4,626 research outputs found
The United States is Obligated to Take All Refugees of a Kind
A Hobbesian Realist position concerning Nation States and their generative grounds in the Social Contract obligates the United States to accept any and all refugees of conflict who are willing to recognize the sovereign power of the United States by submitting to citizenship requirements determined by the United States
Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field
In earlier works we pointed out that the disk's surface layers are
non-turbulent and thus highly conducting (or non-diffusive) because the
hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high
in the disk where the magnetic and radiation pressures are larger than the
plasma thermal pressure. Here, we calculate the vertical profiles of the {\it
stationary} accretion flows (with radial and azimuthal components), and the
profiles of the large-scale, magnetic field taking into account the turbulent
viscosity and diffusivity and the fact that the turbulence vanishes at the
surface of the disk.
Also, here we require that the radial accretion speed be zero at the disk's
surface and we assume that the ratio of the turbulent viscosity to the
turbulent magnetic diffusivity is of order unity. Thus at the disk's surface
there are three boundary conditions. As a result, for a fixed dimensionless
viscosity -value, we find that there is a definite relation between the
ratio of the accretion power going into magnetic disk winds to the
viscous power dissipation and the midplane plasma-, which is the ratio
of the plasma to magnetic pressure in the disk. For a specific disk model with
of order unity we find that the critical value required for a
stationary solution is , where the disk's
half thickness. For weaker magnetic fields, , we argue that
the poloidal field will advect outward while for it will
advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with .Comment: 5 pages, 3 figure
Relativistic Jets from Accretion Disks
The jets observed to emanate from many compact accreting objects may arise
from the twisting of a magnetic field threading a differentially rotating
accretion disk which acts to magnetically extract angular momentum and energy
from the disk. Two main regimes have been discussed, hydromagnetic jets, which
have a significant mass flux and have energy and angular momentum carried by
both matter and electromagnetic field and, Poynting jets, where the mass flux
is small and energy and angular momentum are carried predominantly by the
electromagnetic field. Here, we describe recent theoretical work on the
formation of relativistic Poynting jets from magnetized accretion disks.
Further, we describe new relativistic, fully-electromagnetic, particle-in-cell
simulations of the formation of jets from accretion disks. Analog Z-pinch
experiments may help to understand the origin of astrophysical jets.Comment: 7 pages, 3 figures, Proc. of High Energy Density Astrophysics Conf.,
200
Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars
This work investigates the short wavelength stability of the magnetopause
between a rapidly-rotating, supersonic, dense accretion disc and a
slowly-rotating low-density magnetosphere of a magnetized star. The
magnetopause is a strong shear layer with rapid changes in the azimuthal
velocity, the density, and the magnetic field over a short radial distance and
thus the Kelvin-Helmholtz (KH) instability may be important. The plasma
dynamics is treated using non-relativistic, compressible (isentropic)
magnetohydrodynamics. It is necessary to include the displacement current in
order that plasma wave velocities remain less than the speed of light. We focus
mainly on the case of a star with an aligned dipole magnetic field so that the
magnetic field is axial in the disc midplane and perpendicular to the disc flow
velocity. However, we also give results for cases where the magnetic field is
at an arbitrary angle to the flow velocity. For the aligned dipole case the
magnetopause is most unstable for KH waves propagating in the azimuthal
direction perpendicular to the magnetic field which tends to stabilize waves
propagating parallel to it. The wave phase velocity is that of the disc matter.
A quasi-linear theory of the saturation of the instability leads to a
wavenumber () power spectrum of the density and temperature
fluctuations of the magnetopause, and it gives the mass accretion and angular
momentum inflow rates across the magnetopause. For self-consistent conditions
this mass accretion rate will be equal to the disc accretion rate at large
distances from the magnetopause.Comment: 8 pages, 7 figure
Detecting a Higgs Pseudoscalar with a Boson at the LHC
We have adopted two Higgs doublet models to study the production of a Higgs
pseudoscalar () in association with a gauge boson from gluon fusion
() at the CERN Large Hadron Collider. The prospects for the
discovery of are investigated with physics
backgrounds and realistic cuts. Promising results are found for m_A \alt 260
GeV in two Higgs doublet models when the heavier Higgs scalar () can decay
into a boson and a Higgs pseudoscalar (). Although the cross section
of is usually small in the minimal supersymmetric standard model,
it can be significantly enhanced in general two Higgs doublet models. This
discovery channel might provide an opportunity to search for a Higgs scalar and
a Higgs pseudoscalar simultaneously at the LHC and could lead to new physics
beyond the Standard Model and the minimal supersymmetric model.Comment: Two tables added for the ratio of signal/backgroun
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