1,330 research outputs found
Neurophysiological findings relevant to echolocation in marine animals
A review of echolocation mechanisms in marine mammals, chiefly porpoises, is given. Data cover peripheral auditory and central neurophysiological specializations favorable to the analysis of echolocating clicks and their echoes. Conclusions show (1) signals are received from 50 up to at least 135 kHz, (2) sound is received through the mandible skin, and (3) the midbrain sites are insensitive to low frequencies (below 6 kHz)
Are All Static Black Hole Solutions Spherically Symmetric?
The static black hole solutions to the Einstein-Maxwell equations are all
spherically symmetric, as are many of the recently discovered black hole
solutions in theories of gravity coupled to other forms of matter. However,
counterexamples demonstrating that static black holes need not be spherically
symmetric exist in theories, such as the standard electroweak model, with
electrically charged massive vector fields. In such theories, a magnetically
charged Reissner-Nordstrom solution with sufficiently small horizon radius is
unstable against the development of a nonzero vector field outside the horizon.
General arguments show that, for generic values of the magnetic charge, this
field cannot be spherically symmetric. Explicit construction of the solution
shows that it in fact has no rotational symmetry at all.Comment: 6 pages, plain TeX. Submitted to GRF Essay Competitio
Description of a Poorly Differentiated Carcinoma Within the Brainstem of a White Whale (Delphinapterus leucas) From Magnetic Resonance Images and Histological Analysis
In this study we used magnetic resonance imaging (MRI) to investigate neuroanatomical structure in the brain of a white whale (Delphinapterus leucas) that died from a large tumor within the brainstem. This specimen was also compared with a normal white whale brain using MRI. MRI scans of the white whale specimen show how the tumor deformed surrounding brain structure. Histopathological analysis indicated a poorly differentiated carcinoma of uncertain origin. These analyses demonstrate the usefulness of supplementing histological analyses of pathology with studies of gross morphology facilitated by MRI
Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure
We present an experimental lattice location study of Ga atoms in Ge after ion implantation at elevated temperature (250°C). Using extended x-rayabsorption fine structure (EXAFS) experiments and a dedicated sample preparation method, we have studied the lattice location of Ga atoms in Ge with a concentration ranging from 0.5 at. % down to 0.005 at. %. At Ga concentrations ≤0.05 at.%, all Ga dopants are substitutional directly after ion implantation, without the need for post-implantation thermal annealing. At higher Ga concentrations, a reduction in the EXAFS amplitude is observed, indicating that a fraction of the Ga atoms is located in a defective environment. The local strain induced by the Ga atoms in the Ge matrix is independent of the Ga concentration and extends only to the first nearest neighbor Ge shell, where a 1% contraction in bond length has been measured, in agreement with density functional theory calculations.We acknowledge the support from the Research Foundation
Flanders, the epi-team from imec, the KU Leuven
GOA 09/06 project, the IUAP program P6/42 and the Australian
Research Council. S.C. acknowledges support from
OCAS NV by an OCAS-endowed chair at Ghent University
Static Black Hole Solutions without Rotational Symmetry
We construct static black hole solutions that have no rotational symmetry.
These arise in theories, including the standard electroweak model, that include
charged vector mesons with mass . In such theories, a magnetically
charged Reissner-Nordstrom black hole with horizon radius less than a critical
value of the order of is classically unstable against the development
of a nonzero vector meson field just outside the horizon, indicating the
existence of static black hole solutions with vector meson hair. For the case
of unit magnetic charge, spherically symmetric solutions of this type have
previously been studied. For other values of the magnetic charge, general
arguments show that any new solution with hair cannot be spherically symmetric.
In this paper we develop and apply a perturbative scheme (which may have
applicability in other contexts) for constructing such solutions in the case
where the Reissner-Nordstrom solution is just barely unstable. For a few low
values of the magnetic charge the black holes retain a rotational symmetry
about a single axis, but this axial symmetry disappears for higher charges.
While the vector meson fields vanish exponentially fast at distances greater
than , the magnetic field and the metric have higher multipole
components that decrease only as powers of the distance from the black hole.Comment: 42 pages, phyzzx. 4 figures (PostScript, 1.7 MB when uncompressed)
available by email from the Authors on reques
The Ages of A-Stars I: Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group
We have observed and spatially resolved a set of seven A-type stars in the
nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners
on the CHARA Array. At least four of these stars have large rotational
velocities ( 170 ) and are expected to
be oblate. These interferometric measurements, the stars' observed photometric
energy distributions, and values are used to computationally
construct model oblate stars from which stellar properties (inclination,
rotational velocity, and the radius and effective temperature as a function of
latitude, etc.) are determined. The results are compared with MESA stellar
evolution models (Paxton et al. 2011, 2013) to determine masses and ages. The
value of this new technique is that it enables the estimation of the
fundamental properties of rapidly rotating stars without the need to fully
image the star. It can thus be applied to stars with sizes comparable to the
interferometric resolution limit as opposed to those that are several times
larger than the limit. Under the assumption of coevality, the spread in ages
can be used as a test of both the prescription presented here and the MESA
evolutionary code for rapidly rotating stars. With our validated technique, we
combine these age estimates and determine the age of the moving group to be 414
23 Myr, which is consistent with, but much more precise than previous
estimates.Comment: Accepted by Ap
CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging
Two red supergiants of the Per OB1 association, RS Per and T Per, have been
observed in H band using the MIRC instrument at the CHARA array. The data show
clear evidence of departure from circular symmetry. We present here new
techniques specially developed to analyze such cases, based on state-of-the-art
statistical frameworks. The stellar surfaces are first modeled as limb-darkened
discs based on SATLAS models that fit both MIRC interferometric data and
publicly available spectrophotometric data. Bayesian model selection is then
used to determine the most probable number of spots. The effective surface
temperatures are also determined and give further support to the recently
derived hotter temperature scales of red su- pergiants. The stellar surfaces
are reconstructed by our model-independent imaging code SQUEEZE, making use of
its novel regularizer based on Compressed Sensing theory. We find excellent
agreement between the model-selection results and the reconstructions. Our
results provide evidence for the presence of near-infrared spots representing
about 3-5% of the stellar flux
High-Resolution Infrared Spectroscopy of the Brown Dwarf Epsilon Indi Ba
We report on the analysis of high-resolution infrared spectra of the newly
discovered brown dwarf Epsilon Indi Ba. This is the closest known brown dwarf
to the solar system, with a distance of 3.626 pc. Spectra covering the ranges
of 2.308-2.317 microns and 1.553-1.559 microns were observed at a spectral
resolution of R=50,000 with the Phoenix spectrometer on the Gemini South
telescope. The physical paramters of effective temperature and surface gravity
are derived by comparison to model spectra calculated from atmospheres computed
using unified cloudy models. An accurate projected rotational velocity is also
derived.Comment: 9 pages, 3 figures. Astrophysical Journal Letters, in pres
- …