The sensitivity of the next generation of lunar Cherenkov observations to UHE neutrinos and cosmic rays

We present simulation results for the detection of ultra-high energy (UHE) cosmic ray (CR) and neutrino interactions in the Moon by radio-telescopes. We simulate the expected radio signal at Earth from such interactions, expanding on previous work to include interactions in the sub-regolith layer for single dish and multiple telescope systems. For previous experiments at Parkes, Goldstone (GLUE), and Kalyazin we recalculate the sensitivity to an isotropic flux of UHE neutrinos. We find the published sensitivity for the GLUE experiment to be too high (too optimistic) by an order of magnitude, and consequently the GLUE limit to be too low by an order of magnitude. Our predicted sensitivity for future experiments using the Australia Telescope Compact Array (ATCA) and the Australian SKA Pathfinder (ASKAP) indicate these instruments will be able to detect the more optimistic UHE neutrino flux predictions, while the square kilometre array (SKA) will also be sensitive to all bar one prediction of a diffuse ‘cosmogenic’, or ‘GZK’, neutrino flux.Outstanding theoretical uncertainties at both high-frequency and low-frequency limits currently prevent a reliable estimate of the sensitivity of the lunar Cherenkov technique for UHE cosmic ray (CR) astronomy. Here, we place limits on the effects of large-scale surface roughness on UHE CR detection, and find that when near-surface ‘formation-zone’ effects are ignored, the proposed SKA low-frequency aperture array could detect CR events above 56 EeV at a rate between 15 and 40 times that of the current Pierre Auger Observatory. Should further work indicate that formation-zone effects have little impact on UHE CR sensitivity, observations of the Moon with the SKA would allow directional analysis of UHE cosmic rays, and investigation of correlations with putative cosmic ray source populations, to be conducted with very high statistics.     

The effect of hypothetical diproton stability on the universe

By calculation of the proton-proton capture cross-section, it is shown that the existence of a bound diproton state would not lead to significant production of diprotons during big bang nucleosynthesis, contrary to popular belief. In typical stellar interiors, the stability of diprotons would lead to a reaction pathway for converting protons to deuterons perhaps ∼10¹⁰ times faster than the usual weak capture reaction. This would prevent stars of the familiar hot, dense type from occurring in the universe. However, if diproton stability is achieved by an increase in the low-energy strong coupling, gs, then stars with temperatures and densities sufficiently reduced so as to offset the faster reaction pathway to deuterium will appear to meet elementary stability criteria. The claim that there is a fine-tuned, anthropic upper bound to the strong force which ensures diproton instability therefore appears to be unfounded.     

Trace constituents of Europa's atmosphere

Europa is bombarded by intense radiation that erodes the surface, launching molecules into a thin “atmosphere” representative of surface composition. In addition to atoms and molecules created in the mostly water ice surface such as H₂O, O₂, H₂, the atmosphere is known to have species representative of trace surface materials. These trace species are carried off with the 10-10⁴ H₂O molecules ejected by each energetic heavy ion, a process we have simulated using molecular dynamics. Using the results of those simulations, we found that a neutral mass spectrometer orbiting ∼100 km above the surface could detect species with surface concentrations above ∼0.03%. We have also modeled the atmospheric spatial structure of the volatile species CO₂ and SO₂ under a variety of assumptions. Detections of these species with moderate time and space resolution would allow us to constrain surface composition, chemistry and to study space weathering processes.     

Origin of ice diapirism, true polar wander, subsurface ocean, and tiger stripes of Enceladus driven by compositional convection

We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an “ammonia feedback” mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer above. We then consider a large scale instability arising from the pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Enceladus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region.     

Comparison of parallel group velocity of ECH waves with electron resonant velocity: Implication for electron diffusion by ECH waves

We have investigated the role of group velocity in the calculation of pitch-angle diffusion coefficients by electron cyclotron harmonic (ECH) waves in planetary magnetospheres. The assumption which is generally made that the parallel group velocity can be neglected in comparison with particle parallel velocity is examined in detail. It is found that for lowest harmonic band this assumption is quite good. It is found that in general it is not possible to ignore the parallel group velocity. However, for lowest harmonic band this assumption is quite good at low electron temperatures.     

How do binaries affect the derived dynamical mass of a star cluster?

The dynamical mass of a star cluster can be derived from the virial theorem, using the measured half-mass radius and line-of-sight velocity dispersion of the cluster. However, this dynamical mass may be a significant overestimation of the cluster mass if the contribution of the binary orbital motion is not taken into account. Here, we describe the mass overestimation as a function of cluster properties and binary population properties, and briefly touch on the issue of selection effects. We find that for clusters with a measured velocity dispersion of σ _los?10 km?s^?1 the presence of binaries does not affect the dynamical mass significantly. For clusters with σ _los?1 km?s^?1 (i.e., low-density clusters), the contribution of binaries to σ _los is significant, and may result in a major dynamical mass overestimation. The presence of binaries may introduce a downward shift of Δlog?(L _V /M _dyn)=0.05–0.4 (in solar units) in the log?(L _V /M _dyn) versus age diagram.     

Be star candidates in the Large Magellanic Cloud: the catalogue and comparison with the Small Magellanic Cloud sample

We present a catalogue with coordinates and photometric data of 2446 Be star candidates in the Large Magellanic Cloud (LMC), based on a search of the OGLE II data base. The I -band light curves of these stars show outbursts in 24 per cent of the sample (Type-1 stars), high and low states in 10 per cent, periodic variations in 6 per cent (Type-3 stars), and stochastic variations in 60 per cent of the cases. We report on the result of the statistical study of light curves of Type-1 and Type-3 stars in the LMC, and the comparison with the previously reported results of the Small Magellanic Cloud (SMC) sample. We find a statistically significant difference between amplitude, duration and asymmetry distributions of outbursts in both galaxies. Outbursts of SMC Type-1 stars are usually brighter, longer and with a slower decline. We find a bimodal distribution of periods of Type-3 stars in both galaxies, probably related to the recently discovered double periodic blue variables. We find also period and amplitude distributions of Type-3 LMC stars statistically different from those of the SMC stars. Our findings above suggest that the mechanisms causing the observed photometric variability of Type-1 and Type-3 stars could depend on metallicity. Moreover, they suggest that the outbursts are not primarily caused by stellar winds.