Morphology of lunar craters: A test of lunar erosional models

This paper is a test of published theoretical and experimental studies of crater erosion by micrometeorite bombardment which predict systematic variations in the morphology of lunar craters as a function of crater diameter and crater age. Numerical, ranking-type degradation classifications indicate that the craters on Mare Imbrium and Mare Tranquillitatus confirm these predictions by showing a systematic increase in degradation with decreasing diameter for craters smaller than a few kilometers in diameter but larger than the equilibrium diameter, and by showing fixed proportions of fresh, moderately degraded and very degraded craters under equilibrium conditions. Furthermore, the relative ages of the two mare surfaces may be determined using a diameter/mean-degradation-number curve. These determinations of relative age and process of crater erosion are both essentially independent of the traditionally studied crater diameter/frequency relationships. Morphologies of terra craters near Mare Humorum suggest a young, non-equilibrium crater population superposed on a perimordial population with about equilibrium proportions of fresh, moderately degraded and very degraded craters. The primordial population has been modified by pre-Imbrian or early Imbrian deposition of blanketing deposits. A comparative study of several crater degradation classifications indicates that all are essentially interchangeable.     

Strong shock-phenomena at petawatt-picosecond laser side-on ignition fusion of uncompressed hydrogen-boron11

An extreme anomaly of laser-plasma interaction with petawatt-picosecond (PW-ps) pulses of very high contrast ratio for suppression of relativistic self-focusing permitted a come-back of the Bobin-Chu side-on ignition of uncompressed deuterium-tritium (DT) fusion fuel. The plasma blocks for the side-on ignition have to be produced by the well confirmed nonlinear force acceleration which is about 100,000 times higher than thermo-kinetic fluid-dynamic acceleration for comparison with astrophysical cases. It is essential that the dielectric plasma properties within the nonlinear force are used. Using the measured ion beam densities above 10¹¹ A s/cm² the ignition mechanism needed numerical and theoretical studies of extremely strong shock phenomena. When extending these results to the side-on ignition of uncompressed hydrogen-boron11 (HB11), surprisingly, the ignition by this shock mechanism was only about 10 times more difficult than for DT in contrast to ignition by spherical laser driven compression using thermo-kinetic conditions in which case HB11 ignition is 100,000 times more difficult than DT.     

Background simulations for the Large Area Detector onboard LOFT

The Large Observatory For X-ray Timing (LOFT), currently in an assessment phase in the framework the ESA M3 Cosmic Vision programme, is an innovative medium-class mission specifically designed to answer fundamental questions about the behaviour of matter, in the very strong gravitational and magnetic fields around compact objects and in supranuclear density conditions. Having an effective area of ～10 m² at 8 keV, LOFT will be able to measure with high sensitivity very fast variability in the X-ray fluxes and spectra. A good knowledge of the in-orbit background environment is essential to assess the scientific performance of the mission and optimize the design of its main instrument, the Large Area Detector (LAD). In this paper the results of an extensive Geant-4 simulation of the instrumentwillbe discussed, showing the main contributions to the background and the design solutions for its reduction and control. Our results show that the current LOFT/LAD design is expected to meet its scientific requirement of a background rate equivalent to 10 mCrab in 2?30 keV, achieving about 5 mCrab in the most important 2–10 keV energy band. Moreover, simulations show an anticipated modulation of the background rate as small as 10 % over the orbital timescale. The intrinsic photonic origin of the largest background component also allows for an efficient modelling, supported by an in-flight active monitoring, allowing to predict systematic residuals significantly better than the requirement of 1 %, and actually meeting the 0.25 % science goal.     

Planar periodic orbits in exterior resonances with Neptune

In the framework of the planar restricted three-body problem we study a considerable number of resonances associated to the basic dynamical features of Kuiper belt and located between 30 and 48 a.u. Our study is based on the computation of resonant periodic orbits and their stability. Stable periodic orbits are surrounded by regular librations in phase space and in such domains the capture of trans-Neptunian object is possible. All the periodic orbits found are symmetric and there is an indication of the existence of asymmetric ones only in a few cases. In the present work first, second and third order resonances are under consideration. In the planar circular case we found that most of the periodic orbits are stable. The families of periodic orbits are temporarily interrupted by collisions but they continue up to relatively large values of the Jacobi constant and highly eccentric regular motion exists for all cases. In the elliptic problem and for a particular eccentricity value of the primary bodies, the periodic orbits are isolated. The corresponding families, where they belong to, bifurcate from specific periodic orbits of the circular problem and seem to continue up to the rectilinear problem. Both stable and unstable orbits are obtained for each case. In the elliptic problem, the unstable orbits found are associated with narrow chaotic domains in phase space. The evolution of the orbits, which are located in such chaotic domains, seems to be practically regular and bounded for long time intervals.     

Simulations of multimode bolometric interferometers

We describe a procedure for the numerical modelling of astronomical interferometers, with particular relevance to far-infrared and submillimetre wavelengths. The scheme is based on identifying a set of modes that carry power from the sky to the detector. The procedure is extremely general, and can be used to model scalar or vector fields, in any state of coherence and polarization, the only limitation being that the propagation of a coherent field through the system be described by an integral transform, a constraint that is in practise always met.
We present simulations of ideal, multimode two-dimensional interferometers, and show that the modal theory reproduces the correct behaviour of both Michelson and Fizeau interferometers. We calculate simulated visibility data for a multimode bolometric Michelson interferometer, with a synthesized source, and produce a dirty map, recovering the original source with the usual artefacts associated with interferometers.     

Generalized likelihood uncertainty estimation (GLUE) using adaptive Markov Chain Monte Carlo sampling

In the last few decades hydrologists have made tremendous progress in using dynamic simulation models for the analysis and understanding of hydrologic systems. However, predictions with these models are often deterministic and as such they focus on the most probable forecast, without an explicit estimate of the associated uncertainty. This uncertainty arises from incomplete process representation, uncertainty in initial conditions, input, output and parameter error. The generalized likelihood uncertainty estimation (GLUE) framework was one of the first attempts to represent prediction uncertainty within the context of Monte Carlo (MC) analysis coupled with Bayesian estimation and propagation of uncertainty. Because of its flexibility, ease of implementation and its suitability for parallel implementation on distributed computer systems, the GLUE method has been used in a wide variety of applications. However, the MC based sampling strategy of the prior parameter space typically utilized in GLUE is not particularly efficient in finding behavioral simulations. This becomes especially problematic for high-dimensional parameter estimation problems, and in the case of complex simulation models that require significant computational time to run and produce the desired output. In this paper we improve the computational efficiency of GLUE by sampling the prior parameter space using an adaptive Markov Chain Monte Carlo scheme (the Shuffled Complex Evolution Metropolis (SCEM-UA) algorithm). Moreover, we propose an alternative strategy to determine the value of the cutoff threshold based on the appropriate coverage of the resulting uncertainty bounds. We demonstrate the superiority of this revised GLUE method with three different conceptual watershed models of increasing complexity, using both synthetic and real-world streamflow data from two catchments with different hydrologic regimes.     

Evolution of the mass function of dark matter haloes

We use a high-resolution ΛCDM numerical simulation to calculate the mass function of dark matter haloes down to the scale of dwarf galaxies, back to a redshift of 15, in a  50 h ⁻¹ Mpc  volume containing 80 million particles. Our low-redshift results allow us to probe low-σ density fluctuations significantly beyond the range of previous cosmological simulations. The Sheth & Tormen mass function provides an excellent match to all of our data except for redshifts of 10 and higher, where it overpredicts halo numbers increasingly with redshift, reaching roughly 50 per cent for the  10¹⁰–10¹¹ M_⊙  haloes sampled at redshift 15. Our results confirm previous findings that the simulated halo mass function can be described solely by the variance of the mass distribution, and thus has no explicit redshift dependence. We provide an empirical fit to our data that corrects for the overprediction of extremely rare objects by the Sheth & Tormen mass function. This overprediction has implications for studies that use the number densities of similarly rare objects as cosmological probes. For example, the number density of high-redshift  ( z ≃ 6) QSOs  , which are thought to be hosted by haloes at 5σ peaks in the fluctuation field, are likely to be overpredicted by at least a factor of 50 per cent. We test the sensitivity of our results to force accuracy, starting redshift and halo-finding algorithm.     

Radiative transfer

Chandrasekhar’s work in radiative transfer theory began in 1944 and culminated with the publication of his influential treatiseRadiative Transfer in 1950. In this review his major contributions to radiative transfer will be recounted and evaluated. These include his development of the discrete ordinates method, the invariance principles, and his formulation and solution of the transfer equation for polarized light.