27,125 research outputs found
The Detectability of Transit Depth Variations due to Exoplanetary Oblateness and Spin Precession
Knowledge of an exoplanet's oblateness and obliquity would give clues about
its formation and internal structure. In principle, a light curve of a
transiting planet bears information about the planet's shape, but previous work
has shown that the oblateness-induced signal will be extremely difficult to
detect. Here we investigate the potentially larger signals due to planetary
spin precession. The most readily detectable effects are transit depth
variations (TV) in a sequence of light curves. For a planet as oblate
as Jupiter or Saturn, the transit depth will undergo fractional variations of
order 1%. The most promising systems are those with orbital periods of
approximately 15--30 days, which is short enough for the precession period to
be less than about 40 years, and long enough to avoid spin-down due to tidal
friction. The detectability of the TV signal would be enhanced by moons
(which would decrease the precession period) or planetary rings (which would
increase the amplitude). The Kepler mission should find several planets for
which precession-induced TV signals will be detectable. Due to modeling
degeneracies, Kepler photometry would yield only a lower bound on oblateness.
The degeneracy could be lifted by observing the oblateness-induced asymmetry in
at least one transit light curve, or by making assumptions about the planetary
interior.Comment: Accepted for publication in The Astrophysical Journa
Information standards to support application and enterprise interoperability for the smart grid
Copyright @ 2012 IEEE.Current changes in the European electricity industry are driven by regulatory directives to reduce greenhouse gas emissions, at the same time as replacing aged infrastructure and maintaining energy security. There is a wide acceptance of the requirement for smarter grids to support such changes and accommodate variable injections from renewable energy sources. However the design templates are still emerging to manage the level of information required to meet challenges such as balancing, planning and market dynamics under this new paradigm. While secure and scalable cloud computing architectures may contribute to supporting the informatics challenges of the smart grid, this paper focuses on the essential need for business alignment with standardised information models such as the IEC Common Information Model (CIM), to leverage data value and control system interoperability. In this paper we present details of use cases being considered by National Grid, the GB transmission system operator for information interoperability in pan-network system management and planning.This study is financially supported by the National Grid, UK
Smart grid interoperability use cases for extending electricity storage modeling within the IEC Common Information Model
Copyright @ 2012 IEEEThe IEC Common Information Model (CIM) is recognized as a core standard, supporting electricity transmission system interoperability. Packages of UML classes make up its domain ontology to enable a standardised abstraction of network topology and proprietary power system models. Since the early days of its design, the CIM has grown to reflect the widening scope and detail of utility information use cases as the desire to interoperate between a greater number of systems has increased. The cyber-physical nature of the smart grid places even greater demand upon the CIM to model future scenarios for power system operation and management that are starting to arise. Recent developments of modern electricity networks have begun to implement electricity storage (ES) technologies to provide ancillary balancing services, useful to grid integration of large-scale renewable energy systems. In response to this we investigate modeling of grid-scale electricity storage, by drawing on information use cases for future smart grid operational scenarios at National Grid, the GB Transmission System Operator. We find current structures within the CIM do not accommodate the informational requirements associated with novel ES systems and propose extensions to address this requirement.This study is supported by the UK National Grid and Brunel Universit
Transonic Elastic Model for Wiggly Goto-Nambu String
The hitherto controversial proposition that a ``wiggly" Goto-Nambu cosmic
string can be effectively represented by an elastic string model of exactly
transonic type (with energy density inversely proportional to its tension
) is shown to have a firm mathematical basis.Comment: 8 pages, plain TeX, no figure
Cosmic Vortons and Particle Physics Constraints
We investigate the cosmological consequences of particle physics theories
that admit stable loops of superconducting cosmic string - {\it vortons}.
General symmetry breaking schemes are considered, in which strings are formed
at one energy scale and subsequently become superconducting in a secondary
phase transition at what may be a considerably lower energy scale. We estimate
the abundances of the ensuing vortons, and thereby derive constraints on the
relevant particle physics models from cosmological observations. These
constraints significantly restrict the category of admissible Grand Unified
theories, but are quite compatible with recently proposed effects whereby
superconducting strings may have been formed close to the electroweak phase
transition.Comment: 33 pages, 2 figures, RevTe
Interaction between Faraday rotation and Cotton-Mouton effects in polarimetry modeling for NSTX
The evolution of electromagnetic wave polarization is modeled for propagation
in the major radial direction in the National Spherical Torus Experiment (NSTX)
with retroreflection from the center stack of the vacuum vessel. This modeling
illustrates that the Cotton-Mouton effect-elliptization due to the magnetic
field perpendicular to the propagation direction-is shown to be strongly
weighted to the high-field region of the plasma. An interaction between the
Faraday rotation and Cotton-Mouton effects is also clearly identified.
Elliptization occurs when the wave polarization direction is neither parallel
nor perpendicular to the local transverse magnetic field. Since Faraday
rotation modifies the polarization direction during propagation, it must also
affect the resultant elliptization. The Cotton-Mouton effect also intrinsically
results in rotation of the polarization direction, but this effect is less
significant in the plasma conditions modeled. The interaction increases at
longer wavelength, and complicates interpretation of polarimetry measurements.Comment: Contributed paper published as part of the Proceedings of the 18th
Topical Conference on High-Temperature Plasma Diagnostics, Wildwood, New
Jersey, May, 201
Analytic Approximations for Transit Light Curve Observables, Uncertainties, and Covariances
The light curve of an exoplanetary transit can be used to estimate the
planetary radius and other parameters of interest. Because accurate parameter
estimation is a non-analytic and computationally intensive problem, it is often
useful to have analytic approximations for the parameters as well as their
uncertainties and covariances. Here we give such formulas, for the case of an
exoplanet transiting a star with a uniform brightness distribution. We also
assess the advantages of some relatively uncorrelated parameter sets for
fitting actual data. When limb darkening is significant, our parameter sets are
still useful, although our analytic formulas underpredict the covariances and
uncertainties.Comment: 33 pages, 14 figure
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