 M ⊕ . Such discs are well within the range estimated for initial planetesimals discs and well below that estimated for our own solar system within the context of the Nice model. However, their long term survival to the white dwarf stage is uncertain as estimates for the collisional evolution of planetesimal discs suggest they should be ground down below the required levels on Gyr timescales. Thus, planetary scattering by eccentric, sub-Jovian planets can explain the observed levels of pollution in white dwarfs, but only if current estimates of the collisional erosion of planetesimal discs are in error.

We present Hubble Space Telescope observations of the upper part (T eff > 10 4 K) of the white dwarf cooling sequence in the globular cluster 47 Tucanae and measure a luminosity function of hot white dwarfs. Comparison with previous determinations from large scale field surveys indicates that the previously determined plateau at high effective temperatures is likely a selection effect, as no such feature is seen in this sample. Comparison with theoretical models suggests that the current estimates of white dwarf neutrino emission (primarily by the plasmon channel) are accurate, and variations are restricted to no more than a factor of two globally, at 95% confidence. We use these constraints to place limits on various proposed exotic emission mechanisms, including a non-zero neutrino magnetic moment, formation of axions, and emission of Kaluza-Klein modes into extra dimensions.

Warm jupiters are an unexpected population of extrasolar planets that are too near to their host to have formed in situ, but distant enough to retain a significant eccentricity in the face of tidal damping. These planets are curiously absent around stars larger than two solar radii. We hypothesize that the warm jupiters are migrating due to Kozai-Lidov oscillations, which leads to transient episodes of high eccentricity and a consequent tidal decay. As their host evolves, such planets would be rapidly dragged in or engulfed at minimum periapse, leading to a rapid depletion of the population with increasing stellar radius, as is observed. Using numerical simulations, we determine the relationship between periapse distance and orbital migration rate for planets 0.1 to 10 Jupiter masses and with orbital periods between 10 and 100 days. We find that Kozai-Lidov oscillations effectively result in planetary removal early in the evolution of the host star, possibly accounting for the observed deficit. While the observed eccentricity distribution is inconsistent with the simulated distribution for an oscillating and migrating warm jupiter population, observational biases may explain the discrepancy.

The magnetic field of the solar corona has a large-scale dipole character, which maps into the bipolar field in the solar wind. Using standard representations of the coronal field, we show that high-energy ions can be trapped stably in these large-scale closed fields. The drift shells that describe the conservation of the third adiabatic invariant may have complicated geometries. Particles trapped in these zones would resemble the Van Allen belts and could have detectable consequences. We discuss potential sources of trapped particles.

No abstract