332 research outputs found
On a mechanism for enhancing magnetic activity in tidally interacting binaries
We suggest a mechanism for enhancing magnetic activity in tidally interacting
binaries. We suppose that the deviation of the primary star from spherical
symmetry due to the tidal influence of the companion leads to stellar pulsation
in its fundamental mode. It is shown that stellar radial pulsation amplifies
torsional Alfv{\'e}n waves in a dipole-like magnetic field, buried in the
interior, according to the recently proposed swing wave-wave interaction
(Zaqarashvili 2001). Then amplified Alfv{\'e}n waves lead to the onset of
large-scale torsional oscillations, and magnetic flux tubes arising towards the
surface owing to magnetic buoyancy diffuse into the atmosphere producing
enhanced chromospheric and coronal emission.Comment: Accepted in Ap
Sexual violence in post-conflict Liberia: survivors and their care.
Using routine data from three clinics offering care to survivors of sexual violence (SV) in Monrovia, Liberia, we describe the characteristics of SV survivors and the pattern of SV and discuss how the current approach could be better adapted to meet survivors' needs. There were 1500 survivors seeking SV care between January 2008 and December 2009. Most survivors were women (98%) and median age was 13âyears (Interquartile range: 9-17âyears). Sexual aggression occurred during day-to-day activities in 822 (55%) cases and in the survivor's home in 552 (37%) cases. The perpetrator was a known civilian in 1037 (69%) SV events. Only 619 (41%) survivors sought care within 72âh. The current approach could be improved by: effectively addressing the psychosocial needs of child survivors, reaching male survivors, targeting the perpetrators in awareness and advocacy campaigns and reducing delays in seeking care
A comparison of incompressible limits for resistive plasmas
The constraint of incompressibility is often used to simplify the
magnetohydrodynamic (MHD) description of linearized plasma dynamics because it
does not affect the ideal MHD marginal stability point. In this paper two
methods for introducing incompressibility are compared in a cylindrical plasma
model: In the first method, the limit is taken, where
is the ratio of specific heats; in the second, an anisotropic mass
tensor is used, with the component parallel to the magnetic
field taken to vanish, . Use of resistive MHD reveals
the nature of these two limits because the Alfv\'en and slow magnetosonic
continua of ideal MHD are converted to point spectra and moved into the complex
plane. Both limits profoundly change the slow-magnetosonic spectrum, but only
the second limit faithfully reproduces the resistive Alfv\'en spectrum and its
wavemodes. In ideal MHD, the slow magnetosonic continuum degenerates to the
Alfv\'en continuum in the first method, while it is moved to infinity by the
second. The degeneracy in the first is broken by finite resistivity. For
numerical and semi-analytical study of these models, we choose plasma
equilibria which cast light on puzzling aspects of results found in earlier
literature.Comment: 14 pages, 10 figure
Bounds on the Magnetic Fields in the Radiative Zone of the Sun
We discuss bounds on the strength of the magnetic fields that could be buried
in the radiative zone of the Sun. The field profiles and decay times are
computed for all axisymmetric toroidal Ohmic decay eigenmodes with lifetimes
exceeding the age of the Sun. The measurements of the solar oblateness yield a
bound <~ 7 MG on the strength of the field. A comparable bound is expected to
come from the analysis of the splitting of the solar oscillation frequencies.
The theoretical analysis of the double diffusive instability also yields a
similar bound. The oblateness measurements at their present level of
sensitivity are therefore not expected to measure a toroidal field
contribution.Comment: 15 pages, 6 figure
Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars
We investigate the stability of differentially rotating proto-neutron stars
(PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric
MHD instabilities are derived using a local linear analysis. PNSs are expected
to have much stronger radial shear in the rotation velocity compared to normal
stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a
large azimuthal wavenumber is dominant over the kink mode () in
differentially rotating PNSs. The growth rate of the NMRI is of the order of
the angular velocity which is faster than that of the kink-type
instability by several orders of magnitude. The stability criteria are
analogous to those of the axisymmetric magnetorotational instability with a
poloidal field, although the effects of leptonic gradients are considered in
our analysis. The NMRI can grow even in convectively stable layers if the
wavevectors of unstable modes are parallel to the restoring force by the
Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify
the magnetic fields and drive MHD turbulence in PNSs, which may lead to
enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical
Journal (December 12, 2005
X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars
A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075
sources (Feigelson et al. 2002), providing a uniquely large and well-defined
sample to study the dependence of magnetic activity on bulk properties for
stars descending the Hayashi tracks. The following results are obtained: (1)
X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with
bolometric luminosity with = -3.8 for stars with masses 0.7<M<2
Mo, an order of magnitude below the main sequence saturation level; (2) the
X-ray emission drops rapidly below this level in some or all stars with 2<M<3
Mo; (3) the presence or absence of infrared circumstellar disks has no apparent
relation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise as
rotational periods increase from 0.4 to 20 days. This last finding stands in
dramatic contrast to the strong anticorrelation between X-rays and period seen
in main sequence stars.
The absence of a strong X-ray/rotation relationship in PMS stars, and
particularly the high X-ray values seen in some very slowly rotating stars, is
a clear indication that the mechanisms of magnetic field generation differ from
those operating in main sequence stars. The most promising possibility is a
turbulent dynamo distributed throughout the deep convection zone, but other
models such as alpha-Omega dynamo with `supersaturation' or relic core fields
are not immediately excluded. The drop in magnetic activity in
intermediate-mass stars may reflect the presence of a significant radiative
core. The evidence does not support X-ray production in large-scale star-disk
magnetic fields.Comment: 51 pages, 8 figures. To appear in the Astrophysical Journa
Structure, Deformations and Gravitational Wave Emission of Magnetars
Neutron stars can have, in some phases of their life, extremely strong
magnetic fields, up to 10^15-10^16 G. These objects, named magnetars, could be
powerful sources of gravitational waves, since their magnetic field could
determine large deformations. We discuss the structure of the magnetic field of
magnetars, and the deformation induced by this field. Finally, we discuss the
perspective of detection of the gravitational waves emitted by these stars.Comment: 11 pages, 2 figures, prepared for 19th International Conference on
General Relativity and Gravitation (GR19), Mexico City, Mexico, July 5-9,
201
Stellar jitter from variable gravitational redshift: implications for RV confirmation of habitable exoplanets
A variation of gravitational redshift, arising from stellar radius
fluctuations, will introduce astrophysical noise into radial velocity
measurements by shifting the centroid of the observed spectral lines. Shifting
the centroid does not necessarily introduce line asymmetries. This is
fundamentally different from other types of stellar jitter so far identified,
which do result from line asymmetries. Furthermore, only a very small change in
stellar radius, ~0.01%, is necessary to generate a gravitational redshift
variation large enough to mask or mimic an Earth-twin. We explore possible
mechanisms for stellar radius fluctuations in low-mass stars. Convective
inhibition due to varying magnetic field strengths and the Wilson depression of
starspots are both found to induce substantial gravitational redshift
variations. Finally, we investigate a possible method for monitoring/correcting
this newly identified potential source of jitter and comment on its impact for
future exoplanet searches.Comment: 6 pages, 1 figure, 1 tabl
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
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