446 research outputs found
Recommended from our members
The Stac Fada “impact ejecta” layer: not what it seems
The Stac Fada Member (SFM) forms part of the Stoer Group of the Torridonian of NW Scotland. The SFM is unique in the Torridonian, being characterized by the presence of greenish altered glass clasts. Its origin has been debated for decades with several hypotheses being proposed but all invoking some connection with volcanic activity in the region. More recently, Amor et al. suggested that the SFM represents “a chord section through the continuous ejecta blanket surrounding an impact crater”. Here, we confirm the presence of shocked material within the SFM and then discuss its origin
Cosmic ray moderation of the thermal instability
We apply the Hermite-Bieler theorem in the analysis of the effect of cosmic rays on the thermal stability of an initially uniform, static background. The cosmic rays were treated in a fluid approximation and the diffusion coefficient was assumed to be constant in time and space. The inclusion of cosmic rays does not alter the criterion for the thermal stability of a medium subjected to isobaric perturbations. It does alter the criteria for the stability of a medium perturbed by small amplitude sound waves. In the limit of a high background cosmic ray pressure to thermal pressure ratio, the instability in response to high frequency sound waves is suppressed
Random mobility and spatial structure often enhance cooperation
The effects of an unconditional move rule in the spatial Prisoner's Dilemma,
Snowdrift and Stag Hunt games are studied. Spatial structure by itself is known
to modify the outcome of many games when compared with a randomly mixed
population, sometimes promoting, sometimes inhibiting cooperation. Here we show
that random dilution and mobility may suppress the inhibiting factors of the
spatial structure in the Snowdrift game, while enhancing the already larger
cooperation found in the Prisoner's dilemma and Stag Hunt games.Comment: Submitted to J. Theor. Bio
Assessing the risks of changing ongoing management of endangered species
Recovery programmes for endangered species can become increasingly demanding over time, but managers may be reluctant to change ongoing actions that are believed to be assisting recovery. We used a quantitative risk assessment to choose support strategies for a reintroduced population of Mauritius olive white‐eyes Zosterops chloronothos. Facing increasing costs, managers considered changing the ongoing supplementary feeding strategy, but at the same time worried this could jeopardize the observed positive population trend. We used a feeding experiment to compare the current feeding regime and a cheaper alternative (a simple sugar/water mix). Results suggested the cheaper alternative would only marginally reduce population vital rates. We assessed the influence of these results and the associated uncertainty on population recovery and management costs using two decision‐analytic criteria, incremental cost‐effectiveness ratio and stochastic dominance. The new feeding regime was expected to be, on average, more cost‐effective than the status quo. Moreover, even negative outcomes would only likely mean a slower growing population, not a declining one, whereas not changing feeding regime actually entailed greater risk. Because shifting from the current regime to a cheaper sugar/water mixture was both a risk‐averse and a cost‐effective choice, we decided to implement this change. Four years after the experiment, the population continues to grow and costs have been contained, matching predictions almost exactly. In this case, the field experiment provided useful empirical information about prospective actions; the risk analysis then helped us understand the real implications of changing the feeding regime. We encourage managers of recovery plans facing a similar situation to explicitly recognize trade‐offs and risk aversion, and address them by combining targeted research and formal decision analysis
Inflation-Produced Magnetic Fields in Nonlinear Electrodynamics
We study the generation of primeval magnetic fields during inflation era in
nonlinear theories of electrodynamics. Although the intensity of the produced
fields strongly depends on characteristics of inflation and on the form of
electromagnetic Lagrangian, our results do not exclude the possibility that
these fields could be astrophysically interesting.Comment: 6 page
Two-dimensional AMR simulations of colliding flows
Colliding flows are a commonly used scenario for the formation of molecular
clouds in numerical simulations. Due to the thermal instability of the warm
neutral medium, turbulence is produced by cooling. We carry out a
two-dimensional numerical study of such colliding flows in order to test
whether statistical properties inferred from adaptive mesh refinement (AMR)
simulations are robust with respect to the applied refinement criteria. We
compare probability density functions of various quantities as well as the
clump statistics and fractal dimension of the density fields in AMR simulations
to a static-grid simulation. The static grid with 2048^2 cells matches the
resolution of the most refined subgrids in the AMR simulations. The density
statistics is reproduced fairly well by AMR. Refinement criteria based on the
cooling time or the turbulence intensity appear to be superior to the standard
technique of refinement by overdensity. Nevertheless, substantial differences
in the flow structure become apparent. In general, it is difficult to separate
numerical effects from genuine physical processes in AMR simulations.Comment: 6 pages, 6 figures, submitted to A&
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
The mesoproterozoic Stac Fada Member, NW Scotland : an impact origin confirmed but refined
Funding to GRO from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program and the Canadian Space Agency (CSA) Canadian Analogue Research Network and Field Investigation programs is gratefully acknowledged. Part of LF’s work was supported by the Department of Foreign Affairs and International Trade (DFAIT), Government of Canada.The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.PostprintPeer reviewe
A Survey of Extragalactic Faraday Rotation at High Galactic Latitude: The Vertical Magnetic Field of the Milky Way towards the Galactic Poles
We present a study of the vertical magnetic field of the Milky Way towards
the Galactic poles, determined from observations of Faraday rotation toward
more than 1000 polarized extragalactic radio sources at Galactic latitudes |b|
> 77 degs, using the Westerbork Radio Synthesis Telescope and the Australia
Telescope Compact Array. We find median rotation measures (RMs) of 0.0 +/- 0.5
rad/m^2 and +6.3 +/- 0.7 rad/m^2 toward the north and south Galactic poles,
respectively, demonstrating that there is no coherent vertical magnetic field
in the Milky Way at the Sun's position. If this is a global property of the
Milky Way's magnetism, then the lack of symmetry across the disk rules out pure
dipole or quadrupole geometries for the Galactic magnetic field. The angular
fluctuations in RM seen in our data show no preferred scale within the range ~
0.1 to 25 degs. The observed standard deviation in RM of ~ 9 rad/m^2 then
implies an upper limit of ~1microGauss on the strength of the random magnetic
field in the warm ionized medium at high Galactic latitudes.Comment: 38 pages, 7 figures, 3 tables Accepted for publication in ApJ,
Electronic versions of Tables 1 and 2 are available via email from the first
autho
- …