3,479 research outputs found
Tidal Excitation of Oscillation Modes in Compact White Dwarf Binaries: I. Linear Theory
We study the tidal excitation of gravity modes (g-modes) in compact white
dwarf binary systems with periods ranging from minutes to hours. As the orbit
of the system decays via gravitational radiation, the orbital frequency
increases and sweeps through a series of resonances with the g-modes of the
white dwarf. At each resonance, the tidal force excites the g-mode to a
relatively large amplitude, transferring the orbital energy to the stellar
oscillation. We calculate the eigenfrequencies of g-modes and their coupling
coefficients with the tidal field for realistic non-rotating white dwarf
models. Using these mode properties, we numerically compute the excited mode
amplitude in the linear approximation as the orbit passes though the resonance,
including the backreaction of the mode on the orbit. We also derive analytical
estimates for the mode amplitude and the duration of the resonance, which
accurately reproduce our numerical results for most binary parameters. We find
that the g-modes can be excited to a dimensionless (mass-weighted) amplitude up
to 0.1, with the mode energy approaching of the gravitational binding
energy of the star. This suggests that thousands of years prior to the binary
merger, the white dwarf may be heated up significantly by tidal interactions.
However, more study is needed since the physical amplitudes of the excited
oscillation modes become highly nonlinear in the outer layer of the star, which
can reduce the mode amplitude attained by tidal excitation.Comment: 10 pages, 8 figure
An Impossible Job? The View From the Urban Superintendent's Chair
Presents the results of a survey of superintendents of the 100 largest urban and ex-urban districts in the U.S. Examines how school leaders define their challenges and potential solutions
New roles for Fc receptors in neurodegeneration-the impact on immunotherapy for Alzheimer's disease
There are an estimated 18 million Alzheimer's disease (AD) sufferers worldwide and with no disease modifying treatment currently available, development of new therapies represents an enormous unmet clinical need. AD is characterized by episodic memory loss followed by severe cognitive decline and is associated with many neuropathological changes. AD is characterized by deposits of amyloid beta (A?), neurofibrillary tangles, and neuroinflammation. Active immunization or passive immunization against A? leads to the clearance of deposits in transgenic mice expressing human A?. This clearance is associated with reversal of associated cognitive deficits, but these results have not translated to humans, with both active and passive immunotherapy failing to improve memory loss. One explanation for these observations is that certain anti-A? antibodies mediate damage to the cerebral vasculature limiting the top dose and potentially reducing efficacy. Fc gamma receptors (Fc?R) are a family of immunoglobulin-like receptors which bind to the Fc portion of IgG, and mediate the response of effector cells to immune complexes. Data from both mouse and human studies suggest that cross-linking Fc?R by therapeutic antibodies and the subsequent pro-inflammatory response mediates the vascular side effects seen following immunotherapy. Increasing evidence is emerging that Fc?R expression on CNS resident cells, including microglia and neurons, is increased during aging and functionally involved in the pathogenesis of age-related neurodegenerative diseases. Therefore, we propose that increased expression and ligation of Fc?R in the CNS, either by endogenous IgG or therapeutic antibodies, has the potential to induce vascular damage and exacerbate neurodegeneration. To produce safe and effective immunotherapies for AD and other neurodegenerative diseases it will be vital to understand the role of Fc?R in the healthy and diseased brain. Here we review the literature on Fc?R expression, function and proposed roles in multiple age-related neurological diseases. Lessons can be learnt from therapeutic antibodies used for the treatment of cancer where antibodies have been engineered for optimal efficacy
Neutrino Flavor Evolution in Neutron Star Mergers
We examine the flavor evolution of neutrinos emitted from the disk-like
remnant (hereafter called \lq\lq neutrino disk\rq\rq) of a binary neutron star
(BNS) merger. We specifically follow the neutrinos emitted from the center of
the disk, along the polar axis perpendicular to the equatorial plane. We
carried out two-flavor simulations using a variety of different possible
initial neutrino luminosities and energy spectra, and for comparison,
three-flavor simulations in specific cases. In all simulations, the normal
neutrino mass hierarchy was used. The flavor evolution was found to be highly
dependent on the initial neutrino luminosities and energy spectra; in
particular, we found two broad classes of results depending on the sign of the
initial net electron neutrino lepton number (i.e., the number of neutrinos
minus the number of antineutrinos). In the antineutrino dominated case, we
found that the Matter-Neutrino Resonance (MNR) effect dominates, consistent
with previous results, whereas in the neutrino dominated case, a bipolar
spectral swap develops. The neutrino dominated conditions required for this
latter result have been realized, e.g, in a BNS merger simulation that employs
the \lq\lq DD2\rq\rq\ equation of state for neutron star matter[Phys. Rev. D
93, 044019 (2016)]. For this case, in addition to the swap at low energies, a
collective Mikheyev-Smirnov-Wolfenstein (MSW) mechanism generates a high-energy
electron neutrino tail. The enhanced population of high-energy electron
neutrinos in this scenario could have implications for the prospects of
-process nucleosynthesis in the material ejected outside the plane of the
neutrino disk.Comment: Version published in Physical Review D. 22 pages, 16 figures, 9
tables. For movies see Ancillary files in version
Translation techniques for distributed-shared memory programming models
This thesis argues that a modular, source-to-source translation system for distributed-shared memory programming models would be beneficial to the high-performance computing community. It goes on to present a proof-of-concept example in detail, translating between Global Arrays (GA) and Unified Parallel C (UPC). Some useful extensions to UPC are discussed, along with how they are implemented in the proof-of-concept translator
The Religious Life Of Robert E. Lee
At a time when Confederate memorials to him are being removed from public spaces, Robert E. Lee\u27s memory continues to be controversial. In the heated debates over statues of him, much of the myth of Lee created by the Lost Cause still resonates, including his Christianity. Every historian and biogra...
First Chaplain of the Confederacy: Father Darius Hubert, S.J.
In this slim-but-significant book, independent scholar Katherine Bentley Jeffrey adds tremendously to our understanding of the religious life of the Civil War Era. This definitive biography of Father Darius Hubert . . . offers a view of a working Roman Catholic priest in the South before, during, and after the Civil War. And in the process, adds to the growing literature on religious life in the military during the war, and explores the ways in which this Catholic priest became part of the Lost Cause
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