12,419 research outputs found
Comment on "Pulsar Velocities and Neutrino Oscillations"
In a recent Letter, Kusenko and Segre proposed a new mechanism to explain the
observed proper motions of pulsars. Their mechanism was based on the asymmetric
neutrino emission induced by neutrino oscillations in the protoneutron star
magnetic field. In this note I point out that their estimate of the asymmetry
in the neutrino emission is incorrect. A proper calculation shows that their
mechanism at least requires a magnetic field of 10**16 G in order to produce
the observed average pulsar velocity.Comment: 4 pages, RevTe
Modelling relationships between habitat and dynamics of a wild brown trout (Salmo trutta L.) population in the River Piddle, Dorset, UK
The status of "wild" brown trout (Salmo trutta, L. 1758) populations in the UK is increasingly giving cause for concern (Giles, 1989; Crisp; 1993). Declines in freshwater stocks are often associated with anthropogenic influences destructive to river channel structure and ecosystem function which are contributing to widespread loss of salmonid habitats (Crisp, 1989; White, 2002). Chalk streams are subject to considerable habitat degradation such that rehabilitation requires management actions which better integrate habitat and ecological processes operating to influence fish populations. The influence of local meso-scale habitats upon brown trout population dynamics in two contrasting sectors of the River Piddle, Dorset, UK was quantified using the Physical Habitat Simulation Model (PHABSIM). Sectors examined represented 'typical' semi-natural chalk-stream conditions in the Piddle/Frome catchment area. Spatial availability and temporal variations in habitat quality (WUA) were modelled and tested for correlation against age-specific trout densities obtained from eight years quantitative electro-fishing data. Analyses indicated; (1) availability of marginal habitats associated with abundant bank-side cover was critical to adult over-winter survival and was a key factor determining local carrying capacity
, (2) abundance of juvenile trout was strongly related to critical periods for spawning and rearing habitats, (3) low habitat durations during the first summer acted as a bottleneck at the juvenile life stage and (4) a variety of different meso-habitat types was important to juvenile recruitment dynamics. The implications of these findings are explored in the context of management of chalk rivers with respect to;- (i) habitat factors limiting trout recruitment dynamics (ii) utility of PHABSIM as a management tool for identifying population bottlenecks and (iii) potential for river rehabilitation strategies to effectively manipulate natural mechanisms regulating brown trout populations in chalk streams
The Mechanism of Core-Collapse Supernova Explosions: A Status Report
We review the status of the current quest to understand the mechanism of
core-collapse supernovae, if neutrino-driven. In the process, we discuss the
spherical explosion paradigm and its problems, some results from our new suite
of collapse calculations performed using a recently-developed 1D implicit,
multi-group, Feautrier/tangent-ray, Boltzmann solver coupled to explicit
predictor/corrector hydrodynamics, the basic energetics of supernova
explosions, and the promise of multi-D radiation/hydro simulations to explain
why the cores of massive stars explode.Comment: 10 pages, LaTeX, 4 JPEGs included. To be published in the proceedings
to the ESO/MPA/MPE Workshop (an ESO Astrophysics Symposium) entitled "From
Twilight to Highlight: The Physics of Supernovae," held in Garching bei
M\"unchen, Germany, July 29-31, 2002, eds. Bruno Leibundgut and Wolfgang
Hillebrandt (Springer-Verlag
Probing the galactic halo with ROSAT
We discuss the current status of ROSAT shadowing observations designed to search for emission from million degree gas in the halo of the Milky Way galaxy. Preliminary results indicate that million degree halo gas is observed in the 1/4 keV band in some directions, most notably toward the Draco cloud at (l,b) = (92 deg, +38 deg), but that the halo emission is patchy and highly anisotropic. Our current understanding of this halo emission is based on a small handful of observations which have been analyzed to date. Many more observations are currently being analyzed or are scheduled for observation within the next year, and we expect our understanding of this component of the galactic halo to improve dramatically in the near future
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales
We investigate the rates of production and thermalization of and
neutrinos at temperatures and densities relevant to core-collapse
supernovae and protoneutron stars. Included are contributions from electron
scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and
nucleon scattering. For the scattering processes, in order to incorporate the
full scattering kinematics at arbitrary degeneracy, the structure function
formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is
employed. Furthermore, we derive formulae for the total and differential rates
of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in
asymmetric matter. We find that electron scattering dominates nucleon
scattering as a thermalization process at low neutrino energies
( MeV), but that nucleon scattering is always faster
than or comparable to electron scattering above MeV. In
addition, for g cm, MeV, and
neutrino energies MeV, nucleon-nucleon bremsstrahlung always
dominates electron-positron annihilation as a production mechanism for
and neutrinos.Comment: 29 pages, LaTeX (RevTeX), 13 figures, submitted to Phys. Rev. C. Also
to be found at anonymous ftp site http://www.astrophysics.arizona.edu; cd to
pub/thompso
How can a multimodal approach to primate communication help us understand the evolution of communication?
Scientists studying the communication of non-human animals are often aiming to better understand the evolution of human communication, including human language. Some scientists take a phylogenetic perspective, where the goal is to trace the evolutionary history of communicative traits, while others take a functional perspective, where the goal is to understand the selection pressures underpinning specific traits. Both perspectives are necessary to fully understand the evolution of communication, but it is important to understand how the two perspectives differ and what they can and cannot tell us. Here, we suggest that integrating phylogenetic and functional questions can be fruitful in better understanding the evolution of communication. We also suggest that adopting a multimodal approach to communication might help to integrate phylogenetic and functional questions, and provide an interesting avenue for research into language evolution
Experimental study of acoustic displays of flight parameters in a simulated aerospace vehicle
Evaluating acoustic displays of target location in target detection and of flight parameters in simulated aerospace vehicle
The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
There is no universally acknowledged criterion to distinguish brown dwarfs
from planets. Numerous studies have used or suggested a definition based on an
object's mass, taking the ~13-Jupiter mass (M_J) limit for the ignition of
deuterium. Here, we investigate various deuterium-burning masses for a range of
models. We find that, while 13 M_J is generally a reasonable rule of thumb, the
deuterium fusion mass depends on the helium abundance, the initial deuterium
abundance, the metallicity of the model, and on what fraction of an object's
initial deuterium abundance must combust in order for the object to qualify as
having burned deuterium. Even though, for most proto-brown dwarf conditions,
50% of the initial deuterium will burn if the object's mass is ~(13.0 +/-
0.8)M_J, the full range of possibilities is significantly broader. For models
ranging from zero-metallicity to more than three times solar metallicity, the
deuterium burning mass ranges from ~11.0 M_J (for 3-times solar metallicity,
10% of initial deuterium burned) to ~16.3 M_J (for zero metallicity, 90% of
initial deuterium burned).Comment: "Models" section expanded, references added, accepted by Ap
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