The total number of giant planets in debris discs with central clearings

Infrared spectra from the Spitzer Space Telescope ( SSC ) of many debris discs are well fit with a single blackbody temperature which suggest clearings within the disc. We assume that clearings are caused by orbital instability in multiple planet systems with similar configurations to our own. These planets remove dust-generating planetesimal belts as well as dust generated by the outer disc that is scattered or drifts into the clearing. From numerical integrations, we estimate a minimum planet spacing required for orbital instability (and so planetesimal and dust removal) as a function of system age and planet mass. We estimate that a 10⁸ yr old debris disc with a dust disc edge at a radius of 50 au hosted by an A star must contain approximately five Neptune mass planets between the clearing radius and the iceline in order to remove all primordial objects within it. We infer that known debris disc systems contain at least a fifth of a Jupiter mass in massive planets. The number of planets and spacing required is insensitive to the assumed planet mass. However, an order of magnitude higher total mass in planets could reside in these systems if the planets are more massive.     

Damped Oscillations of Coronal Loops

A mechanism of damped oscillations of a coronal loop is investigated. The loop is treated as a thin toroidal flux rope with two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics are described within the framework of ideal magnetohydrodynamics (MHD). The main features of the theory are the following: i) Oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and ii) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The oscillation is restricted to the vertical plane of the flux rope. The initial equilibrium flux rope is set into oscillation by a pulse of upflow of the ambient plasma. The theory is applied to two events of oscillating loops observed by the Transition Region and Coronal Explorer (TRACE). It is shown that the Lorentz force and drag with a reasonable value of the coupling coefficient (c _d ) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows that the variations in the observed intensity can be explained by the minor radial expansion and contraction. For the two events, the values of the drag coefficient consistent with the observed damping times are in the range c _d ≈2 – 5, with specific values being dependent on parameters such as the loop density, ambient magnetic field, and the loop geometry. This range is consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories of coronal mass ejections (CMEs).     

Sedimentological investigations of metamorphic clastics: the basal clastic rocks of the Brenner Mesozoic (Stubai Alps, Austria/Italy)

Sedimentological investigations were carried out in the basal clastic rocks of the Brenner Mesozoic in the Stubai Alps. These are Scythian in age and metamorphosed to a greenschist grade. The increase of temperature and pressure from north to south allows the metamorphic effects on Sedimentological features to be gradually traced into higher grade metamorphic areas. The use of various laboratory methods as well as profiling with lithofacies types shows the limitation of using only single methods in sediments suffering thermal and deformational overprint. Only a combined application of different methods allows the extraction of useful sedimentological information. Based on the results of these investigations, a fan delta to shelf succession is proposed for the basal clastic rocks. Petrographic analyses show the relation of the sediments to an underlying weathering horizon.     

High-performance direct gravitational N-body simulations on graphics processing units

We present the results of gravitational direct N-body simulations using the commercial graphics processing units (GPU) NVIDIA Quadro FX1400 and GeForce 8800GTX, and compare the results with GRAPE-6Af special purpose hardware. The force evaluation of the N-body problem was implemented in Cg using the GPU directly to speed-up the calculations. The integration of the equations of motions were, running on the host computer, implemented in C using the 4th order predictor–corrector Hermite integrator with block time steps. We find that for a large number of particles (N  10⁴) modern graphics processing units offer an attractive low cost alternative to GRAPE special purpose hardware. A modern GPU continues to give a relatively flat scaling with the number of particles, comparable to that of the GRAPE. The GRAPE is designed to reach double precision, whereas the GPU is intrinsically single-precision. For relatively large time steps, the total energy of the N-body system was conserved better than to one in 10⁶ on the GPU, which is impressive given the single-precision nature of the GPU. For the same time steps, the GRAPE gave somewhat more accurate results, by about an order of magnitude. However, smaller time steps allowed more energy accuracy on the grape, around 10⁻¹¹, whereas for the GPU machine precision saturates around 10⁻⁶ For N  10⁶ the GeForce 8800GTX was about 20 times faster than the host computer. Though still about a factor of a few slower than GRAPE, modern GPUs outperform GRAPE in their low cost, long mean time between failure and the much larger onboard memory; the GRAPE-6Af holds at most 256k particles whereas the GeForce 8800GTX can hold 9 million particles in memory.     

Patterns in abundance and size of two deep-water gorgonian octocorals,in relation to depth and substrate features off Nova Scotia

Deep-water corals form unique ecosystems, yet very little is known about factors that regulate their distribution and growth. The abundance and size of two deep-water gorgonian coral species, Paragorgia arborea and Primnoa resedaeformis, and their relationship with depth and substratum cover, were investigated at Northeast Channel, off Nova Scotia, in July 2006. This is the first study to measure abundance and size of these two coral species at depths >500 m in the Canadian Atlantic region. A total of 5 transects between 500 and 1000 m depth were examined using video collected by the remotely operated vehicle ROPOS. Abundance of both species was patchy, but higher at these deeper depths than at <500 m. Abundance generally declined with depth, and was moderately correlated with cover of hard substratum (cobble, boulder, bedrock). These relationships were stronger and less variable for P. resedaeformis than for P. arborea, suggesting that factors such as topographic relief may play an additional role in regulating distributions of P. arborea. Maximum colony height was 125 and 240 cm for P. resedaeformis and P. arborea, respectively, and much greater than recorded for depths <500 m. Overall, colony height and depth relationships were strong for both species, but variable among transects. P. resedaeformis showed a negative relationship with depth, while the opposite was observed for P. arborea, suggesting that the two species are affected differently by factors that vary with depth (e.g. temperature, fishing disturbance). Relationships between colony size and size of attachment stone were stronger for P. arborea, especially for overturned colonies, than for P. resedaeformis, suggesting that availability of suitably coarse substrate may be more important for the long-term persistence of P. arborea colonies.     

Military geology and the Battle of Gettysburg, July 1863

Gettysburg – the most costly of all battles of the American Civil War, and one of the most significant battles ever fought. Contested in rural Pennsylvania, the Battle of Gettysburg took place after a chance encounter by the Confederate Army of Northern Virginia (General Robert E. Lee) with the Union Army of the Potomac (Major-General George Meade). It was to prove the turning point in the war, the so-called 'high tide' of the Confederacy, as after Gettysburg the fortunes of war turned in favour of the Union. Fought over three days in early July 1863, the battle continues to be analyzed and re-analyzed from all perspectives. Significantly, it was the first battle to be re-interpreted from the perspective of its geology, by Andrew Brown of the Pennsylvania State Survey in the early 1960s. It continues to fascinate geologists interested in the overall impact of geology and terrain in determining the outcome of battle. This article examines why Gettysburg has been so influential in shaping our understanding of geology in warfare.     

On the sublimation of ice particles on the surface of Mars; with applications to the 2007/8 Phoenix Scout mission

Experimental studies related to the sublimation of ice, in bulk or as small particles, alone or mixed with dust similar to that expected on the surface of Mars, are reported. The experiments, a cloud physics particle sublimation model, and a convection model presented by Ingersoll, all indicate a strong dependence of sublimation rate on temperature, and this appears to be the dominant factor, assuming that the relative humidity of the air is fairly low. In addition the rate of loss of water vapour appears to depend primarily on exposed surface area and less on particle size and the total mass of the sample, or the mass of ice in the sample. The 2007/8 Phoenix Scout mission plans to obtain and analyse samples of sub-surface ice from about 70° N on Mars. A concern is that these samples, in the form of ice chips of size about 1 mm diameter, could be prone to sublimation when exposed for prolonged periods (many hours) to a relatively warm and dry atmosphere. Our laboratory simulations confirm that this could be a problem if particles are simply left lying on the surface, but also indicate that samples kept suitably cold and collected together in confined piles will survive long enough for the collection and delivery (to the analysis instruments) procedure to be completed.