1,193 research outputs found
Michelson Interferometry with the Keck I Telescope
We report the first use of Michelson interferometry on the Keck I telescope
for diffraction-limited imaging in the near infrared JHK and L bands. By using
an aperture mask located close to the f/25 secondary, the 10 m Keck primary
mirror was transformed into a separate-element, multiple aperture
interferometer. This has allowed diffraction-limited imaging of a large number
of bright astrophysical targets, including the geometrically complex dust
envelopes around a number of evolved stars. The successful restoration of these
images, with dynamic ranges in excess of 200:1, highlights the significant
capabilities of sparse aperture imaging as compared with more conventional
filled-pupil speckle imaging for the class of bright targets considered here.
In particular the enhancement of the signal-to-noise ratio of the Fourier data,
precipitated by the reduction in atmospheric noise, allows high fidelity
imaging of complex sources with small numbers of short-exposure images relative
to speckle. Multi-epoch measurements confirm the reliability of this imaging
technique and our whole dataset provides a powerful demonstration of the
capabilities of aperture masking methods when utilized with the current
generation of large-aperture telescopes. The relationship between these new
results and recent advances in interferometry and adaptive optics is briefly
discussed.Comment: Accepted into Publications of the Astronomical Society of the
Pacific. To appear in vol. 112. Paper contains 10 pages, 8 figure
Integration of the atmospheric fluctuations in a dual-field optical interferometer: the short exposure regime
Spatial phase-referencing in dual-field optical interferometry is
reconsidered. Our analysis is based on the 2-sample variance of the
differential phase between target and reference star. We show that averaging
over time of the atmospheric effects depends on this 2-sample phase variance
(Allan variance) rather than on the true variance. The proper expression for
fringe smearing beyond the isoplanatic angle is derived. With simulations of
atmospheric effects, based on a Paranal turbulence model, we show how the
performances of a dual-field optical interferometer can be evaluated in a
diagram 'separation angle' versus 'magnitude of faint object'. In this diagram,
a domain with short exposure is found to be most useful for interferometry,
with about the same magnitude limits in the H and K bands. With star counts
from a Galaxy model, we evaluate the sky coverage for differential astrometry
and detection of exoplanets, i.e. likelihood of faint reference stars in the
vicinity of a bright target. With the 2mass survey, we evaluate sky coverage
for phase-referencing, i.e. avaibility of a bright enough star for main delay
tracking in the vicinity of any target direction.Comment: 9 pages, 8 figures, accepted for publication in A&
Unifying gauge couplings at the string scale
Using the current precision electroweak data, we look for the minimal
particle content which is necessary to add to the standard model in order to
have a complete unification of gauge couplings and gravity at the weakly
coupled heterotic string scale. We find that the addition of a vector-like
fermion at an intermediate scale and a non-standard hypercharge normalization
are in general sufficient to achieve this goal at two-loop level. Requiring the
extra matter scale to be below the TeV scale, it is found that the addition of
three vector-like fermion doublets with a mass around 700 GeV yields a perfect
string-scale unification, provided that the affine levels are , as in the string-GUT. Furthermore,
if supersymmetry is broken at the unification scale, the Higgs mass is
predicted in the range 125 GeV - 170 GeV, depending on the precise values of
the top quark mass and parameter.Comment: 11 pages, 4 eps figures, using jpconf style, talk given at CORFU2005,
RTN meeting ``The Quest for Unification: Theory Confronts Experiment'', 11 -
18 September 2005, Corfu, Greec
T-Duality in 2-D Integrable Models
The non-conformal analog of abelian T-duality transformations relating pairs
of axial and vector integrable models from the non abelian affine Toda family
is constructed and studied in detail.Comment: 14 pages, Latex, v.2 misprints corrected, reference added, to appear
in J. Phys.
Supersymmetric Quantum Corrections and Poisson-Lie T-Duality
The quantum actions of the (4,4) supersymmetric non-linear sigma model and
its dual in the Abelian case are constructed by using the background superfield
method. The propagators of the quantum superfield and its dual and the gauge
fixing actions of the original and dual (4,4) supersymmetric sigma models are
determined. On the other hand, the BRST transformations are used to obtain the
quantum dual action of the (4,4) supersymmetric non-linear sigma model in the
sense of Poisson-Lie T-dualityComment: 18 page
Supersymmetric String Waves
We present plane-wave-type solutions of the lowest order superstring
effective action which have unbroken space-time supersymmetries. They describe
dilaton, axion and gauge fields in a stringy generalization of the Brinkmann
metric. Some conspiracy between the metric and the axion field is required. We
show that there exists a special class of these solutions, for which
stringy corrections to the effective on-shell action, to the
equations of motion (and therefore to the solutions themselves), and to the
supersymmetry transformations vanish. We call these solutions supersymmetric
string waves (SSW).Comment: 19 pages, LaTeX, SU-ITP-92-30 and UG-10/9
Planet Formation Imager (PFI): Introduction and Technical Considerations
Complex non-linear and dynamic processes lie at the heart of the planet
formation process. Through numerical simulation and basic observational
constraints, the basics of planet formation are now coming into focus. High
resolution imaging at a range of wavelengths will give us a glimpse into the
past of our own solar system and enable a robust theoretical framework for
predicting planetary system architectures around a range of stars surrounded by
disks with a diversity of initial conditions. Only long-baseline interferometry
can provide the needed angular resolution and wavelength coverage to reach
these goals and from here we launch our planning efforts. The aim of the
"Planet Formation Imager" (PFI) project is to develop the roadmap for the
construction of a new near-/mid-infrared interferometric facility that will be
optimized to unmask all the major stages of planet formation, from initial dust
coagulation, gap formation, evolution of transition disks, mass accretion onto
planetary embryos, and eventual disk dispersal. PFI will be able to detect the
emission of the cooling, newly-formed planets themselves over the first 100
Myrs, opening up both spectral investigations and also providing a vibrant look
into the early dynamical histories of planetary architectures. Here we
introduce the Planet Formation Imager (PFI) Project
(www.planetformationimager.org) and give initial thoughts on possible facility
architectures and technical advances that will be needed to meet the
challenging top-level science requirements.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June
2014, Paper ID 9146-35, 10 pages, 2 Figure
First Results from the CHARA Array. II. A Description of the Instrument
The CHARA Array is a six 1-m telescope optical/IR interferometric array
located on Mount Wilson California, designed and built by the Center for High
Angular Resolution Astronomy of Georgia State University. In this paper we
describe the main elements of the Array hardware and software control systems
as well as the data reduction methods currently being used. Our plans for
upgrades in the near future are also described
Exact Models of Extremal Dyonic 4D Black Hole Solutions of Heterotic String Theory
Families of exact supersymmetric conformal field theories of
magnetically and electrically charged extremal 4D black hole solutions of
heterotic string theory are presented. They are constructed using a
supersymmetric WZW model where anomalously embedded
subgroups are gauged. Crucial cancelations of the
anomalies coming from the supersymmetric fermions, the current algebra fermions
and the gauging ensure that there is a consistency of these models at the
quantum level. Various 2D models, which may be considered as building blocks
for extremal 4D constructions, are presented. They generalise the class of 2D
models which might be obtained from gauging and coincide with known
heterotic string backgrounds. The exact conformal field theory presented by
Giddings, Polchinski and Strominger describing the angular sector of the
extremal magnetically charged black hole is a special case of this
construction. An example where the radial and angular theories are mixed
non--trivially is studied in detail, resulting in an extremal dilatonic
Taub--NUT--like dyon.Comment: 42 pages (Plain TEX), IASSNS-HEP-94/20 (Revised version has minor
corrections, references and a note added and is now identical to published
version in Phys Rev D.
Solution--Generating Transformations and the String Effective Action
We study exhaustively the solution-generating transformations (dualities)
that occur in the context of the low-energy effective action of superstring
theory. We first consider target-space duality (``T duality'') transformations
in absence of vector fields. We find that for one isometry the full duality
group is (SO^{\uparrow}(1,1))^{3} x D_{4}, the discrete part (D_{4}) being
non-Abelian. We, then, include non-Abelian Yang--Mills fields and find the
corresponding generalization of the T duality transformations. We study the
\alpha^{\prime} corrections to these transformations and show that the T
duality rules considerably simplify if the gauge group is embedded in the
holonomy group. Next, in the case in which there are Abelian vector fields, we
consider the duality group that includes the transformation introduced by Sen
that rotates among themselves components of the metric, axion and vector field.
Finally we list the duality symmetries of the Type II theories with one
isometry.Comment: latex file, 42 pages (less if you use optional commands) No changes
at all. Resubmited due to mailer problem
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