The presence of planets around solar-type stars suggests that many white dwarfs should have relic planetary systems. While planets closer than $5 AU will most likely not survive the post-main-sequence lifetime of their parent star, any planet with semimajor axis greater than 5 AU will survive, and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central star's mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases significantly with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38, and may also be an important factor in explaining the existence of DAZ white dwarfs. The onset of instability through changing mass ratios will also be a significant effect for planetary embryos gaining mass in protoplanetary disks.

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Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different 'dynamical ages'). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.

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The localization of the short-duration, hard-spectrum gamma-ray burst GRB 050509b by the Swift satellite was a watershed event. Never before had a member of this mysterious subclass of classic GRBs been rapidly and precisely positioned in a sky accessible to the bevy of ground-based follow-up facilities. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. Though the Swift X-ray Telescope (XRT) discovered an X-ray afterglow of GRB 050509b, the first ever of a short-hard burst, no convincing optical/infrared candidate afterglow or supernova was found for the object. We present a re-analysis of the XRT afterglow and find an absolute position of R.A. = 12 h 36 m 13. s 59, Decl. = +28 • 59 ′ 04. ′′ 9 (J2000), with a 1σ uncertainty of 3. ′′ 68 in R.A., 3. ′′ 52 in Decl.; this is about 4 ′′ to the west of the XRT position reported previously. Close to this position is a bright elliptical galaxy with redshift z = 0.2248 ± 0.0002, about 1 ′ from the center of a rich cluster of galaxies. This cluster has detectable diffuse emission, with a temperature of kT = 5.25 +3.36 −1.68 keV. We also find several (∼11) much fainter galaxies consistent with the XRT position from deep Keck imaging and have obtained Gemini spectra of several of these sources. Nevertheless we argue, based on positional coincidences, that the GRB and the bright elliptical are likely to be physically related. We thus have discovered evidence that supports the notion that at least some short-duration, hard-spectra GRBs are at cosmological distances.We also explore the connection of the properties of the burst and the afterglow, finding that GRB 050509b was underluminous in both of these relative to long-duration GRBs. However, we also demonstrate that the ratio of the blast-wave energy to the γ-ray energy is consistent with that of long-duration GRBs. This suggests a comparably high efficiency of γ-ray conversion as in long GRBs as might be expected if the same emission mechanism is at work in short and long GRBs. Based on this analysis, on the location of the GRB (40 ± 13 kpc from a bright galaxy), on the galaxy type (elliptical), and the lack of a coincident supernova, we suggest that there is now observational consistency with the hypothesis that short-hard bursts arise during the merger of a compact binary (two neutron stars, or a neutron star and a black hole). In this context, we limit the properties of a Li-Paczyński "mini-supernova" that is predicted to arise on ∼day timescales. Other progenitor models are still viable, and additional rapidly localized bursts from the Swift mission will undoubtedly help to further clarify the progenitor picture.