6,165 research outputs found
The X-Ray Crystal Analyses of Some Organic and Charge-Transfer Compounds, and the Development of Related Computer Programs
The thesis is divided into three main parts; the first is devoted to a brief review of the theory and methods of X-ray Crystallography, the second describes a number of computer programs devised wholly or in part by the author, and the third presents four structural analyses. The computer programs found in Part II are written in KDF 9 - ALGOL, a subset of ALGOL 60. The ASS system of crystallographic computer programs is described and two programs from its structure solution aspect are reported. These are a Fourier-coefficient weighting program which weights a structure factor, for use in a Fourier summation, depending on the probability that its phase is correct, and a program which will provide a complete analysis of structure-factor data. Two other programs, more connected with structure refinement, are then discussed, an isotropic structure-factor least-squares program and a general anisotropic structure-factor program. Part III consists of the application of the X-ray method to organic structural problems. The crystal-structure analyses of the molecular complexes of anthracene and 1,12-dimethylbenzophenanthrene with 4-bromo-2,5,7-trinitrofluorenone are described and an account of the charge-transfer bonding Involved In the compounds is given. Two further structural analyses, those of a derivative of a degradation product of a pigment obtained from the bacterium Pseudomonas lemmonieri, and of the derivative of a tertiary alcohol, obtained from a reaction designed to yield a doubly bridged tricyclic molecule, are then presented and the results discussed
Several types of types in programming languages
Types are an important part of any modern programming language, but we often
forget that the concept of type we understand nowadays is not the same it was
perceived in the sixties. Moreover, we conflate the concept of "type" in
programming languages with the concept of the same name in mathematical logic,
an identification that is only the result of the convergence of two different
paths, which started apart with different aims. The paper will present several
remarks (some historical, some of more conceptual character) on the subject, as
a basis for a further investigation. The thesis we will argue is that there are
three different characters at play in programming languages, all of them now
called types: the technical concept used in language design to guide
implementation; the general abstraction mechanism used as a modelling tool; the
classifying tool inherited from mathematical logic. We will suggest three
possible dates ad quem for their presence in the programming language
literature, suggesting that the emergence of the concept of type in computer
science is relatively independent from the logical tradition, until the
Curry-Howard isomorphism will make an explicit bridge between them.Comment: History and Philosophy of Computing, HAPOC 2015. To appear in LNC
Non-conservative evolution in Algols: where is the matter?
There is gathering indirect evidence suggesting non-conservative evolutions
in Algols. However, the systemic mass-loss rate is poorly constrained by
observations and generally set as a free parameter in binary-star evolution
simulations. Moreover, systemic mass loss may lead to observational signatures
that are still to be found. We investigate the impact of the outflowing gas and
the possible presence of dust grains on the spectral energy distribution (SED).
We used the 1D plasma code Cloudy and compared the results with the 3D
Monte-Carlo radiative transfer code Skirt for dusty simulations. The
circumbinary mass-distribution and binary parameters are computed with
state-of-the-art binary calculations done with the Binstar evolution code. The
outflowing material reduces the continuum flux-level of the stellar SED in the
optical and UV. Due to the time-dependence of this effect, it may help to
distinguish between different ejection mechanisms. Dust, if present, leads to
observable infrared excesses even with low dust-to-gas ratios and traces the
cold material at large distances from the star. By searching for such dust
emission in the WISE catalogue, we found a small number of Algols showing
infrared excesses, among which the two rather surprising objects SX Aur and CZ
Vel. We find that some binary B[e] stars show the same strong Balmer continuum
as we predict with our models. However, direct evidence of systemic mass loss
is probably not observable in genuine Algols, since these systems no longer
eject mass through the hotspot mechanism. Furthermore, owing to its high
velocity, the outflowing material dissipates in a few hundred years. If hot
enough, the hotspot may produce highly ionised species such as SiIV and
observable characteristics that are typical of W Ser systems.Comment: Accepted for piblications in A&A; 21 pages, 19 figure
Radio Astrometry Of The Triple Systems Algol And UX Arietis
We have used multi-epoch long-baseline radio interferometry to determine the
proper motion and orbital elements of Algol and UX Arietis, two radio-bright,
close binary stellar systems with distant tertiary components. For Algol, we
refine the proper motion and outer orbit solutions, confirming the recent
result of Zavala et al. (2010) that the inner orbit is retrograde. The radio
centroid closely tracks the motion of the KIV secondary. In addition, the radio
morphology varies from double-lobed at low flux level to crescent-shaped during
active periods. These results are most easily interpreted as synchrotron
emission from a large, co-rotating meridional loop centered on the K-star. If
this is correct, it provides a radio-optical frame tie candidate with an
uncertainty {\pm}0.5 mas. For UX Arietis, we find a outer orbit solution that
accounts for previous VLBI observations of an acceleration term in the proper
motion fit. The outer orbit solution is also consistent with previously
published radial velocity curves and speckle observations of a third body. The
derived tertiary mass, 0.75 solar masses, is consistent with the K1
main-sequence star detected spectroscopically. The inner orbit solution favors
radio emission from the active K0IV primary only. The radio morphology,
consisting of a single, partially resolved emission region, may be associated
with the persistent polar spot observed using Doppler imaging
Automated Classification of Periodic Variable Stars detected by the Wide-field Infrared Survey Explorer
We describe a methodology to classify periodic variable stars identified
using photometric time-series measurements constructed from the Wide-field
Infrared Survey Explorer (WISE) full-mission single-exposure Source Databases.
This will assist in the future construction of a WISE Variable Source Database
that assigns variables to specific science classes as constrained by the WISE
observing cadence with statistically meaningful classification probabilities.
We have analyzed the WISE light curves of 8273 variable stars identified in
previous optical variability surveys (MACHO, GCVS, and ASAS) and show that
Fourier decomposition techniques can be extended into the mid-IR to assist with
their classification. Combined with other periodic light-curve features, this
sample is then used to train a machine-learned classifier based on the random
forest (RF) method. Consistent with previous classification studies of variable
stars in general, the RF machine-learned classifier is superior to other
methods in terms of accuracy, robustness against outliers, and relative
immunity to features that carry little or redundant class information. For the
three most common classes identified by WISE: Algols, RR Lyrae, and W Ursae
Majoris type variables, we obtain classification efficiencies of 80.7%, 82.7%,
and 84.5% respectively using cross-validation analyses, with 95% confidence
intervals of approximately +/-2%. These accuracies are achieved at purity (or
reliability) levels of 88.5%, 96.2%, and 87.8% respectively, similar to that
achieved in previous automated classification studies of periodic variable
stars.Comment: 48 pages, 17 figures, 1 table, accepted by A
On the basis for ELF - An Extensible Language Facility
Computer language for data processing and information retrieva
Spin Angular Momentum Evolution of the Long Period Algols
We consider the spin angular momentum evolution of the accreting components
of Algol-type binary stars. In wider Algols the accretion is through a disc so
that the accreted material can transfer enough angular momentum to the gainer
that material at its equator should be spinning at break-up. We demonstrate
that even a small amount of mass transfer, much less than required to produce
today's mass ratios, transfers enough angular momentum to spin the gainer up to
this critical rotation velocity. However the accretors in these systems have
spins typically between 10 and per cent of the critical rate. So some
mechanism for angular momentum loss from the gainers is required. We consider
generation of magnetic fields in the radiative atmospheres in a differentially
rotating star and the possibility of angular momentum loss driven by strong
stellar winds in the intermediate mass stars, such as the primaries of the
Algols. Differential rotation, induced by the accretion itself, may produce
such winds which carry away enough angular momentum to reduce their rotational
velocities to the today's observed values. We apply this model to two systems
with initial periods of 5\,d, one with initial masses 5 and
and the other with 3.2 and . Our calculations show that, if
the mass outflow rate in the stellar wind is about per cent of the
accretion rate and the dipole magnetic field is stronger than about kG,
the spin rate of the gainer is reduced to below break-up velocity even in the
fast phase of mass transfer. Larger mass loss is needed for smaller magnetic
fields. The slow rotation of the gainers in the classical Algol systems is
explained by a balance between the spin-up by mass accretion and spin-down by a
stellar wind linked to a magnetic field.Comment: 12 figures, 26 pages, accepted in MNRA
Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei
We present an investigation of H-alpha emission line variations observed in
the massive Algol binary, RY Per. We give new radial velocity data for the
secondary based upon our optical spectra and for the primary based upon high
dispersion UV spectra. We present revised orbital elements and an estimate of
the primary's projected rotational velocity (which indicates that the primary
is rotating 7 times faster than synchronous). We use a Doppler tomography
algorithm to reconstruct the individual primary and secondary spectra in the
region of H-alpha, and we subtract the latter from each of our observations to
obtain profiles of the primary and its disk alone. Our H-alpha observations of
RY Per show that the mass gaining primary is surrounded by a persistent but
time variable accretion disk. The profile that is observed outside-of-eclipse
has weak, double-peaked emission flanking a deep central absorption, and we
find that these properties can be reproduced by a disk model that includes the
absorption of photospheric light by the band of the disk seen in projection
against the face of the star. We developed a new method to reconstruct the disk
surface density distribution from the ensemble of H-alpha profiles observed
around the orbit, and this method accounts for the effects of disk occultation
by the stellar components, the obscuration of the primary by the disk, and flux
contributions from optically thick disk elements. The resulting surface density
distribution is elongated along the axis joining the stars, in the same way as
seen in hydrodynamical simulations of gas flows that strike the mass gainer
near trailing edge of the star. This type of gas stream configuration is
optimal for the transfer of angular momentum, and we show that rapid rotation
is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu
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