7,680 research outputs found
Magnetically warped discs in close binaries
We demonstrate that measurable vertical structure can be excited in the
accretion disc of a close binary system by a dipolar magnetic field centred on
the secondary star. We present the first high resolution hydrodynamic
simulations to show the initial development of a uniform warp in a tidally
truncated accretion disc. The warp precesses retrogradely with respect to the
inertial frame. The amplitude depends on the phase of the warp with respect to
the binary frame. A warped disc is the best available explanation for negative
superhumps.Comment: 11 pages, 10 figures, MNRAS accepte
The dynamics of eccentric accretion discs in superhump systems
We have applied an eccentric accretion disc theory in simplified form to the
case of an accretion disc in a binary system, where the disc contains the 3:1
Lindblad resonance. This is relevant to the case of superhumps in SU Ursae
Majoris cataclysmic variables and other systems, where it is thought that this
resonance leads to growth of eccentricity and a modulation in the light curve
due to the interaction of a precessing eccentric disc with tidal stresses. A
single differential equation is formulated which describes the propagation,
resonant excitation and viscous damping of eccentricity. The theory is first
worked out in the simple case of a narrow ring and leads to the conclusion that
the eccentricity distribution is locally suppressed by the presence of the
resonance, creating a dip in the eccentricity at the resonant radius.
Application of this theory to the superhump case confirms this conclusion and
produces a more accurate expression for the precession rate of the disc than
has been previously accomplished with simple dynamical estimates.Comment: 10 pages, 11 figures. Accepted for publication in MNRA
Aggregate eco-efficiency indices for New Zealand – a Principal Components Analysis
Eco-efficiency has emerged as a management response to waste issues associated with current production processes. Despite the popularity of the term in both business and government circles, limited attention has been paid to measuring and reporting eco-efficiency to government policy makers. Aggregate measures of eco-efficiency are needed, to complement existing measures and to help highlight important patterns in eco-efficiency data. This paper aims to develop aggregate measures of eco-efficiency for use by policy makers. Specifically, this paper provides a unique analysis by applying principal components analysis (PCA) to eco-efficiency indicators in New Zealand. This study reveals that New Zealand's overall eco-efficiency improved for two out of the five aggregate measures over the period 1994/95 to 1997/98. The worsening of the other aggregate measures reflects, among other things, the relatively poor performance of the primary production and related processing sectors. These results show PCA is an effective approach for aggregating eco-efficiency indicators and assisting decision makers by reducing redundancy in an eco-efficiency indicators matrix.Policy development, policy evaluation, Aggregate indices, Agricultural and Food Policy, Community/Rural/Urban Development, Crop Production/Industries, Environmental Economics and Policy, Farm Management, Land Economics/Use,
Simulations of spectral lines from an eccentric precessing accretion disc
Two dimensional SPH simulations of a precessing accretion disc in a q=0.1
binary system (such as XTE J1118+480) reveal complex and continuously varying
shape, kinematics, and dissipation. The stream-disc impact region and disc
spiral density waves are prominent sources of energy dissipation.The dissipated
energy is modulated on the period P_{sh} = ({P_{orb}}^{-1}-{P_{prec}}^{-1}^{-1}
with which the orientation of the disc relative to the mass donor repeats. This
superhump modulation in dissipation energy has a variation in amplitude of ~10%
relative to the total dissipation energy and evolves, repeating exactly only
after a full disc precession cycle. A sharp component in the light curve is
associated with centrifugally expelled material falling back and impacting the
disc. Synthetic trailed spectrograms reveal two distinct "S-wave" features,
produced respectively by the stream gas and the disc gas at the stream-disc
impact shock. These S-waves are non-sinusoidal, and evolve with disc precession
phase. We identify the spiral density wave emission in the trailed spectrogram.
Instantaneous Doppler maps show how the stream impact moves in velocity space
during an orbit. In our maximum entropy Doppler tomogram the stream impact
region emission is distorted, and the spiral density wave emission is
uppressed. A significant radial velocity modulation of the whole line profile
occurs on the disc precession period. We compare our SPH simulation with a
simple 3D model: the former is appropriate for comparison with emission lines
while the latter is preferable for skewed absorption lines from precessing
discs.Comment: See http://physics.open.ac.uk/FHMR/ for associated movie (avi) files.
The full paper is in MNRAS press. Limited disk space limit of 650k, hence low
resolution figure file
Detection of superhumps in the VY Scl-type nova-like variable KR Aur
We report on detection of negative superhumps in KR Aur which is the
representative member of the VY Scl stars. The observations were obtained with
the multi-channel photometer during 107 h. The analysis of the data clearly
revealed brightness variations with a period of 3.771 +/- 0.005 h. This is 3.5
per cent less than the orbital period, suggesting it is a negative superhump.
Negative superhumps in VY Scl stars are widely spread. The discovery of
powerful soft X-rays from V751 Cyg demonstrates that VY Scl stars may contain
white dwarfs, at which nuclear burning of the accreted material occur. If this
suspicion is correct, we then can suppose that the powerful radiation emerging
from the white dwarf may cause the tilt of the accretion disk to the orbital
plane, and its retrograde precession may produce negative superhumps in VY Scl
stars.Comment: 6 pages, 6 figures, will be published in MNRA
Superhumps: Confronting Theory with Observation
We review the theory and observations related to the ``superhump'' precession
of eccentric accretion discs in close binary sytems. We agree with earlier
work, although for different reasons, that the discrepancy between observation
and dynamical theory implies that the effect of pressure in the disc cannot be
neglected. We extend earlier work that investigates this effect to include the
correct expression for the radius at which resonant orbits occur. Using
analytic expressions for the accretion disc structure, we derive a relationship
between the period excess and mass-ratio with the pressure effects included.
This is compared to the observed data, recently derived results for detailed
integration of the disc equations and the equivalent empirically derived
relations and used to predict values for the mass ratio based on measured
values of the period excess for 88 systems.Comment: 11 pages, 7 figures, 4 tables, accepted for publication in MNRA
SPH Simulations of Negative (Nodal) Superhumps: A Parametric Study
Negative superhumps in cataclysmic variable systems result when the accretion
disc is tilted with respect to the orbital plane. The line of nodes of the
tilted disc precesses slowly in the retrograde direction, resulting in a
photometric signal with a period slightly less than the orbital period. We use
the method of smoothed particle hydrodynamics to simulate a series of models of
differing mass ratio and effective viscosity to determine the retrograde
precession period and superhump period deficit as a function of
system mass ratio . We tabulate our results and present fits to both
and versus , as well as compare the
numerical results with those compiled from the literature of negative superhump
observations. One surprising is that while we find negative superhumps most
clearly in simulations with an accretion stream present, we also find evidence
for negative superhumps in simulations in which we shut off the mass transfer
stream completely, indicating that the origin of the photometric signal is more
complicated than previously believed.Comment: 14 pages, 15 figures. Accepted for publication in MNRA
Anderson Acceleration For Bioinformatics-Based Machine Learning
Anderson acceleration (AA) is a well-known method for accelerating the
convergence of iterative algorithms, with applications in various fields
including deep learning and optimization. Despite its popularity in these
areas, the effectiveness of AA in classical machine learning classifiers has
not been thoroughly studied. Tabular data, in particular, presents a unique
challenge for deep learning models, and classical machine learning models are
known to perform better in these scenarios. However, the convergence analysis
of these models has received limited attention. To address this gap in
research, we implement a support vector machine (SVM) classifier variant that
incorporates AA to speed up convergence. We evaluate the performance of our SVM
with and without Anderson acceleration on several datasets from the biology
domain and demonstrate that the use of AA significantly improves convergence
and reduces the training loss as the number of iterations increases. Our
findings provide a promising perspective on the potential of Anderson
acceleration in the training of simple machine learning classifiers and
underscore the importance of further research in this area. By showing the
effectiveness of AA in this setting, we aim to inspire more studies that
explore the applications of AA in classical machine learning.Comment: Accepted in KDH-2023: Knowledge Discovery in Healthcare Data (IJCAI
Workshop
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