Occurrence and extent of hybridisation between the invasive Mallard Duck and native Yellow-billed Duck in South Africa

Hybridisation between invasive and native species represents a significant threat to biodiversity. The Mallard Duck (Anas platyrhynchos) is known to hybridise with numerous closely related Anas species in regions where they have been introduced, threatening the genetic integrity of native ducks and in some instances contributing to their extinction risk. Mallard Ducks were introduced into South Africa in the 1940s and are now naturalised and widespread in the country. It has been speculated that Mallard Ducks are hybridising with native Yellow-billed Ducks (A. undulata) in South Africa, but evidence for this remains observational or purely anecdotal. Here we use data from nuclear microsatellite markers and mitochondrial DNA sequencing to show that hybridisation is indeed occuring between these two species. We found evidence for the occurance of hybridisation, mostly as crosses between Mallard Duck hens and Yellow-billed Duck drakes. Surprisingly, our results suggest that introgressive hybridisation is primarily occuring into the invasive Mallard Duck population (mostly Mallard Duck backcrosses were detected), evidenced by directionally-skewed gene flow and sex-biased mating. Whether these findings reflect true assortative mating or a case of Haldane’s rule remains unknown. We also found evidence of high connectivity between Yellow-billed Duck populations, as far as 1000 km apart, in South Africa. Taken together these results suggest that hybrid genotypes can disperse over vast distances between populations and lead to genetic pollution, even in the absence of invasive Mallard Ducks. Active management of Mallard Duck populations has been met by public resistance in some areas in South Africa, partly because of a lack of evidence showing clear impacts by these birds. This study provides some of the first scientifically-documented evidence for such impacts

Multidimensional relativistic MHD simulations of Pulsar Wind Nebulae: dynamics and emission

Pulsar Wind Nebulae, and the Crab nebula in particular, are the best cosmic laboratories to investigate the dynamics of magnetized relativistic outflows and particle acceleration up to PeV energies. Multidimensional MHD modeling by means of numerical simulations has been very successful at reproducing, to the very finest details, the innermost structure of these synchrotron emitting nebulae, as observed in the X-rays. Therefore, the comparison between the simulated source and observations can be used as a powerful diagnostic tool to probe the physical conditions in pulsar winds, like their composition, magnetization, and degree of anisotropy. However, in spite of the wealth of observations and of the accuracy of current MHD models, the precise mechanisms for magnetic field dissipation and for the acceleration of the non-thermal emitting particles are mysteries still puzzling theorists to date. Here we review the methodologies of the computational approach to the modeling of Pulsar Wind Nebulae, discussing the most relevant results and the recent progresses achieved in this fascinating field of high-energy astrophysics.Comment: 29 pages review, preliminary version. To appear in the book "Modelling Nebulae" edited by D. Torres for Springer, based on the invited contributions to the workshop held in Sant Cugat (Barcelona), June 14-17, 201

An Anglo-Saxon execution cemetery at Walkington Wold, Yorkshire

This paper presents a re-evaluation of a cemetery excavated over 30 years ago at Walkington Wold in east Yorkshire. The cemetery is characterized by careless burial on diverse alignments, and by the fact that most of the skeletons did not have associated crania. The cemetery has been variously described as being the result of an early post-Roman massacre, as providing evidence for a ‘Celtic’ head cult or as an Anglo-Saxon execution cemetery. In order to resolve the matter, radiocarbon dates were acquired and a re-examination of the skeletal remains was undertaken. It was confirmed that the cemetery was an Anglo-Saxon execution cemetery, the only known example from northern England, and the site is set into its wider context in the paper

Non-stationary Rayleigh-Taylor instability in supernovae ejecta

The Rayleigh-Taylor instability plays an important role in the dynamics of several astronomical objects, in particular, in supernovae (SN) evolution. In this paper we develop an analytical approach to study the stability analysis of spherical expansion of the SN ejecta by using a special transformation in the co-moving coordinate frame. We first study a non-stationary spherical expansion of a gas shell under the pressure of a central source. Then we analyze its stability with respect to a no radial, non spherically symmetric perturbation of the of the shell. We consider the case where the polytropic constant of the SN shell is $\gamma=5/3$ and we examine the evolution of a arbitrary shell perturbation. The dispersion relation is derived. The growth rate of the perturbation is found and its temporal and spatial evolution is discussed. The stability domain depends on the ejecta shell thickness, its acceleration, and the perturbation wavelength.Comment: 16 page

Magnetic fields in supernova remnants and pulsar-wind nebulae

We review the observations of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe) that give information on the strength and orientation of magnetic fields. Radio polarimetry gives the degree of order of magnetic fields, and the orientation of the ordered component. Many young shell supernova remnants show evidence for synchrotron X-ray emission. The spatial analysis of this emission suggests that magnetic fields are amplified by one to two orders of magnitude in strong shocks. Detection of several remnants in TeV gamma rays implies a lower limit on the magnetic-field strength (or a measurement, if the emission process is inverse-Compton upscattering of cosmic microwave background photons). Upper limits to GeV emission similarly provide lower limits on magnetic-field strengths. In the historical shell remnants, lower limits on B range from 25 to 1000 microGauss. Two remnants show variability of synchrotron X-ray emission with a timescale of years. If this timescale is the electron-acceleration or radiative loss timescale, magnetic fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition arguments and dynamical modeling can be used to infer magnetic-field strengths anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field geometries often suggest a toroidal structure around the pulsar, but this is not universal. Viewing-angle effects undoubtedly play a role. MHD models of radio emission in shell SNRs show that different orientations of upstream magnetic field, and different assumptions about electron acceleration, predict different radio morphology. In the remnant of SN 1006, such comparisons imply a magnetic-field orientation connecting the bright limbs, with a non-negligible gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording change in Abstrac

Thermal compression of atomic hydrogen on helium surface

We describe experiments with spin-polarized atomic hydrogen gas adsorbed on liquid $^{4}$He surface. The surface gas density is increased locally by thermal compression up to $5.5\times10^{12}$ cm$^{-2}$ at 110 mK. This corresponds to the onset of quantum degeneracy with the thermal de-Broglie wavelength being 1.5 times larger than the mean interatomic spacing. The atoms were detected directly with a 129 GHz electron-spin resonance spectrometer probing both the surface and the bulk gas. This, and the simultaneous measurement of the recombination power, allowed us to make accurate studies of the adsorption isotherm and the heat removal from the adsorbed hydrogen gas. From the data, we estimate the thermal contact between 2D hydrogen gas and phonons of the helium film. We analyze the limitations of the thermal compression method and the possibility to reach the superfluid transition in 2D hydrogen gas.Comment: 20 pages, 11 figure

Magnetic fields in cosmic particle acceleration sources

We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. We discuss a number of CR-driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review

On the stability and uniqueness of the flow of a fluid through a porous medium

© 2016, The Author(s). In this short note, we study the stability of flows of a fluid through porous media that satisfies a generalization of Brinkman’s equation to include inertial effects. Such flows could have relevance to enhanced oil recovery and also to the flow of dense liquids through porous media. In any event, one cannot ignore the fact that flows through porous media are inherently unsteady, and thus, at least a part of the inertial term needs to be retained in many situations. We study the stability of the rest state and find it to be asymptotically stable. Next, we study the stability of a base flow and find that the flow is asymptotically stable, provided the base flow is sufficiently slow. Finally, we establish results concerning the uniqueness of the flow under appropriate conditions, and present some corresponding numerical results

An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics

For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types