4,439 research outputs found
Silicon Sensors implemented on p-type substrates for high radiation resistance applications
Silicon based micropattern detectors are essential elements of modern high
energy physics experiments. Cost effectiveness and high radiation resistance
are two important requirements for technologies to be used in inner tracking
devices. Processes based on p-type substrates have very strong appeal for these
applications. Recent results and prototype efforts under way are reviewed.Comment: 7 pages, 2 figures; invited paper at Vertex 2006, Perugia, Italy,
september 200
Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory
Multidimensional magneto-hydrodynamical (MHD) simulations coupled with
stochastic differential equations (SDEs) adapted to test particle acceleration
and transport in complex astrophysical flows are presented. The numerical
scheme allows the investigation of shock acceleration, adiabatic and radiative
losses as well as diffusive spatial transport in various diffusion regimes. The
applicability of SDEs to astrophysics is first discussed in regards to the
different regimes and the MHD code spatial resolution. The procedure is then
applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets.
The ability of SDE to reproduce analytical solutions of the
diffusion-convection equation for electrons is tested through the incorporation
of an increasing number of effects: shock acceleration, spatially dependent
diffusion coefficients and synchrotron losses. The SDEs prove to be efficient
in various shock configuration occurring in the inner jet during the
development of the Kelvin-Helmholtz instability. The particle acceleration in
snapshots of strong single and multiple shock acceleration including realistic
spatial transport is treated. In chaotic magnetic diffusion regime, turbulence
levels around are found to
be the most efficient to enable particles to reach the highest energies. The
spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows),
does not exhibit a power-law shape due to transverse momentum dependent
escapes. Out of this range, the confinement is not so efficient and the
spectrum cut-off above few hundreds of GeV, questioning the Chandra
observations of X-ray knots as being synchrotron radiation. The extension to
full time dependent simulations to X-ray extragalactic jets is discussed.Comment: Astronomy & Astrophysics (in press), 18 page
On fan-shaped cold MHD winds from Keplerian accretion discs
We investigate under which conditions cold, fan-shaped winds can be steadily
launched from thin (Keplerian) accretion discs. Such winds are
magneto-centrifugal winds launched from a thin annulus in the disc, along open
magnetic field lines that fan out above the disc. In principle, such winds
could be found in two situations: (1) at the interface between an inner Jet
Emitting Disc, which is itself powering magneto-centrifugally driven winds, and
an outer standard accretion disc; (2) at the interface between an inner closed
stellar magnetosphere and the outer standard accretion disc. We refer to
Terminal or T-winds to the former kind and to Magnetospheric or M-winds to the
latter.
The full set of resistive and viscous steady state MHD equations are analyzed
for the disc (the annulus), which allow us to derive general expressions valid
for both configurations. We find that, under the framework of our analysis, the
only source of energy able to power any kind of fan-shaped winds is the viscous
transport of rotational energy coming below the inner radii. Using standard
local prescriptions for the anomalous (turbulent) transport of angular
momentum and magnetic fields in the disc, we derive the strength of the
transport coefficients that are needed to steadily sustain the global
configuration. It turns out that, in order for these winds to be dynamically
relevant and explain observed jets, the disc coefficients must be far much
larger than values expected from current knowledge of turbulence occurring
inside proto-stellar discs.
Either the current view on MHD turbulence must be deeply reconsidered or
steady-state fan-shaped winds are never realized in Nature. The latter
hypothesis seems to be consistent with current numerical simulations.Comment: Among several possibilites, this paper addresses also the case of the
X-wind Accepted for publication in MNRA
Jet launching and field advection in quasi-Keplerian discs
The fact that self-confined jets are observed around black holes, neutron
stars and young forming stars points to a jet launching mechanism independent
of the nature of the central object, namely the surrounding accretion disc. The
properties of Jet Emitting Discs (JEDs) are briefly reviewed. It is argued
that, within an alpha prescription for the turbulence (anomalous viscosity and
diffusivity), the steady-state problem has been solved. Conditions for
launching jets are very stringent and require a large scale magnetic field
close to equipartition with the total (gas and radiation) pressure. The
total power feeding the jets decreases with the disc thickness: fat ADAF-like
structures with cannot drive super-Alfv\'enic jets. However, there
exist also hot, optically thin JED solutions that would be observationally very
similar to ADAFs.
Finally, it is argued that variations in the large scale magnetic field
is the second parameter required to explain hysteresis cycles seen in LMXBs
(the first one would be ).Comment: 4 pages, 1 figure, proceedings of IAU 275 "Jets at all scales"
(Gustavo E. Romero, Rashid A. Sunyaev and Tomaso Belloni, eds
Lithium-Beryllium-Boron and Oxygen in the early Galaxy
Oxygen is a much better evolutionary index than iron to follow the history of
Lithium-Beryllium-Boron (LiBeB) since it is the main producer of these light
elements at least in the early Galaxy. The O-Fe relation is crucial to the
determination of the exact physical process responsible for the LiBeB
production. Calculated nucleosynthetic yields of massive stars, estimates of
the energy cost of Be production, and above all recent observations reported in
this meeting seem to favor a mechanism in which fast nuclei enriched into He, C
and O arising from supernovae are accelerated in superbubbles and fragment on H
and He in the interstellar medium.Comment: Invited Review, IAU, JD8, Manchester, August 2000, to be published in
Highlights of Astronom
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