10,470 research outputs found
White dwarf masses in cataclysmic variables
The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has
recently been found to dramatically disagree with the predictions of the
standard CV formation model. The high mean WD mass among CVs is not imprinted
in the currently observed sample of CV progenitors and cannot be attributed to
selection effects. Two possibilities have been put forward: either the WD grows
in mass during CV evolution, or in a significant fraction of cases, CV
formation is preceded by a (short) phase of thermal time-scale mass transfer
(TTMT) in which the WD gains a sufficient amount of mass. We investigate if
either of these two scenarios can bring theoretical predictions and
observations into agreement. We employed binary population synthesis models to
simulate the present intrinsic CV population. We incorporated aspects specific
to CV evolution such as an appropriate mass-radius relation of the donor star
and a more detailed prescription for the critical mass ratio for dynamically
unstable mass transfer. We also implemented a previously suggested wind from
the surface of the WD during TTMT and tested the idea of WD mass growth during
the CV phase by arbitrarily changing the accretion efficiency. We compare the
model predictions with the characteristics of CVs derived from observed
samples. We find that mass growth of the WDs in CVs fails to reproduce the
observed WD mass distribution. In the case of TTMT, we are able to produce a
large number of massive WDs if we assume significant mass loss from the surface
of the WD during the TTMT phase. However, the model still produces too many CVs
with helium WDs. Moreover, the donor stars are evolved in many of these
post-TTMT CVs, which contradicts the observations. We conclude that in our
current framework of CV evolution neither TTMT nor WD mass growth can fully
explain either the observed WD mass or the period distribution in CVs.Comment: 15 pages, 7 figures, 1 table, accepted for publication in A&A.
Replaced and added a reference, corrected typo
Nonlinear projective filtering in a data stream
We introduce a modified algorithm to perform nonlinear filtering of a time
series by locally linear phase space projections. Unlike previous
implementations, the algorithm can be used not only for a posteriori processing
but includes the possibility to perform real time filtering in a data stream.
The data base that represents the phase space structure generated by the data
is updated dynamically. This also allows filtering of non-stationary signals
and dynamic parameter adjustment. We discuss exemplary applications, including
the real time extraction of the fetal electrocardiogram from abdominal
recordings.Comment: 8 page
The ultra-compact binary candidate KUV 23182+1007 is a bright quasar
KUV 23182+1007 was identified as a blue object in the Kiso UV Survey in the
1980s. Classification-dispersion spectroscopy showed a featureless continuum
except for a strong emission line in the region of He II 4686 A. This is a
hallmark of the rare AM CVn class of cataclysmic variable star, so we have
obtained a high-S/N blue spectrum of this object to check its classification.
Instead, the spectrum shows a strong quasar-like emission line centred on 4662
A. Comparison with the SDSS quasar template spectra confirms that KUV
23182+1007 is a quasar with a redshift of z = 0.665.Comment: 4 pages, 2 figures, to appear in IBVS. Data are available from
http://www.astro.keele.ac.uk/~jkt
Direct measurement of diurnal polar motion by ring laser gyroscopes
We report the first direct measurements of the very small effect of forced
diurnal polar motion, successfully observed on three of our large ring lasers,
which now measure the instantaneous direction of Earth's rotation axis to a
precision of 1 part in 10^8 when averaged over a time interval of several
hours. Ring laser gyroscopes provide a new viable technique for directly and
continuously measuring the position of the instantaneous rotation axis of the
Earth and the amplitudes of the Oppolzer modes. In contrast, the space geodetic
techniques (VLBI, SLR, GPS, etc.) contain no information about the position of
the instantaneous axis of rotation of the Earth, but are sensitive to the
complete transformation matrix between the Earth-fixed and inertial reference
frame. Further improvements of gyroscopes will provide a powerful new tool for
studying the Earth's interior.Comment: 5 pages, 4 figures, agu2001.cl
Stochastic unraveling of Redfield master equations and its application to electron transfer problems
A method for stochastic unraveling of general time-local quantum master
equations (QMEs) is proposed. The present kind of jump algorithm allows a
numerically efficient treatment of QMEs which are not in Lindblad form, i.e.
are not positive semidefinite by definition. The unraveling can be achieved by
allowing for trajectories with negative weights. Such a property is necessary,
e.g. to unravel the Redfield QME and to treat various related problems with
high numerical efficiency. The method is successfully tested on the damped
harmonic oscillator and on electron transfer models including one and two
reaction coordinates. The obtained results are compared to those from a direct
propagation of the reduced density matrix (RDM) as well as from the standard
quantum jump method. Comparison of the numerical efficiency is performed
considering both the population dynamics and the RDM in the Wigner phase space
representation.Comment: accepted in J. Chem. Phys.; 26 pages, 6 figures; the order of
authors' names on the title page correcte
DDF and Pohlmeyer invariants of (super)string
We show how the Pohlmeyer invariants of the bosonic string are expressible in
terms of DDF invariants. Quantization of the DDF observables in the usual way
yields a consistent quantization of the algebra of Pohlmeyer invariants.
Furthermore it becomes straightforward to generalize the Pohlmeyer invariants
to the superstring as well as to all backgrounds which allow a free field
realization of the worldsheet theory.Comment: 17 pp, minor typos corrected, references to papers by Isaev and
Borodulin added, which contain essentially the same results as reported her
Non-ergodic effects in the Coulomb glass: specific heat
We present a numerical method for the investigation of non-ergodic effects in
the Coulomb glass. For that, an almost complete set of low-energy many-particle
states is obtained by a new algorithm. The dynamics of the sample is mapped to
the graph formed by the relevant transitions between these states, that means
by transitions with rates larger than the inverse of the duration of the
measurement. The formation of isolated clusters in the graph indicates
non-ergodicity. We analyze the connectivity of this graph in dependence on
temperature, duration of measurement, degree of disorder, and dimensionality,
studying how non-ergodicity is reflected in the specific heat.Comment: Submited Phys. Rev.
Impact of Sodium Layer variations on the performance of the E-ELT MCAO module
Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may
exploit Natural Guide Stars to solve intrinsic limitations of artificial
beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the
impact of the sodium layer structure and variability. The sodium layer may also
have transverse structures leading to differential effects among Laser Guide
Stars. Starting from the analysis of the input perturbations related to the
Sodium Layer variability, modeled directly on measured sodium layer profiles,
we analyze, through a simplified end-to-end simulation code, the impact of the
low/medium orders induced on global performance of the European Extremely Large
Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin
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