5,793 research outputs found
Origin of electron cyclotron maser-induced radio emissions at ultra-cool dwarfs: magnetosphere-ionosphere coupling currents
A number of ultra-cool dwarfs emit circularly polarised radio waves generated
by the electron cyclotron maser instability. In the solar system such radio is
emitted from regions of strong auroral magnetic field-aligned currents. We thus
apply ideas developed for Jupiter's magnetosphere, being a well-studied
rotationally-dominated analogue in our solar system, to the case of
fast-rotating UCDs. We explain the properties of the radio emission from UCDs
by showing that it would arise from the electric currents resulting from an
angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by
an extremely powerful analogue of the process which causes Jupiter's auroras.
Such a velocity gradient indicates that these bodies interact significantly
with their space environment, resulting in intense auroral emissions. These
results strongly suggest that auroras occur on bodies outside our solar system.Comment: Accepted for publication in the Astrophysical Journa
Hubble Space Telescope observations of the NUV transit of WASP-12b
We present new observations of four closely-spaced NUV transits of the hot
Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the
phase resolution of the observed NUV light curve relative to previous
observations, while minimising the temporal variation of the system. We observe
significant excess NUV absorption during the transit, with mean normalised
in-transit fluxes of , i.e. 2-5
deeper than the optical transit level of for a uniform stellar
disk (the exact confidence level depending on the normalisation method used).
We further observe an asymmetric transit shape, such that the post-conjunction
fluxes are overall 2-3 higher than pre-conjunction values, and
characterised by rapid variations in count rate between the pre-conjunction and
out of transit levels. We do not find evidence for an early ingress to the NUV
transit as suggested by earlier HST observations. However, we show that the NUV
count rate observed prior to the optical transit is highly variable, but
overall 2.2-3.0 below the post-transit values and comparable
in depth to the optical transit, possibly forming a variable region of NUV
absorption from at least phase 0.83, limited by the data coverage.Comment: Accepted into the Astrophysical Journa
Instability driven fragmentation of nanoscale fractal islands
Formation and evolution of fragmentation instabilities in fractal islands,
obtained by deposition of silver clusters on graphite, are studied. The
fragmentation dynamics and subsequent relaxation to the equilibrium shapes are
controlled by the deposition conditions and cluster composition. Sharing common
features with other materials' breakup phenomena, the fragmentation instability
is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres
A Coordinated X-ray and Optical Campaign of the Nearby Massive Binary Orionis Aa: II. X-ray Variability
We present time-resolved and phase-resolved variability studies of an
extensive X-ray high-resolution spectral dataset of the Orionis Aa
binary system. The four observations, obtained with Chandra ACIS HETGS, have a
total exposure time of ~479 ks and provide nearly complete binary phase
coverage. Variability of the total X-ray flux in the range 5-25 is
confirmed, with maximum amplitude of about +/-15% within a single ~125 ks
observation. Periods of 4.76d and 2.04d are found in the total X-ray flux, as
well as an apparent overall increase in flux level throughout the 9-day
observational campaign. Using 40 ks contiguous spectra derived from the
original observations, we investigate variability of emission line parameters
and ratios. Several emission lines are shown to be variable, including S XV, Si
XIII, and Ne IX. For the first time, variations of the X-ray emission line
widths as a function of the binary phase are found in a binary system, with the
smallest widths at phase=0.0 when the secondary Orionis Aa2 is at
inferior conjunction. Using 3D hydrodynamic modeling of the interacting winds,
we relate the emission line width variability to the presence of a wind cavity
created by a wind-wind collision, which is effectively void of embedded wind
shocks and is carved out of the X-ray-producing primary wind, thus producing
phase-locked X-ray variability.Comment: 36 pages, 14 Tables, 19 Figures, accepted by ApJ, one of 4 related
papers to be published togethe
Post-Newtonian Approximation in Maxwell-Like Form
The equations of the linearized first post-Newtonian approximation to general
relativity are often written in "gravitoelectromagnetic" Maxwell-like form,
since that facilitates physical intuition. Damour, Soffel and Xu (DSX) (as a
side issue in their complex but elegant papers on relativistic celestial
mechanics) have expressed the first post-Newtonian approximation, including all
nonlinearities, in Maxwell-like form. This paper summarizes that DSX
Maxwell-like formalism (which is not easily extracted from their celestial
mechanics papers), and then extends it to include the post-Newtonian
(Landau-Lifshitz-based) gravitational momentum density, momentum flux (i.e.
gravitational stress tensor) and law of momentum conservation in Maxwell-like
form. The authors and their colleagues have found these Maxwell-like momentum
tools useful for developing physical intuition into numerical-relativity
simulations of compact binaries with spin.Comment: v4: Revised for resubmission to Phys Rev D, 6 pages. v3: Reformulated
in terms of DSX papers. Submitted to Phys Rev D, 6 pages. v2: Added
references. Changed definitions & convention
A Coordinated X-ray and Optical Campaign on the Nearest Massive Eclipsing Binary, Delta Ori Aa: I. Overview of the X-ray Spectrum
We present an overview of four phase-constrained Chandra HETGS X-ray
observations of Delta Ori A. Delta Ori A is actually a triple system which
includes the nearest massive eclipsing spectroscopic binary, Delta Ori Aa, the
only such object which can be observed with little phase-smearing with the
Chandra gratings. Since the fainter star, Delta Ori Aa2, has a much lower X-ray
luminosity than the brighter primary, Delta Ori A provides a unique system with
which to test the spatial distribution of the X-ray emitting gas around Delta
Ori Aa1 via occultation by the photosphere of and wind cavity around the X-ray
dark secondary. Here we discuss the X-ray spectrum and X-ray line profiles for
the combined observation, having an exposure time of nearly 500 ksec and
covering nearly the entire binary orbit. Companion papers discuss the X-ray
variability seen in the Chandra spectra, present new space-based photometry and
ground-based radial velocities simultaneous with the X-ray data to better
constrain the system parameters, and model the effects of X-rays on the optical
and UV spectrum. We find that the X-ray emission is dominated by embedded wind
shock emission from star Aa1, with little contribution from the tertiary star
Ab or the shocked gas produced by the collision of the wind of Aa1 against the
surface of Aa2. We find a similar temperature distribution to previous X-ray
spectrum analyses. We also show that the line half-widths are about
the terminal velocity of the wind of star Aa1. We find a strong
anti-correlation between line widths and the line excitation energy, which
suggests that longer-wavelength, lower-temperature lines form farther out in
the wind. Our analysis also indicates that the ratio of the intensities of the
strong and weak lines of \ion{Fe}{17} and \ion{Ne}{10} are inconsistent with
model predictions, which may be an effect of resonance scatteringComment: accepted by ApJ; revised according to ApJ proo
ABCC9/SUR2 in the Brain: Implications for Hippocampal Sclerosis of Aging and a Potential Therapeutic Target
The ABCC9 gene and its polypeptide product, SUR2, are increasingly implicated in human neurologic disease, including prevalent diseases of the aged brain. SUR2 proteins are a component of the ATP-sensitive potassium (“K ATP ”) channel, a metabolic sensor for stress and/or hypoxia that has been shown to change in aging. The K ATP channel also helps regulate the neurovascular unit. Most brain cell types express SUR2, including neurons, astrocytes, oligodendrocytes, microglia, vascular smooth muscle, pericytes, and endothelial cells. Thus it is not surprising that ABCC9 gene variants are associated with risk for human brain diseases. For example, Cantu syndrome is a result of ABCC9 mutations; we discuss neurologic manifestations of this genetic syndrome. More common brain disorders linked to ABCC9 gene variants include hippocampal sclerosis of aging (HS-Aging), sleep disorders, and depression. HS-Aging is a prevalent neurological disease with pathologic features of both neurodegenerative (aberrant TDP-43) and cerebrovascular (arteriolosclerosis) disease. As to potential therapeutic intervention, the human pharmacopeia features both SUR2 agonists and antagonists, so ABCC9 /SUR2 may provide a “druggable target”, relevant perhaps to both HS-Aging and Alzheimer\u27s disease. We conclude that more work is required to better understand the roles of ABCC9 /SUR2 in the human brain during health and disease conditions
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