2,980 research outputs found
Bayesian analysis of interiors of HD 219134b, Kepler-10b, Kepler-93b, CoRoT-7b, 55 Cnc e, and HD 97658b using stellar abundance proxies
Using a generalized Bayesian inference method, we aim to explore the possible
interior structures of six selected exoplanets for which planetary mass and
radius measurements are available in addition to stellar host abundances:
HD~219134b, Kepler-10b, Kepler-93b, CoRoT-7b, 55~Cnc~e, and HD~97658b. We aim
to investigate the importance of stellar abundance proxies for the planetary
bulk composition (namely Fe/Si and Mg/Si) on prediction of planetary interiors.
We performed a full probabilistic Bayesian inference analysis to formally
account for observational and model uncertainties while obtaining confidence
regions of structural and compositional parameters of core, mantle, ice layer,
ocean, and atmosphere. We determined how sensitive our parameter predictions
depend on (1) different estimates of bulk abundance constraints and (2)
different correlations of bulk abundances between planet and host star. [...]
Although the possible ranges of interior structures are large, structural
parameters and their correlations are constrained by the sparse data. The
probability for the tested exoplanets to be Earth-like is generally very low.
Furthermore, we conclude that different estimates of planet bulk abundance
constraints mainly affect mantle composition and core size.Comment: Astronomy & Astrophysics, 597, A38 (15 pages, 9 figures
Development of a carbon fibre composite active mirror: Design and testing
Carbon fibre composite technology for lightweight mirrors is gaining
increasing interest in the space- and ground-based astronomical communities for
its low weight, ease of manufacturing, excellent thermal qualities and
robustness. We present here first results of a project to design and produce a
27 cm diameter deformable carbon fibre composite mirror. The aim was to produce
a high surface form accuracy as well as low surface roughness. As part of this
programme, a passive mirror was developed to investigate stability and coating
issues. Results from the manufacturing and polishing process are reported here.
We also present results of a mechanical and thermal finite element analysis, as
well as early experimental findings of the deformable mirror. Possible
applications and future work are discussed.Comment: Accepted by Optical Engineering. Figures 1-7 on
http://www.star.ucl.ac.uk/~sk/OEpaper_files
A generalized bayesian inference method for constraining the interiors of super Earths and sub-Neptunes
We aim to present a generalized Bayesian inference method for constraining
interiors of super Earths and sub-Neptunes. Our methodology succeeds in
quantifying the degeneracy and correlation of structural parameters for high
dimensional parameter spaces. Specifically, we identify what constraints can be
placed on composition and thickness of core, mantle, ice, ocean, and
atmospheric layers given observations of mass, radius, and bulk refractory
abundance constraints (Fe, Mg, Si) from observations of the host star's
photospheric composition. We employed a full probabilistic Bayesian inference
analysis that formally accounts for observational and model uncertainties.
Using a Markov chain Monte Carlo technique, we computed joint and marginal
posterior probability distributions for all structural parameters of interest.
We included state-of-the-art structural models based on self-consistent
thermodynamics of core, mantle, high-pressure ice, and liquid water.
Furthermore, we tested and compared two different atmospheric models that are
tailored for modeling thick and thin atmospheres, respectively. First, we
validate our method against Neptune. Second, we apply it to synthetic
exoplanets of fixed mass and determine the effect on interior structure and
composition when (1) radius, (2) atmospheric model, (3) data uncertainties, (4)
semi-major axes, (5) atmospheric composition (i.e., a priori assumption of
enriched envelopes versus pure H/He envelopes), and (6) prior distributions are
varied. Our main conclusions are: [...]Comment: Astronomy & Astrophysics, 597, A37, 17 pages, 11 figure
Accumulation horizons and period-adding in optically injected semiconductor lasers
We study the hierarchical structuring of islands of stable periodic
oscillations inside chaotic regions in phase diagrams of single-mode
semiconductor lasers with optical injection. Phase diagrams display remarkable
{\it accumulation horizons}: boundaries formed by the accumulation of infinite
cascades of self-similar islands of periodic solutions of ever-increasing
period. Each cascade follows a specific period-adding route. The riddling of
chaotic laser phases by such networks of periodic solutions may compromise
applications operating with chaotic signals such as e.g. secure communications.Comment: 4 pages, 4 figures, laser phase diagrams, to appear in Phys. Rev. E,
vol. 7
A technology comparison concerning scale dependencies of industrial furnaces. A case study of glass production
The focus of light - linear polarization breaks the rotational symmetry of the focal spot
We experimentally demonstrate for the first time that a linearly polarized
beam is focussed to an asymmetric spot when using a high-numerical aperture
focussing system. This asymmetry was predicted by Richards and Wolf
[Proc.R.Soc.London A, 253, 358 (1959)] and can only be measured when a
polarization insensitive sensor is placed in the focal region. We used a
specially modified photodiode in a knife edge type set up to obtain highly
resolved images of the total electric energy density distribution at the focus.
The results are in good agreement with the predictions of a vectorial focussing
theory.Comment: to be published in "Journal of Modern Optics
Electron impact double ionization of helium from classical trajectory calculations
With a recently proposed quasiclassical ansatz [Geyer and Rost, J. Phys. B 35
(2002) 1479] it is possible to perform classical trajectory ionization
calculations on many electron targets. The autoionization of the target is
prevented by a M\o{}ller type backward--forward propagation scheme and allows
to consider all interactions between all particles without additional
stabilization. The application of the quasiclassical ansatz for helium targets
is explained and total and partially differential cross sections for electron
impact double ionization are calculated. In the high energy regime the
classical description fails to describe the dominant TS1 process, which leads
to big deviations, whereas for low energies the total cross section is
reproduced well. Differential cross sections calculated at 250 eV await their
experimental confirmation.Comment: LaTeX, 22 pages, 10 figures, submitted to J. Phys.
On the AdS Higher Spin / O(N) Vector Model Correspondence: degeneracy of the holographic image
We explore the conjectured duality between the critical O(N) vector model and
minimal bosonic massless higher spin (HS) theory in AdS. In the boundary free
theory, the conformal partial wave expansion (CPWE) of the four-point function
of the scalar singlet bilinear is reorganized to make it explicitly
crossing-symmetric and closed in the singlet sector, dual to the bulk HS gauge
fields. We are able to analytically establish the factorized form of the fusion
coefficients as well as the two-point function coefficient of the HS currents.
We insist in directly computing the free correlators from bulk graphs with the
unconventional branch. The three-point function of the scalar bilinear turns
out to be an "extremal" one at d=3. The four-leg bulk exchange graph can be
precisely related to the CPWs of the boundary dual scalar and its shadow. The
flow in the IR by Legendre transforming at leading 1/N, following the pattern
of double-trace deformations, and the assumption of degeneracy of the hologram
lead to the CPWE of the scalar four-point function at IR. Here we confirm some
previous results, obtained from more involved computations of skeleton graphs,
as well as extend some of them from d=3 to generic dimension 2<d<4.Comment: 22 pages, 5 figure
Double ionization of helium by electron-impact: complete pictures of the four-body breakup dynamics
The dynamics of He double ionization by 2 keV electron impact is studied experimentally for a momentum
transfer of 0.6 a.u. at excess energies of 10 and 40 eV. Complete sets of fivefold differential
cross sections are presented for all electron emission angles in coplanar geometry. Contributions beyond
the first Born approximation are identified comparing experimental data with first order convergent
close-coupling calculations which are in considerably better agreement with the present experiment than
with the earlier measurement of Kheifets et al. [J. Phys. B 32, 5047 (1999)]
Impact of the measured parameters of exoplanets on the inferred internal structure
Exoplanet characterization is one of the main foci of current exoplanetary
science. For super-Earths and sub-Neptunes, we mostly rely on mass and radius
measurements, which allow to derive the body's mean density and give a rough
estimate of the planet's bulk composition. However, the determination of
planetary interiors is a very challenging task. In addition to the uncertainty
in the observed fundamental parameters, theoretical models are limited due to
the degeneracy in determining the planetary composition. We aim to study
several aspects that affect internal characterization of super-Earths and
sub-Neptunes: observational uncertainties, location on the M-R diagram, impact
of additional constraints as bulk abundances or irradiation, and model
assumptions. We use a full probabilistic Bayesian inference analysis that
accounts for observational and model uncertainties. We employ a Nested Sampling
scheme to efficiently produce the posterior probability distributions for all
the planetary structural parameter of interest. We include a structural model
based on self-consistent thermodynamics of core, mantle, high-pressure ice,
liquid water, and H-He envelope. Regarding the effect of mass and radius
uncertainties on the determination of the internal structure, we find three
different regimes: below the Earth-like composition line and above the
pure-water composition line smaller observational uncertainties lead to better
determination of the core and atmosphere mass respectively, and between them
structure characterization only weakly depends on the observational
uncertainties. We show that small variations in the temperature or entropy
profiles lead to radius variations that are comparable to the observational
uncertainty, suggesting that uncertainties linked to model assumptions can
become more relevant to determine the internal structure than observational
uncertainties.Comment: 12 pages, 12 figure
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