3,585 research outputs found
Force-Free Models of Magnetically Linked Star-Disk Systems
Disk accretion onto a magnetized star occurs in a variety of astrophysical
contexts, from young stars to X-ray pulsars. The magnetohydrodynamic
interaction between the stellar field and the accreting matter can have a
strong effect on the disk structure, the transfer of mass and angular momentum
between the disk and the star, and the production of bipolar outflows, e.g.,
plasma jets. We study a key element of this interaction - the time evolution of
the magnetic field configuration brought about by the relative rotation between
the disk and the star - using simplified, largely semianalytic, models. We
first discuss the rapid inflation and opening up of the magnetic field lines in
the corona above the accretion disk, which is caused by the differential
rotation twisting. Then we consider additional physical effects that tend to
limit this expansion, such as the effect of plasma inertia and the possibility
of reconnection in the disk's corona, the latter possibly leading to repeated
cycles in the evolution. We also derive the condition for the existence of a
steady state for a resistive disk and conclude that a steady state
configuration is not realistically possible. Finally, we generalize our
analysis of the opening of magnetic field lines by using a non-self-similar
numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.Comment: 75 pages, 22 figures, 2 tables. Submitted to Astrophysical Journa
Flat galaxies with dark matter halos - existence and stability
We consider a model for a flat, disk-like galaxy surrounded by a halo of dark
matter, namely a Vlasov-Poisson type system with two particle species, the
stars which are restricted to the galactic plane and the dark matter particles.
These constituents interact only through the gravitational potential which
stars and dark matter create collectively. Using a variational approach we
prove the existence of steady state solutions and their nonlinear stability
under suitably restricted perturbations.Comment: 39 page
Coherent Pair State of Pion in Constituent Quark Model
A coherent state of pions is introduced to the nonrelativistic quark model.
The coherent pair approximation is employed for the pion field in order to
maintain the spin-isospin symmetry. In this approximation the pion is localized
in the momentum space, and the vertex form factor in the pion-quark interaction
is derived from this localization. The nucleon masses and wave functions are
calculated using this model, and our results are compared to those of the quark
model with the one pion exchange potential. Similar result is obtained for the
mass spectrum, but there exists a clear difference in the internal structure of
nucleon resonances.Comment: 17 pages, 2 figures, revtex, submitted to Phys. Rev.
A new variational approach to the stability of gravitational systems
We consider the three dimensional gravitational Vlasov Poisson system which
describes the mechanical state of a stellar system subject to its own gravity.
A well-known conjecture in astrophysics is that the steady state solutions
which are nonincreasing functions of their microscopic energy are nonlinearly
stable by the flow. This was proved at the linear level by several authors
based on the pioneering work by Antonov in 1961. Since then, standard
variational techniques based on concentration compactness methods as introduced
by P.-L. Lions in 1983 have led to the nonlinear stability of subclasses of
stationary solutions of ground state type.
In this paper, inspired by pioneering works from the physics litterature
(Lynden-Bell 94, Wiechen-Ziegler-Schindler MNRAS 88, Aly MNRAS 89), we use the
monotonicity of the Hamiltonian under generalized symmetric rearrangement
transformations to prove that non increasing steady solutions are local
minimizer of the Hamiltonian under equimeasurable constraints, and extract
compactness from suitable minimizing sequences. This implies the nonlinear
stability of nonincreasing anisotropic steady states under radially symmetric
perturbations
Formation of Millisecond Pulsars from Accretion Induced Collapse and Constraints on Pulsar Gamma Ray Burst Models
We study accretion induced collapse of magnetized white dwarfs as an origin
of millisecond pulsars. We apply magnetized accretion disk models to the
pre-collapse accreting magnetic white dwarfs and calculate the white dwarf spin
evolution. If the pulsar magnetic field results solely from the flux-frozen
fossil white dwarf field, a typical millisecond pulsar is born with a field
strength . The uncertainty in the field strength is
mainly due to the uncertain physical parameters of the magnetized accretion
disk models. A simple correlation between the pulsar spin and the
magnetic field , , is
derived for a typical accretion rate \sim 5\times 10^{-8}M_{\sun}/yr. This
correlation remains valid for a wide pre-collapse physical conditions unless
the white dwarf spin and the binary orbit are synchronized prior to accretion
induced collapse. We critically examine the possibility of spin-orbit
synchronization in close binary systems. Using idealized homogeneous ellipsoid
models, we compute the electromagnetic and gravitational wave emission from the
millisecond pulsars and find that electromagnetic dipole emission remains
nearly constant while millisecond pulsars may spin up rather than spin down as
a result of gravitational wave emission. We also derive the physical conditions
under which electromagnetic emission from millisecond pulsars formed by
accretion induced collapse can be a source of cosmological gamma-ray bursts. We
find that relativistic beaming of gamma-ray emission and precession of
gamma-ray emitting jets are required unless the dipole magnetic field strengths
are G; such strong dipole fields are in excess of those allowed from
the accretion induced collapse formation process except in spin-orbit
synchronization.Comment: 36 pages, AASLATEX, 4 ps figures, Ap
Thin accretion disc with a corona in a central magnetic field
We study the steady-state structure of an accretion disc with a corona
surrounding a central, rotating, magnetized star. We assume that the
magneto-rotational instability is the dominant mechanism of angular momentum
transport inside the disc and is responsible for producing magnetic tubes above
the disc. In our model, a fraction of the dissipated energy inside the disc is
transported to the corona via these magnetic tubes. This energy exchange from
the disc to the corona which depends on the disc physical properties is
modified because of the magnetic interaction between the stellar magnetic field
and the accretion disc. According to our fully analytical solutions for such a
system, the existence of a corona not only increases the surface density but
reduces the temperature of the accretion disc. Also, the presence of a corona
enhances the ratio of gas pressure to the total pressure. Our solutions show
that when the strength of the magnetic field of the central neutron star is
large or the star is rotating fast enough, profiles of the physical variables
of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc
Interchange Slip-Running Reconnection and Sweeping SEP Beams
We present a new model to explain how particles (solar energetic particles;
SEPs), accelerated at a reconnection site that is not magnetically connected to
the Earth, could eventually propagate along the well-connected open flux tube.
Our model is based on the results of a low-beta resistive magnetohydrodynamics
simulation of a three-dimensional line-tied and initially current-free bipole,
that is embedded in a non-uniform open potential field. The topology of this
configuration is that of an asymmetric coronal null-point, with a closed fan
surface and an open outer spine. When driven by slow photospheric shearing
motions, field lines, initially fully anchored below the fan dome, reconnect at
the null point, and jump to the open magnetic domain. This is the standard
interchange mode as sketched and calculated in 2D. The key result in 3D is
that, reconnected open field lines located in the vicinity of the outer spine,
keep reconnecting continuously, across an open quasi-separatrix layer, as
previously identified for non-open-null-point reconnection. The apparent
slipping motion of these field lines leads to form an extended narrow magnetic
flux tube at high altitude. Because of the slip-running reconnection, we
conjecture that if energetic particles would be traveling through, or be
accelerated inside, the diffusion region, they would be successively injected
along continuously reconnecting field lines that are connected farther and
farther from the spine. At the scale of the full Sun, owing to the super-radial
expansion of field lines below 3 solar radii, such energetic particles could
easily be injected in field lines slipping over significant distances, and
could eventually reach the distant flux tube that is well-connected to the
Earth
Stable Models of Elliptical Galaxies
We construct stable axially symmetric models of elliptical galaxies. The
particle density on phase space for these models depends monotonically on the
particle energy and on the third component of the angular momentum. They are
obtained as minimizers of suitably defined energy-Casimir functionals, and this
implies their nonlinear stability. Since our analysis proceeds from a rigorous
but purely mathematical point of view it should be interesting to determine if
any of our models match observational data in astrophysics. The main purpose of
these notes is to initiate some exchange of information between the
astrophysics and the mathematics communities.Comment: 26 page
Development of Mouse Hepatocyte Lines Permissive for Hepatitis C Virus (HCV)
The lack of a suitable small animal model for the analysis of hepatitis C virus (HCV) infection has hampered elucidation of the HCV life cycle and the development of both protective and therapeutic strategies against HCV infection. Human and mouse harbor a comparable system for antiviral type I interferon (IFN) induction and amplification, which regulates viral infection and replication. Using hepatocytes from knockout (ko) mice, we determined the critical step of the IFN-inducing/amplification pathways regulating HCV replication in mouse. The results infer that interferon-beta promoter stimulator (IPS-1) or interferon A receptor (IFNAR) were a crucial barrier to HCV replication in mouse hepatocytes. Although both IFNARko and IPS-1ko hepatocytes showed a reduced induction of type I interferons in response to viral infection, only IPS-1-/- cells circumvented cell death from HCV cytopathic effect and significantly improved J6JFH1 replication, suggesting IPS-1 to be a key player regulating HCV replication in mouse hepatocytes. We then established mouse hepatocyte lines lacking IPS-1 or IFNAR through immortalization with SV40T antigen. Expression of human (h)CD81 on these hepatocyte lines rendered both lines HCVcc-permissive. We also found that the chimeric J6JFH1 construct, having the structure region from J6 isolate enhanced HCV replication in mouse hepatocytes rather than the full length original JFH1 construct, a new finding that suggests the possible role of the HCV structural region in HCV replication. This is the first report on the entry and replication of HCV infectious particles in mouse hepatocytes. These mouse hepatocyte lines will facilitate establishing a mouse HCV infection model with multifarious applications
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