893 research outputs found
Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines
The similarity of quasar line spectra has been taken as an indication that
the emission line clouds have preferred parameters, suggesting that the
environment is subject to a fine tuning process. We show here that the observed
spectrum is a natural consequence of powerful selection effects. We computed a
large grid of photoionization models covering the widest possible range of
cloud gas density and distance from the central continuum source. For each line
only a narrow range of density and distance from the continuum source results
in maximum reprocessing efficiency, corresponding to ``locally
optimally-emitting clouds'' (LOC). These parameters depend on the ionization
and excitation potentials of the line, and its thermalization density. The mean
QSO line spectrum can be reproduced by simply adding together the full family
of clouds, with an appropriate covering fraction distribution. The observed
quasar spectrum is a natural consequence of the ability of various clouds to
reprocess the underlying continuum, and can arise in a chaotic environment with
no preferred pressure, gas density, or ionization parameter.Comment: 9 pages including 1 ps figure. LaTeX format using aaspp4.st
Alignment and preliminary outcomes of an ELT-size instrument to a very large telescope: LINC-NIRVANA at LBT
LINC-NIRVANA (LN) is a high resolution, near infrared imager that uses a
multiple field-of-view, layer-oriented, multi-conjugate AO system, consisting
of four multi-pyramid wavefront sensors (two for each arm of the Large
Binocular Telescope, each conjugated to a different altitude). The system
employs up to 40 star probes, looking at up to 20 natural guide stars
simultaneously. Its final goal is to perform Fizeau interferometric imaging,
thereby achieving ELT-like spatial resolution (22.8 m baseline resolution). For
this reason, LN is also equipped with a fringe tracker, a beam combiner and a
NIR science camera, for a total of more than 250 optical components and an
overall size of approximately 6x4x4.5 meters. This paper describes the
tradeoffs evaluated in order to achieve the alignment of the system to the
telescope. We note that LN is comparable in size to planned ELT
instrumentation. The impact of such alignment strategies will be compared and
the selected procedure, where the LBT telescope is, in fact, aligned to the
instrument, will be described. Furthermore, results coming from early
night-time commissioning of the system will be presented.Comment: 8 pages, 6 pages, AO4ELT5 Proceedings, 201
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
Run Scenarios for the Linear Collider
Scenarios are developed for runs at a Linear Collider, in the case that there
is a rich program of new physics.Comment: 12 pages, 10 tables, Latex; Snowmass 2001 plenary repor
Free Energy Self-Averaging in Protein-Sized Random Heteropolymers
Current theories of heteropolymers are inherently macrpscopic, but are
applied to folding proteins which are only mesoscopic. In these theories, one
computes the averaged free energy over sequences, always assuming that it is
self-averaging -- a property well-established only if a system with quenched
disorder is macroscopic. By enumerating the states and energies of compact 18,
27, and 36mers on a simplified lattice model with an ensemble of random
sequences, we test the validity of the self-averaging approximation. We find
that fluctuations in the free energy between sequences are weak, and that
self-averaging is a valid approximation at the length scale of real proteins.
These results validate certain sequence design methods which can exponentially
speed up computational design and greatly simplify experimental realizations.Comment: 4 pages, 3 figure
Efficient Quantum Computing of Complex Dynamics
We propose a quantum algorithm which uses the number of qubits in an optimal
way and efficiently simulates a physical model with rich and complex dynamics
described by the quantum sawtooth map. The numerical study of the effect of
static imperfections in the quantum computer hardware shows that the main
elements of the phase space structures are accurately reproduced up to a time
scale which is polynomial in the number of qubits. The errors generated by
these imperfections are more dangerous than the errors of random noise in gate
operations.Comment: revtex, 4 pages, 3 figure
Sfermion Precision Measurements at a Linear Collider
At future e+- e- linear colliders, the event rates and clean signals of
scalar fermion production - in particular for the scalar leptons - allow very
precise measurements of their masses and couplings and the determination of
their quantum numbers. Various methods are proposed for extracting these
parameters from the data at the sfermion thresholds and in the continuum. At
the same time, NLO radiative corrections and non-zero width effects have been
calculated in order to match the experimental accuracy. The substantial mixing
expected for the third generation sfermions opens up additional opportunities.
Techniques are presented for determining potential CP-violating phases and for
extracting tan(beta) from the stau sector, in particular at high values. The
consequences of possible large mass differences in the stop and sbottom system
are explored in dedicated analyses.Comment: Expanded version of contributions to the proceedings of ICHEP'02
(Amsterdam) and LCWS 2002 (Jeju Island
The stability of axisymmetric Taylor-Couette flow in hydromagnetics
The linear marginal instability of an axisymmetric MHD Taylor-Couette flow of
infinite vertical extension is considered. The dependence of the flow stability
on magnetic Prandtl number, Pm, and gap-width between rotating cylinders is
investigated. There is an unstability for flows with resting outer cylinder
even without a magnetic field. Nevertheless, there are solutions with smaller
critical Reynolds numbers for certain (weak) magnetic fields. These solutions,
however, exist for not too small Pm or large enough gap. For hydrodynamically
stable flow, we found that a magnetic field amplitude always exsits where the
critical Reynolds number is minimal. In all calculated cases the minimal
critical Reynolds numbers are running with 1/Pm for small Pm and the critical
Reynolds numbers exceed value of 10^6 for Pm of sodium (10^{-5}) or gallium
(10^{-6}).Comment: 8 pages, 17 figure
Ingot-like class of wavefront sensors for laser guide stars
Context. Full sky coverage adaptive optics (AO) on extremely large telescopes requires the adoption of several laser guide stars as references. With such large apertures, the apparent elongation of the beacons is absolutely significant. With a few exceptions, wavefront sensors (WFSs) designed for natural guide stars can be adapted and used in suboptimal mode in this context. Aims. We analyse and describe the geometrical properties of a class of WFSs that are specifically designed to deal with laser guide stars propagated from a location in the immediate vicinity of the telescope aperture. Methods. We describe, in three dimensions, the loci where the light of the laser guide stars would focus in the focal volume located behind the focal plane where astronomical objects are reimaged. We also describe the properties of several types of optomechanical devices that act as perturbers for this new class of pupil plane sensors, through refraction and reflections. We refer to these as ingot WFSs. Results. We provide the recipes both for the most reasonably complex version of these WFSs, with six pupils and, for the simplest one, only three pupils. Both of them are referred to on the basis of the European Extremely Large Telescope (ELT) case. We outlined elements that are meant to give a qualitative idea of how the sensitivity of this new class of sensors compares to conventional ones. Conclusions. We present a new class of WFSs, based on an extension to the case of elongated sources at a finite distance of the pyramid WFS. We point out which advantages of the pyramid can be retained and how it may be adopted to optimize the sensing procedure
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