101 research outputs found
Dark mammoth trunks in the merging galaxy NGC 1316 and a mechanism of cosmic double helices
NGC 1316 is a giant, elliptical galaxy containing a complex network of dark,
dust features. The morphology of these features has been examined in some
detail using a Hubble Space Telescope, Advanced Camera for Surveys image. It is
found that most of the features are constituted of long filaments. There also
exist a great number of dark structures protruding inwards from the filaments.
Many of these structures are strikingly similar to elephant trunks in H II
regions in the Milky Way Galaxy, although much larger. The structures, termed
mammoth trunks, generally are filamentary and often have shapes resembling the
letters V or Y. In some of the mammoth trunks the stem of the Y can be resolved
into two or more filaments, many of which showing signs of being intertwined. A
model of the mammoth trunks, related to a recent theory of elephant trunks, is
proposed. Based on magnetized filaments, the model is capable of giving an
account of the various shapes of the mammoth trunks observed, including the
twined structures.Comment: Accepted for publication in Astrophysics & Space Scienc
Electrodynamics of Magnetars IV: Self-Consistent Model of the Inner Accelerator, with Implications for Pulsed Radio Emission
We consider the voltage structure in the open-field circuit and outer
magnetosphere of a magnetar. The standard polar-cap model for radio pulsars is
modified significantly when the polar magnetic field exceeds 1.8x10^{14} G.
Pairs are created by accelerated particles via resonant scattering of thermal
X-rays, followed by the nearly instantaneous conversion of the scattered photon
to a pair. A surface gap is then efficiently screened by e+- creation, which
regulates the voltage in the inner part of the circuit to ~10^9 V. We also
examine the electrostatic gap structure that can form when the magnetic field
is somewhat weaker, and deduce a voltage 10-30 times larger over a range of
surface temperatures. We examine carefully how the flow of charge back to the
star above the gap depends on the magnitude of the current that is extracted
from the surface of the star, on the curvature of the magnetic field lines, and
on resonant drag. The rates of different channels of pair creation are
determined self-consistently, including the non-resonant scattering of X-rays,
and collisions between gamma rays and X-rays. We find that the electrostatic
gap solution has too small a voltage to sustain the observed pulsed radio
output of magnetars unless i) the magnetic axis is nearly aligned with the
rotation axis and the light of sight; or ii) the gap is present on the closed
as well as the open magnetic field lines. Several properties of the radio
magnetars -- their rapid variability, broad pulses, and unusually hard radio
spectra -- are consistent with a third possibility, that the current in the
outer magnetosphere is strongly variable, and a very high rate of pair creation
is sustained by a turbulent cascade.Comment: 32 pages, submitted to the Astrophysical Journa
Structure of the interstellar medium around Cas A
We present a three-year series of observations at 24 microns with the Spitzer
Space Telescope of the interstellar material in a 200 x 200 arcmin square area
centered on Cassiopeia A. Interstellar dust heated by the outward light pulse
from the supernova explosion emits in the form of compact, moving features.
Their sequential outward movements allow us to study the complicated
three-dimensional structure of the interstellar medium (ISM) behind and near
Cassiopeia A. The ISM consists of sheets and filaments, with many structures on
a scale of a parsec or less. The spatial power spectrum of the ISM appears to
be similar to that of fractals with a spectral index of 3.5. The filling factor
for the small structures above the spatial wavenumber k ~ 0.5 cycles/pc is only
~ 0.4%.Comment: 29 pages including 10 figures; accepted by The Astrophysical Journa
The magnetar emission in the IR band: the role of magnetospheric currents
There is a general consensus about the fact that the magnetar scenario
provides a convincing explanation for several of the observed properties of the
Anomalous X-ray Pulsars and the Soft Gamma Repeaters. However, the origin of
the emission observed at low energies is still an open issue. We present a
quantitative model for the emission in the optical/infrared band produced by
curvature radiation from magnetospheric charges, and compare results with
current magnetars observations.Comment: 6 Pages, 2 Figures. Astrophysics and Space Science, in press.
Proceedings of the ICREA Workshop on The High-Energy Emission from Pulsars
and their Systems, Sant Cugat, April 12-16 201
Magnetically Dominated Strands of Cold Hydrogen in the Riegel-Crutcher Cloud
We present new high resolution (100 arcsec) neutral hydrogen (H I)
self-absorption images of the Riegel-Crutcher cloud obtained with the Australia
Telescope Compact Array and the Parkes Radio Telescope. The Riegel-Crutcher
cloud lies in the direction of the Galactic center at a distance of 125 +/- 25
pc. Our observations resolve the very large, nearby sheet of cold hydrogen into
a spectacular network of dozens of hair-like filaments. Individual filaments
are remarkably elongated, being up to 17 pc long with widths of less than ~0.1
pc. The strands are reasonably cold, with spin temperatures of 40 K and in many
places appearing to have optical depths larger than one. Comparing the H I
images with observations of stellar polarization we show that the filaments are
very well aligned with the ambient magnetic field. We argue that the structure
of the cloud has been determined by its magnetic field. In order for the cloud
to be magnetically dominated the magnetic field strength must be > 30
microGauss.Comment: To appear in the Astrophysical Journal. 26 pages, 6 figures. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/rc_cloud.pd
Magnetic Fields in Star-Forming Molecular Clouds II. The Depolarization Effect in the OMC-3 Filament of Orion A
Polarized 850 micron thermal emission data of the region OMC-3 in the Orion A
molecular cloud are presented. These data, taken in 1998 with the SCUBA
polarimeter mounted on the James Clerk Maxwell Telescope, have been re-reduced
using improved software. The polarization pattern is not suggestive of a
uniform field structure local to OMC-3, nor does the orientation of the vectors
align with existing polarimetry maps of the OMC-1 core 20' to the south. The
depolarization toward high intensity regions cannot be explained by uniform
field geometry except in the presence of changing grain structure, which is
most likely to occur in regions of high density or temperature (i.e. the
embedded cores). The depolarization in fact occurs along the length of the
filamentary structure of OMC-3 and is not limited to the vicinity of the bright
cores. Such a polarization pattern is predicted by helical field models for
filamentary clouds. We present three scenarios to explain the observed
polarization pattern of OMC-3 in terms of a helical field geometry. Qualitative
models incorporating a helical field geometry are presented for two cases.Comment: 57 pages, 12 figures, 3 tables; accepted for publication in Ap
Electric current circuits in astrophysics
Cosmic magnetic structures have in common that they are anchored
in a dynamo, that an external driver converts kinetic energy into internal
magnetic energy, that this magnetic energy is transported as Poynting fl ux across the magnetically dominated structure, and that the magnetic energy
is released in the form of particle acceleration, heating, bulk motion,
MHD waves, and radiation. The investigation of the electric current system is
particularly illuminating as to the course of events and the physics involved.
We demonstrate this for the radio pulsar wind, the solar flare, and terrestrial
magnetic storms
Paediatric non-progression following grandmother-to-child HIV transmission
Background
In contrast to adult HIV infection, where slow disease progression is strongly linked to immune control of HIV mediated by protective HLA class I molecules such as HLA-B*81:01, the mechanisms by which a minority of HIV-infected children maintain normal-for-age CD4 counts and remain clinically healthy appear to be HLA class I-independent and are largely unknown. To better understand these mechanisms, we here studied a HIV-infected South African female, who remained a non-progressor throughout childhood.
Results
Phylogenetic analysis of viral sequences in the HIV-infected family members, together with the history of grand-maternal breast-feeding, indicated that, unusually, the non-progressor child had been infected via grandmother-to-child transmission. Although HLA-B*81:01 was expressed by both grandmother and grand-daughter, autologous virus in each subject encoded an escape mutation L188F within the immunodominant HLA-B*81:01-restricted Gag-specific epitope TL9 (TPQDLNTML, Gag 180–188). Since the transmitted virus can influence paediatric and adult HIV disease progression, we investigated the impact of the L188F mutant on replicative capacity. When this variant was introduced into three distinct HIV clones in vitro, viral replicative capacity was abrogated altogether. However, a virus constructed using the gag sequence of the non-progressor child replicated as efficiently as wildtype virus.
Conclusion
These findings suggest alternative sequences of events: the transmission of the uncompensated low fitness L188F to both children, potentially contributing to slow progression in both, consistent with previous studies indicating that disease progression in children can be influenced by the replicative capacity of the transmitted virus; or the transmission of fully compensated virus, and slow progression here principally the result of HLA-independent host-specific factors, yet to be defined
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