445 research outputs found
Profile Monitor SEM's for the NuMI Beam at FNAL
The Neutrinos at the Main Injector (NuMI) project will extract 120 GeV
protons from the FNAL Main Injector in 8.56usec spills of 4E13 protons every
1.9 sec. We have designed secondary emission monitor (SEM) detectors to measure
beam profile and halo along the proton beam transport line. The SEM?s are Ti
foils 5um in thickness segmented in either 1?mm or 0.5?mm pitch strips,
resulting in beam loss ~5E-6. We discuss aspects of the mechanical design,
calculations of expected beam heating, and results of a beam test at the 8 GeV
transport line to MiniBoone at FNAL.Comment: to appear in proceedings of 11th Beam Instrumentation Workshop, Oak
Ridge, T
Anisotropic winds from close-in extra-solar planets
We present two-dimensional hydrodynamic models of thermally driven winds from
highly irradiated, close-in extra-solar planets. We adopt a very simple
treatment of the radiative heating processes at the base of the wind, and
instead focus on the differences between the properties of outflows in
multidimensions in comparison to spherically symmetric models computed with the
same methods. For hot (T > 2 x 10^{4} K) or highly ionized gas, we find strong
(supersonic) polar flows are formed above the planet surface which produce weak
shocks and outflow on the night-side. In comparison to a spherically symmetric
wind with the same parameters, the sonic surface on the day-side is much closer
to the planet surface in multidimensions, and the total mass loss rate is
reduced by almost a factor of four. We also compute the steady-state structure
of interacting planetary and stellar winds. Both winds end in a termination
shock, with a parabolic contact discontinuity which is draped over the planet
separating the two shocked winds. The planetary wind termination shock and the
sonic surface in the wind are well separated, so that the mass loss rate from
the planet is essentially unaffected. However, the confinement of the planetary
wind to the small volume bounded by the contact discontinuity greatly enhances
the column density close to the planet, which might be important for the
interpretation of observations of absorption lines formed by gas surrounding
transiting planets.Comment: ApJ accepte
MHD simulations of the collapsar model for GRBs
We present results from axisymmetric, time-dependent magnetohydrodynamic
(MHD) simulations of the collapsar model for gamma-ray bursts. Our main
conclusion is that, within the collapsar model, MHD effects alone are able to
launch, accelerate and sustain a strong polar outflow. We also find that the
outflow is Poynting flux-dominated, and note that this provides favorable
initial conditions for the subsequent production of a baryon-poor fireball.Comment: 4 pages, to appear in proceedings of "2003 GRB Conference" (Santa Fe,
NM, September 8-12, 2003), needs aipprocs LaTeX class, movies are available
at http://rocinante.colorado.edu/~proga
Radiation-Driven Outflows in Active Galactic Nuclei
We review the results from multi-dimensional, time-dependent simulations of
gas dynamics in AGN. We will focus on two types of outflows powered by
radiation emitted from the AGN central engine: (i) outflows driven from the
innermost part of an accretion disk and (2) outflows driven from a large-scale
inflow that is likely the main supplier of material to the central engine. We
discuss the relevance of both types of outflows to the so-called AGN feedback
problem. However, the AGN feedback should not be considered separately from the
AGN physics. Therefore, we also discuss the issue whether the properties of the
same outflows are consistent with the gas properties in broad- and narrow-line
regions.Comment: 9 pages, 5 figures, in Recent Directions In Astrophysical
Quantitative Spectroscopy And Radiation Hydrodynamics: Proceedings of the
International Conference in Honor of Dimitri Mihalas for His Lifetime
Scientific Contributions on the Occasion of His 70th Birthday (AIP Conference
Proceedings 1171
A non-hydrodynamical model for acceleration of line-driven winds in Active Galactic Nuclei
We present a study of the acceleration phase of line-driven winds in AGNs, in
order to examine the physical conditions for the existence of such winds for a
wide variety of initial conditions. We built a simple and fast non-hydrodynamic
model, QWIND, where we assume that a wind is launched from the accretion disc
at supersonic velocities of the order of a few 10^2 km/s and we concentrate on
the subsequent supersonic phase. We show that this model can produce a wind
with terminal velocities of the order of 10^4 km/s. There are three zones in
the wind, only the middle one of which can launch a wind: in the inner zone the
wind is too ionized and so experiences only the Compton radiation force which
is not effective in accelerating gas. This inner failed wind however plays an
important role in shielding the next zone, lowering the ionization parameter
there. In the middle zone the lower ionization of the gas leads to a much
larger radiation force and the gas achieves escape velocity This middle zone is
quite thin (about 100 gravitational radii). The outer, third, zone is shielded
from the UV radiation by the central wind zone and so does not achieve a high
enough acceleration to reach escape velocity. We also describe a simple
analytic approximation of our model, based on neglecting the effects of gravity
during the acceleration phase. This analytic approach is in agreement with the
results of the numerical code, and is a powerful way to check whether a
radiation driven wind can be accelerated with a given set of initial
parameters. Our analytical analysis and the fast QWIND model are in agreement
with more complex hydrodynamical models, and allow an exploration of the
dependence of the wind properties for a wide set of initial parameters: black
hole mass, Eddington ratio, initial density profile, X-ray to UV ratio.Comment: 15 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
Photoionized features in the X-ray spectrum of Ex Hydrae
We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae ( EX Hya). In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely, O VIII lambda 18.97, Mg XII lambda 8.42, Si XIV lambda 6.18, and Fe XVII lambda 16.78. The broad and narrow components have widths of similar to 1600 km s(-1) and similar to 150 km s(-1), respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions such as O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables
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