1,656 research outputs found
Interferometer predictions with triangulated images: solving the multi-scale problem
Interferometers play an increasingly important role for spatially resolved
observations. If employed at full potential, interferometry can probe an
enormous dynamic range in spatial scale. Interpretation of the observed
visibilities requires the numerical compu- tation of Fourier integrals over the
synthetic model images. To get the correct values of these integrals, the model
images must have the right size and resolution. Insufficient care in these
choices can lead to wrong results. We present a new general-purpose scheme for
the computation of visibilities of radiative transfer images. Our method
requires a model image that is a list of intensities at arbitrarily placed
positions on the image-plane. It creates a triangulated grid from these
vertices, and assumes that the intensity inside each triangle of the grid is a
linear function. The Fourier integral over each triangle is then evaluated with
an analytic expression and the complex visibility of the entire image is then
the sum of all triangles. The result is a robust Fourier trans- form that does
not suffer from aliasing effects due to grid regularities. The method
automatically ensures that all structure contained in the model gets reflected
in the Fourier transform.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Labour Supply Effects of an Early Retirement Programme
In 1988, an early retirement program (AFP) was introduced in Norway for the 66-years-old. Since then, AFP has gradually been extended and by now it covers workers aged 62-66. In this paper we employ a multinominal logit model to study the transition between states in the labour market. The model is estimated on a large panel data set covering the period 1988-2 to 1999-4. The estimated model tracks the development quite well, as also outside sample predictions do. The model is used to assess the future labour market impact of abolishing AFP. We find that by abolishing AFP may increase the labour force participation among older men (55-67) in 2005 from 72 percent in the baseline projection to 83 percent. For females the corresponding increase is from 62 to 67 percent.Early retirement, large panel data sets, econometric models
VALUING AGRICULTURAL MORTGAGE-BACKED SECURITIES
A model to value Federal Agricultural Mortgage Corporation (Farmer Mac) agricultural mortgage-backed securities (AMBS) is developed and numerically solved. The results suggest prepayment penalties currently being used by Farmer Mac reduce yields on AMBS considerably. Even with prepayment penalties, it can be advantageous for profit maximizing mortgagors to optimally prepay or even default on agricultural mortgages. The model is used to quantify prepayment and default risk by valuing the embedded options in the mortgages. Monte Carlo simulation is also used to determine the probability to optimal prepayment given the term structure assumption used to develop the model.agricultural mortgage-backed securities, default, dynamic programming, simulation, prepayment, Agricultural Finance, G13, G21,
Multiple-Length Division Revisited: A Tour of the Minefield
Long division of natural numbers plays a crucial role in Cobol arithmetic, cryptography, and primality testing. Only a handful of textbooks discuss the theory and practice of long division, and none of them do it satisfactorily. This tutorial attempts to fill this surprising gap in the literature on computer algorithms. We illustrate the subtleties of long division by examples, define the problem concisely, summarize the theory, and develop a complete Pascal algorithm using a consistent terminology
Analysis of a Parallel Mergesort
The paper describes a performance model of a fine-grained, parallel mergesort which sorts N elements in 0(N) time using 0(logN) processors. The model predicts both the communication time involved in merging the elements and the decomposition time required to activate and terminate the binary tree of processes. The parallel algorithm is written in Joyce and runs on an Encore Multimax
The face of the factory girl: Educational rhetoric in the mediation of an historical place
World heritage cities and sites convey culture and form understandings of the past as well as show a way to relate to it in a present context. These cities and sites are concrete examples of what to remember; and their mediated rhetoric, as found inexhibitions and on websites, gives a view into the management of our collective, cultural memory. This article will analyse key elements of the website for New Lanark, a Scottish heritage site, and in so doing will observe how rhetorical agency is shaped through the use of iconic photographs, visual rhetoric and the foregrounding of specific topics. Predominant on this website is the face of a young factory girl, smiling at the viewer. The heritage site thus has its identity formed, but so does the public addressed by this rhetoric. The institutional position of the historical place and its mediated rhetoric point to the fact that world heritage sites are rhetorical educational texts, far from being as neutral as might be expected
Interferometric view of the circumstellar envelopes of northern FU Orionis-type stars
FU Orionis-type objects are young, low-mass stars with large outbursts in
visible light that last for several years or decades. They are thought to
represent an evolutionary phase during the life of every young star when
accretion from the circumstellar disk is enhanced during recurring time
periods. These outbursts are able to rapidly build up the star while affecting
the circumstellar disk and thus the ongoing or future planet formation. In many
models infall from a circumstellar envelope seems to be necessary to trigger
the outbursts. We observed the J=10 rotational transition of CO and
CO towards eight northern FU Orionis-type stars (V1057 Cyg, V1515 Cyg,
V2492 Cyg, V2493 Cyg, V1735 Cyg, V733 Cep, RNO 1B and RNO 1C) and derive
temperatures and envelope masses and discuss the morphology and kinematics of
the circumstellar material. We detected extended CO emission associated with
all our targets. Smaller scale CO clumps were found to be associated with five
objects with radii of 20005000 AU and masses of 0.020.5 ;
these are clearly heated by the central stars. Three of these envelopes are
also strongly detected in the 2.7 mm continuum. No central CO clumps were
detected around V733 Cep and V710 Cas but there are many other clumps in their
environments. Traces of outflow activity were observed towards V1735 Cyg, V733
Cep and V710 Cas. The diversity of the observed envelopes enables us to set up
an evolutionary sequence between the objects. We find their evolutionary state
to range from early, embedded Class I stage to late, Class II-type objects with
very low-mass circumstellar material. The results reinforce the idea of FU
Orionis-type stars as representatives of a transitory stage between embedded
Class I young stellar objects and classical T-Tauri stars.Comment: 17 pages, 11 figures; accepted for publication in A&
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