3,009 research outputs found
LiBeB, Cosmic Rays and Gamma-Ray Line Astronomy
This article is a summary of a recently held conference on the light
elements, Li, Be and B, and their relationship to cosmic-ray origin and
gamma-ray astronomy. The proceedings will be published by the PASP.Comment: latex 6 pages, uses aasms4.sty To appear in the Publications of the
Astronomical Society of the Pacific (PASP
Time-dependent magnetohydrodynamic self-similar extragalactic jets
Extragalactic jets are visualized as dynamic erruptive events modelled by
time-dependent magnetohydrodynamic (MHD) equations. The jet structure comes
through the temporally self-similar solutions in two-dimensional axisymmetric
spherical geometry. The two-dimensional magnetic field is solved in the finite
plasma pressure regime, or finite regime, and it is described by an
equation where plasma pressure plays the role of an eigenvalue. This allows a
structure of magnetic lobes in space, among which the polar axis lobe is
strongly peaked in intensity and collimated in angular spread comparing to the
others. For this reason, the polar lobe overwhelmes the other lobes, and a jet
structure arises in the polar direction naturally. Furthermore, within each
magnetic lobe in space, there are small secondary regions with closed
two-dimensional field lines embedded along this primary lobe. In these embedded
magnetic toroids, plasma pressure and mass density are much higher accordingly.
These are termed as secondary plasmoids. The magnetic field lines in these
secondary plasmoids circle in alternating sequence such that adjacent plasmoids
have opposite field lines. In particular, along the polar primary lobe, such
periodic plasmoid structure happens to be compatible with radio observations
where islands of high radio intensities are mapped
Stability and structure of analytical MHD jet formation models with a finite outer disk radius
(Abridged) Finite radius accretion disks are a strong candidate for launching
astrophysical jets from their inner parts and disk-winds are considered as the
basic component of such magnetically collimated outflows. The only available
analytical MHD solutions for describing disk-driven jets are those
characterized by the symmetry of radial self-similarity. Radially self-similar
MHD models, in general, have two geometrical shortcomings, a singularity at the
jet axis and the non-existence of an intrinsic radial scale, i.e. the jets
formally extend to radial infinity. Hence, numerical simulations are necessary
to extend the analytical solutions towards the axis and impose a physical
boundary at finite radial distance. We focus here on studying the effects of
imposing an outer radius of the underlying accreting disk (and thus also of the
outflow) on the topology, structure and variability of a radially self-similar
analytical MHD solution. The initial condition consists of a hybrid of an
unchanged and a scaled-down analytical solution, one for the jet and the other
for its environment. In all studied cases, we find at the end steady
two-component solutions.Comment: 14 pages, 15 figures, accepted for publication in A &
On the Phenomenology of Hydrodynamic Shear Turbulence
The question of a purely hydrodynamic origin of turbulence in accretion disks
is reexamined, on the basis of a large body of experimental and numerical
evidence on various subcritical (i.e., linearly stable) hydrodynamic flows.
One of the main points of this paper is that the length scale and velocity
fluctuation amplitude which are characteristic of turbulent transport in these
flows scale like , where is the minimal Reynolds number for
the onset of fully developed turbulence. From this scaling, a simple
explanation of the dependence of with relative gap width in subcritical
Couette-Taylor flows is developed. It is also argued that flows in the shearing
sheet limit should be turbulent, and that the lack of turbulence in all such
simulations performed to date is most likely due to a lack of resolution, as a
consequence of the effect of the Coriolis force on the large scale fluctuations
of turbulent flows.
These results imply that accretion flows should be turbulent through
hydrodynamic processes. If this is the case, the Shakura-Sunyaev
parameter is constrained to lie in the range in accretion
disks, depending on unknown features of the mechanism which sustains
turbulence. Whether the hydrodynamic source of turbulence is more efficient
than the MHD one where present is an open question.Comment: 31 pages, 3 figures. Accepted for publication in Ap
Testing Spallation Processes With Beryllium and Boron
The nucleosynthesis of Be and B by spallation processes provides unique
insight into the origin of cosmic rays. Namely, different spallation schemes
predict sharply different trends for the growth of LiBeB abundances with
respect to oxygen. ``Primary'' mechanisms predict BeB O, and are well
motivated by the data if O/Fe is constant at low metallicity. In contrast,
``secondary'' mechanisms predict BeB O and are consistent with
the data if O/Fe increases towards low metallicity as some recent data suggest.
Clearly, any primary mechanism, if operative, will dominate early in the
history of the Galaxy. In this paper, we fit the BeB data to a two-component
scheme which includes both primary and secondary trends. In this way, the data
can be used to probe the period in which primary mechanisms are effective. We
analyze the data using consistent stellar atmospheric parameters based on
Balmer line data and the continuum infrared flux. Results depend sensitively on
Pop II O abundances and, unfortunately, on the choice of stellar parameters.
When using recent results which show O/Fe increasing toward lower metallicity,
a two-component Be-O fits indicates that primary and secondary components
contribute equally at [O/H] = -1.8 for Balmer line data; and
[O/H] = -1.4 to -1.8 for IRFM. We apply these constraints to recent
models for LiBeB origin. The Balmer line data does not show any evidence for
primary production. On the other hand, the IRFM data does indicate a preference
for a two-component model, such as a combination of standard GCR and
metal-enriched particles accelerated in superbubbles. These conclusions rely on
a detailed understanding of the abundance data including systematic effects
which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe
Towards a New Standard Theory for Astrophysical Disk Accretion
We briefly review recent developments in black hole accretion disk theory,
placing new emphasis on the vital role played by magnetohydrodynamic (MHD)
stresses in transporting angular momentum. The apparent universality of
accretion-related outflow phenomena is a strong indicator that vertical
transport of angular momentum by large-scale MHD torques is important and may
even dominate radial transport by small-scale MHD turbulence. This leads to an
enhanced overall rate of angular momentum transport and allows accretion of
matter to proceed at an interesting rate. Furthermore, we argue that when
vertical transport is important, the radial structure of the accretion disk is
modified and this affects the disk emission spectrum. We present a simple model
demonstrating that energetic, magnetically-driven outflows give rise to a disk
spectrum that is dimmer and redder than a standard accretion disk accreting at
the same rate. We briefly discuss the implications of this key result for
accreting black holes in different astrophysical systems.Comment: Accepted for publication as brief review in Mod. Phys. Let.
VELO Module Production - Laser Test and Noise Analysis
This note describes the algorithms used to detect problems by analyzing datasets taken at different stages of module production using the hybrid readout systems
Waves and Instabilities in Accretion Disks: MHD Spectroscopic Analysis
A complete analytical and numerical treatment of all magnetohydrodynamic
waves and instabilities for radially stratified, magnetized accretion disks is
presented. The instabilities are a possible source of anomalous transport.
While recovering results on known hydrodynamicand both weak- and strong-field
magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a
realistic accretion disk model demonstrates a much richer variety of
instabilities accessible to the plasma than previously realized. We show that
both weakly and strongly magnetized accretion disks are prone to strong
non-axisymmetric instabilities.The ability to characterize all waves arising in
accretion disks holds great promise for magnetohydrodynamic spectroscopic
analysis.Comment: FOM-Institute for plasma physics "Rijnhuizen", Nieuwegein, the
Netherlands 12 pages, 3 figures, Accepted for publication in ApJ
Development of planar pixel modules for the ATLAS high luminosity LHC tracker upgrade
The high-luminosity LHC will present significant challenges for tracking systems. ATLAS is preparing to upgrade the entire tracking system, which will include a significantly larger pixel detector. This paper reports on the development of large area planar detectors for the outer pixel layers and the pixel endcaps. Large area sensors have been fabricated and mounted onto 4 FE-I4 readout ASICs, the so-called quad-modules, and their performance evaluated in the laboratory and testbeam. Results from characterisation of sensors prior to assembly, experience with module assembly, including bump-bonding and results from laboratory and testbeam studies are presented
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