3,749 research outputs found
The Magellanic System: What have we learnt from FUSE?
I review some of the findings on the Magellanic System produced by the Far
Ultraviolet Spectroscopic Explorer (FUSE) during and after its eight years of
service. The Magellanic System with its high-velocity complexes provides a
nearby laboratory that can be used to characterize phenomena that involve
interaction between galaxies, infall and outflow of gas and metals in galaxies.
These processes are crucial for understanding the evolution of galaxies and the
intergalactic medium. Among the FUSE successes I highlight are the coronal gas
about the LMC and SMC, and beyond in the Stream, the outflows from these
galaxies, the discovery of molecules in the diffuse gas of the Stream and the
Bridge, an extremely sub-solar and sub-SMC metallicity of the Bridge, and a
high-velocity complex between the Milky Way and the Clouds.Comment: A contributed paper to the FUSE Annapolis Conference "Future
Directions in Ultraviolet Spectroscopy.", 5 pages. To appear as an AIP
Conference Proceedin
Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110
We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110
(z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of
intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11.
The O VI systems unambiguously trace hot gas only in one case. For the 4 other
O VI systems, photoionization and collisional ionization models are viable
options to explain the observed column densities of the O VI and the other
ions. If the O VI systems are mostly photoionized, only a fraction of the
observed O VI will contribute to the baryonic density of the warm-hot ionized
medium (WHIM) along this line of sight. Combining our results with previous
ones, we show that there is a general increase of N(O VI) with increasing b(O
VI). Cooling flow models can reproduce the N-b distribution but fail to
reproduce the observed ionic ratios. A comparison of the number of O I, O II, O
III, O IV, and O VI systems per unit redshift show that the low-z IGM is more
highly ionized than weakly ionized. We confirm that photoionized O VI systems
show a decreasing ionization parameter with increasing H I column density. O VI
absorbers with collisional ionization/photoionization degeneracy follow this
relation, possibly suggesting that they are principally photoionized. We find
that the photoionized O VI systems in the low redshift IGM have a median
abundance of 0.3 solar. We do not find additional Ne VIII systems other than
the one found by Savage et al., although our sensitivity should have allowed
the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE
applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6
K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line
transitions.Comment: Accepted for publication in the ApJS. Full resolution figures
available at
http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd
Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission
Far Ultraviolet Spectroscopic Explorer observations are presented for
WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a
distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40
per 20 km/s resolution element and cover the wavelength range 905-1187 \AA.
LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II,
N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an
ionized nitrogen fraction of > 0.23. We determine the ratio (2). Assuming a standard interstellar
oxygen abundance, we derive . Using the
value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I
ratio is (2).Comment: accepted for publication in the ApJ
3D simulations of Einstein's equations: symmetric hyperbolicity, live gauges and dynamic control of the constraints
We present three-dimensional simulations of Einstein equations implementing a
symmetric hyperbolic system of equations with dynamical lapse. The numerical
implementation makes use of techniques that guarantee linear numerical
stability for the associated initial-boundary value problem. The code is first
tested with a gauge wave solution, where rather larger amplitudes and for
significantly longer times are obtained with respect to other state of the art
implementations. Additionally, by minimizing a suitably defined energy for the
constraints in terms of free constraint-functions in the formulation one can
dynamically single out preferred values of these functions for the problem at
hand. We apply the technique to fully three-dimensional simulations of a
stationary black hole spacetime with excision of the singularity, considerably
extending the lifetime of the simulations.Comment: 21 pages. To appear in PR
The discrete energy method in numerical relativity: Towards long-term stability
The energy method can be used to identify well-posed initial boundary value
problems for quasi-linear, symmetric hyperbolic partial differential equations
with maximally dissipative boundary conditions. A similar analysis of the
discrete system can be used to construct stable finite difference equations for
these problems at the linear level. In this paper we apply these techniques to
some test problems commonly used in numerical relativity and observe that while
we obtain convergent schemes, fast growing modes, or ``artificial
instabilities,'' contaminate the solution. We find that these growing modes can
partially arise from the lack of a Leibnitz rule for discrete derivatives and
discuss ways to limit this spurious growth.Comment: 18 pages, 22 figure
Mode coupling in the nonlinear response of black holes
We study the properties of the outgoing gravitational wave produced when a
non-spinning black hole is excited by an ingoing gravitational wave.
Simulations using a numerical code for solving Einstein's equations allow the
study to be extended from the linearized approximation, where the system is
treated as a perturbed Schwarzschild black hole, to the fully nonlinear regime.
Several nonlinear features are found which bear importance to the data analysis
of gravitational waves. When compared to the results obtained in the linearized
approximation, we observe large phase shifts, a stronger than linear generation
of gravitational wave output and considerable generation of radiation in
polarization states which are not found in the linearized approximation. In
terms of a spherical harmonic decomposition, the nonlinear properties of the
harmonic amplitudes have simple scaling properties which offer an economical
way to catalog the details of the waves produced in such black hole processes.Comment: 17 pages, 20 figures, abstract and introduction re-writte
AMR, stability and higher accuracy
Efforts to achieve better accuracy in numerical relativity have so far
focused either on implementing second order accurate adaptive mesh refinement
or on defining higher order accurate differences and update schemes. Here, we
argue for the combination, that is a higher order accurate adaptive scheme.
This combines the power that adaptive gridding techniques provide to resolve
fine scales (in addition to a more efficient use of resources) together with
the higher accuracy furnished by higher order schemes when the solution is
adequately resolved. To define a convenient higher order adaptive mesh
refinement scheme, we discuss a few different modifications of the standard,
second order accurate approach of Berger and Oliger. Applying each of these
methods to a simple model problem, we find these options have unstable modes.
However, a novel approach to dealing with the grid boundaries introduced by the
adaptivity appears stable and quite promising for the use of high order
operators within an adaptive framework
Dominance of a single topological sector in gauge theory on non-commutative geometry
We demonstrate a striking effect of non-commutative (NC) geometry on
topological properties of gauge theory by Monte Carlo simulations. We study 2d
U(1) NC gauge theory for various boundary conditions using a new finite-matrix
formulation proposed recently. We find that a single topological sector
dictated by the boundary condition dominates in the continuum limit. This is in
sharp contrast to the results in commutative space-time based on lattice gauge
theory, where all topological sectors appear with certain weights in the
continuum limit. We discuss possible implications of this effect in the context
of string theory compactifications and in field theory contexts.Comment: 16 pages, 27 figures, typos correcte
On "many black hole" space-times
We analyze the horizon structure of families of space times obtained by
evolving initial data sets containing apparent horizons with several connected
components. We show that under certain smallness conditions the outermost
apparent horizons will also have several connected components. We further show
that, again under a smallness condition, the maximal globally hyperbolic
development of the many black hole initial data constructed by Chrusciel and
Delay, or of hyperboloidal data of Isenberg, Mazzeo and Pollack, will have an
event horizon, the intersection of which with the initial data hypersurface is
not connected. This justifies the "many black hole" character of those
space-times.Comment: several graphic file
A simple proof of the Markoff conjecture for prime powers
We give a simple and independent proof of the result of Jack Button and Paul
Schmutz that the Markoff conjecture on the uniqueness of the Markoff triples
(a,b,c), where a, b, and c are in increasing order, holds whenever is a
prime power.Comment: 5 pages, no figure
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