2,496 research outputs found
Modeling of Closure Phase Measurements with AMBER/VLTI - Towards Characterization of Exoplanetary Atmospheres
Differential phase observations with a near-IR interferometer offer a way to
obtain spectra of extrasolar planets. The method makes use of the wavelength
dependence of the interferometer phase of the planet/star system, which depends
both on the interferometer geometry and on the brightness ratio between the
planet and the star. The differential phase is strongly affected by
instrumental and atmospheric dispersion effects. Difficulties in calibrating
these effects might prevent the application of the differential phase method to
systems with a very high contrast, such as extrasolar planets. A promising
alternative is the use of spectrally resolved closure phases, which are immune
to many of the systematic and random errors affecting the single-baseline
phases. We have modeled the response of the AMBER instrument at the VLTI to
realistic models of known extrasolar planetary systems, taking into account
their theoretical spectra as well as the geometry of the VLTI. We present a
strategy to determine the geometry of the planetary system and the spectrum of
the extrasolar planet from closure phase observations in two steps. We show
that there is a close relation between the nulls in the closure phase and the
nulls in the corresponding single-baseline phases: every second null of a
single-baseline phase is also a null in the closure phase. In particular, the
nulls in the closure phase do not depend on the spectrum but only on the
geometry. Therefore the geometry of the system can be determined by measuring
the nulls in the closure phase, and braking the remaining ambiguity due to the
unknown system orientation by means of observations at different hour angles.
Based on the known geometry, the planet spectrum can then be directly
synthesized from the closure phases.Comment: replaced version with corrected Fig.5; 9 pages, 6 figures, Proceeding
of the SPIE conference, Glasgow, 2004, Proc. SPIE 5491, in pres
Why is the snowflake schema a good data warehouse design?
Database design for data warehouses is based on the notion of the snowflake schema and its important special case, the star schema. The snowflake schema represents a dimensional model which is composed of a central fact table and a set of constituent dimension tables which can be further broken up into subdimension tables. We formalise the concept of a snowflake schema in terms of an acyclic database schema whose join tree satisfies certain structural properties. We then define a normal form for snowflake schemas which captures its intuitive meaning with respect to a set of functional and inclusion dependencies. We show that snowflake schemas in this normal form are independent as well as separable when the relation schemas are pairwise incomparable. This implies that relations in the data warehouse can be updated independently of each other as long as referential integrity is maintained. In addition, we show that a data warehouse in snowflake normal form can be queried by joining the relation over the fact table with the relations over its dimension and subdimension tables. We also examine an information-theoretic interpretation of the snowflake schema and show that the redundancy of the primary key of the fact table is zero
Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares
The mechanism that accelerates particles to the energies required to produce
the observed high-energy impulsive emission in solar flares is not well
understood. Drake et al. (2006) proposed a mechanism for accelerating electrons
in contracting magnetic islands formed by kinetic reconnection in multi-layered
current sheets. We apply these ideas to sunward-moving flux ropes (2.5D
magnetic islands) formed during fast reconnection in a simulated eruptive
flare. A simple analytic model is used to calculate the energy gain of
particles orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the estimated
energy gains in a single island range up to a factor of five. This is higher
than that found by Drake et al. for islands in the terrestrial magnetosphere
and at the heliopause, due to strong plasma compression that occurs at the
flare current sheet. In order to increase their energy by two orders of
magnitude and plausibly account for the observed high-energy flare emission,
the electrons must visit multiple contracting islands. This mechanism should
produce sporadic emission because island formation is intermittent. Moreover, a
large number of particles could be accelerated in each
magnetohydrodynamic-scale island, which may explain the inferred rates of
energetic-electron production in flares. We conclude that island contraction in
the flare current sheet is a promising candidate for electron acceleration in
solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016
The Vadalog System: Datalog-based Reasoning for Knowledge Graphs
Over the past years, there has been a resurgence of Datalog-based systems in
the database community as well as in industry. In this context, it has been
recognized that to handle the complex knowl\-edge-based scenarios encountered
today, such as reasoning over large knowledge graphs, Datalog has to be
extended with features such as existential quantification. Yet, Datalog-based
reasoning in the presence of existential quantification is in general
undecidable. Many efforts have been made to define decidable fragments. Warded
Datalog+/- is a very promising one, as it captures PTIME complexity while
allowing ontological reasoning. Yet so far, no implementation of Warded
Datalog+/- was available. In this paper we present the Vadalog system, a
Datalog-based system for performing complex logic reasoning tasks, such as
those required in advanced knowledge graphs. The Vadalog system is Oxford's
contribution to the VADA research programme, a joint effort of the universities
of Oxford, Manchester and Edinburgh and around 20 industrial partners. As the
main contribution of this paper, we illustrate the first implementation of
Warded Datalog+/-, a high-performance Datalog+/- system utilizing an aggressive
termination control strategy. We also provide a comprehensive experimental
evaluation.Comment: Extended version of VLDB paper
<https://doi.org/10.14778/3213880.3213888
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