13,484 research outputs found
The Formation of Low-Mass Double White Dwarfs through an Initial Phase of Stable Non-Conservative Mass Transfer
Although many double white dwarfs (DWDs) have been observed, the evolutionary
channel by which they are formed from low-mass/long-period
red-giant-main-sequence (RG-MS) binaries remains uncertain. The canonical
explanations involve some variant of double common-envelope (CE) evolution,
however it has been found that such a mechanism cannot produce the observed
distribution. We present a model for the initial episode of mass transfer (MT)
in RG-MS binaries, and demonstrate that their evolution into double white
dwarfs need not arise through a double-CE process, as long as the initial
primary's core mass (Md,c) does not exceed 0.46M. Instead, the first
episode of dramatic mass loss may be stable, non-conservative MT. We find a
lower bound on the fraction of transferred mass that must be lost from the
system in order to provide for MT, and demonstrate the feasibility of this
channel in producing observed low-mass (with M < 0.46M) DWD
systems.Comment: 2 pages, 1 figure, Conference Proceedings for the International
Conference on Binaries, Mykonos, Greec
FearNot! An Anti-Bullying Intervention: Evaluation of an Interactive Virtual Learning Environment
Original paper can be found at: http://www.aisb.org.uk/publications/proceedings.shtm
Charge-exchange limits on low-energy α-particle fluxes in solar flares
This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Ă
, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon<sup>â1</sup>. We study 10 events in total, including the Îł-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism
Non Local Electron-Phonon Correlations in a Dispersive Holstein Model
Due to the dispersion of optical phonons, long range electron-phonon
correlations renormalize downwards the coupling strength in the Holstein model.
We evaluate the size of this effect both in a linear chain and in a square
lattice for a time averaged {\it e-ph} potential, where the time variable is
introduced according to the Matsubara formalism. Mapping the Holstein
Hamiltonian onto the time scale we derive the perturbing source current which
appears to be non time retarded. This property permits to disentangle phonon
and electron coordinates in the general path integral for an electron coupled
to dispersive phonons. While the phonon paths can be integrated out
analytically, the electron path integrations have to be done numerically. The
equilibrium thermodynamic properties of the model are thus obtained as a
function of the electron hopping value and of the phonon spectrum parameters.
We derive the {\it e-ph} corrections to the phonon free energy and show that
its temperature derivatives do not depend on the {\it e-ph} effective coupling
hence, the Holstein phonon heat capacity is strictly harmonic. A significant
upturn in the low temperature total heat capacity over ratio is attributed
to the electron hopping which largely contributes to the action.Comment: Phys.Rev.B (2005
Relativistic kinetic equation for Compton scattering of polarized radiation in strong magnetic field
We derive the relativistic kinetic equation for Compton scattering of
polarized radiation in strong magnetic field using the Bogolyubov method. The
induced scattering and the Pauli exclusion principle are taken into account.
The electron polarization is also considered in the general form of the kinetic
equation. The special forms of the equation for the cases of the non-polarized
electrons, the rarefied electron gas and the two polarization mode description
of radiation are found. The derived equations are valid for any photon and
electron energies and the magnetic field strength below about 10^{16} G. These
equations provide the basis for formulation of the equation for polarized
radiation transport in atmospheres and magnetospheres of strongly magnetized
neutron stars.Comment: 23 pages, accepted for publication in Phys. Rev.
Clinical Features And Laboratory Patterns In A Cohort Of Consecutive Argentinian Patients With Von Willebrand's Disease
Background and Objectives. von Willebrand's disease (vWD) is a bleeding disorder with variable clinical expression. Our aim was to classify patients with vWD and to determine the phenotype in their relatives. Design and Methods. The types and subtypes, blood group frequency and its relevance, bleeding sites, response to the desmopressin (DDAVP) test, transfusion requirements and clinical features in type 1 and 2A families were determined in 1,885 patients. Results. Our findings were: type 1: 91%, type 2A: 3.1%, severe vWD: 1.3%; type 2N: 1.6%; type low intraplatelet: 2.7%; combined 1+2N: 0.3%. Blood group O prevalence was 70.5%. Bleeding and transfusion requirements were not correlated to blood groups. The most frequent symptoms were: ecchymoses-hematomas and epistaxis and, in females over 13 years, also menorrhagia. Normal levels of factor VIII:C were found in 38.4% of the patients. DDAVP was infused in 567 patients with a good response in 80.6%. About 9% of our patients needed transfusion therapy. The diagnosis of von Willebrandâs disease is more likely in subjects belonging to families with type 2A disease than in members of families with type 1 vWD in spite of these being symptomatic. Interpretation and Conclusions. These observations provide a good strategy to identify, classify and treat vWD patients without performing molecular assays.Fil: Woods, Adriana InĂ©s. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Meschengieser, S. S.. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Blanco, A. N.. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Salviu, M. J.. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Farias, Cristina Elena. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Kempfer, Ana Catalina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Academia Nacional de Medicina de Buenos Aires; ArgentinaFil: Lazzari, MarĂa Ăngela. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Academia Nacional de Medicina de Buenos Aires; Argentin
The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005
On 2004 December 27, a highly-energetic giant flare was recorded from the
magnetar candidate SGR 1806-20. In the months preceding this flare, the
persistent X-ray emission from this object began to undergo significant
changes. Here, we report on the evolution of key spectral and temporal
parameters prior to and following this giant flare. Using the Rossi X-ray
Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the
spin-down rate of this SGR varied erratically in the months before and after
the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for
a discrete jump in spin frequency at the time of the December 27th flare
(|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong
correlation between pulsed flux and torque consistent with the model for
magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov &
Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR
1806-20 changes drastically following the flare. Using the Chandra X-ray
Observatory and other publicly available imaging X-ray detector observations,
we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual
magnetar persistent emission spectral model of a power-law plus a blackbody
provides an excellent fit to the data. We confirm the earlier finding by
Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between
1993 and 2004. Contrary to the direct correlation between torque and spectral
hardness proposed by Mereghetti et al., we find evidence for a sudden torque
change that triggered a gradual hardening of the energy spectrum on a timescale
of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and
phase-averaged of SGR 1806-20 all peak months before the flare epoch.Comment: 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To
appear in the Oct 20 2006 editio
Magnetars as cooling neutron stars with internal heating
We study thermal structure and evolution of magnetars as cooling neutron
stars with a phenomenological heat source in a spherical internal layer. We
explore the location of this layer as well as the heating rate that could
explain high observable thermal luminosities of magnetars and would be
consistent with the energy budget of neutron stars. We conclude that the heat
source should be located in an outer magnetar's crust, at densities rho < 5e11
g/cm^3, and should have the heat intensity of the order of 1e20 erg/s/cm^3.
Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the
surface.Comment: 8 pages, 5 figures, submitted to MNRA
A note on leapfrogging vortex rings
In this paper we provide examples, by numerical simulation using the Navier-Stokes equations for axisymmetric laminar flow, of the 'leapfrogging' motion of two, initially identical, vortex rings which share a common axis of symmetry. We show that the number of clear passes that each ring makes through the other increases with Reynolds number, and that as long as the configuration remains stable the two rings ultimately merge to form a single vortex ring
Near-inertial wave scattering by random flows
The impact of a turbulent flow on wind-driven oceanic near-inertial waves is
examined using a linearised shallow-water model of the mixed layer. Modelling
the flow as a homogeneous and stationary random process with spatial scales
comparable to the wavelengths, we derive a transport (or kinetic) equation
governing wave-energy transfers in both physical and spectral spaces. This
equation describes the scattering of the waves by the flow which results in a
redistribution of energy between waves with the same frequency (or,
equivalently, with the same wavenumber) and, for isotropic flows, in the
isotropisation of the wave field. The time scales for the scattering and
isotropisation are obtained explicitly and found to be of the order of tens of
days for typical oceanic parameters. The predictions inferred from the
transport equation are confirmed by a series of numerical simulations.
Two situations in which near-inertial waves are strongly influenced by flow
scattering are investigated through dedicated nonlinear shallow-water
simulations. In the first, a wavepacket propagating equatorwards as a result
from the -effect is shown to be slowed down and dispersed both zonally
and meridionally by scattering. In the second, waves generated by moving
cyclones are shown to be strongly disturbed by scattering, leading again to an
increased dispersion.Comment: Accepted for publication in Phys. Rev. Fluid
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