1,026 research outputs found
Variable turbulent convection as the cause of the Blazhko effect - testing the Stothers model
The amplitude and phase modulation observed in a significant fraction of the
RR Lyrae variables - the Blazhko effect - represents a long-standing enigma in
stellar pulsation theory. No satisfactory explanation for the Blazhko effect
has been proposed so far. In this paper we focus on the Stothers (2006) idea,
in which modulation is caused by changes in the structure of the outer
convective zone, caused by a quasi-periodically changing magnetic field.
However, up to this date no quantitative estimates were made to investigate
whether such a mechanism can be operational and whether it is capable of
reproducing the light variation we observe in Blazhko variables. We address the
latter problem. We use a simplified model, in which the variation of turbulent
convection is introduced into the non-linear hydrodynamic models in an ad hoc
way, neglecting interaction with the magnetic field. We study the light curve
variation through the modulation cycle and properties of the resulting
frequency spectra. Our results are compared with Kepler observations of RR Lyr.
We find that reproducing the light curve variation, as is observed in RR Lyr,
requires a huge modulation of the mixing length, of the order of +/-50 per
cent, on a relatively short time-scale of less than 40 days. Even then, we are
not able to reproduce neither all the observed relations between modulation
components present in the frequency spectrum, nor the relations between Fourier
parameters describing the shape of the instantaneous light curves.Comment: 17 pages, 13 figures, accepted for publication in MNRAS; for
associated animation, see
http://homepage.univie.ac.at/radek.smolec/publications/KASC11a
Near-field imaging and frequency tuning of a high-Q photonic crystal membrane microcavity
We discuss experimental studies of the interaction between a nanoscopic
object and a photonic crystal membrane resonator of quality factor Q=55000. By
controlled actuation of a glass fiber tip in the near-field of a photonic
crystal, we constructed a complete spatio-spectral map of the resonator mode
and its coupling with the fiber-tip. On the one hand, our findings demonstrate
that scanning probes can profoundly influence the optical characteristics and
the near-field images of photonic devices. On the other hand, we show that the
introduction of a nanoscopic object provides a low-loss method for on-command
tuning of a photonic crystal resonator frequency. Our results are in a very
good agreement with the predictions of a combined numerical/analytical theory.Comment: 9 pages, 4 figure
Time- and frequency-domain polariton interference
We present experimental observations of interference between an atomic spin
coherence and an optical field in a {\Lambda}-type gradient echo memory. The
interference is mediated by a strong classical field that couples a weak probe
field to the atomic coherence through a resonant Raman transition. Interference
can be observed between a prepared spin coherence and another propagating
optical field, or between multiple {\Lambda} transitions driving a single spin
coherence. In principle, the interference in each scheme can yield a near unity
visibility.Comment: 11 pages, 5 figure
Structure Space of Model Proteins --A Principle Component Analysis
We study the space of all compact structures on a two-dimensional square
lattice of size . Each structure is mapped onto a vector in
-dimensions according to a hydrophobic model. Previous work has shown that
the designabilities of structures are closely related to the distribution of
the structure vectors in the -dimensional space, with highly designable
structures predominantly found in low density regions. We use principal
component analysis to probe and characterize the distribution of structure
vectors, and find a non-uniform density with a single peak. Interestingly, the
principal axes of this peak are almost aligned with Fourier eigenvectors, and
the corresponding Fourier eigenvalues go to zero continuously at the
wave-number for alternating patterns (). These observations provide a
stepping stone for an analytic description of the distribution of structural
points, and open the possibility of estimating designabilities of realistic
structures by simply Fourier transforming the hydrophobicities of the
corresponding sequences.Comment: 14 pages, 12 figures, Conclusion has been modifie
Symmetry and designability for lattice protein models
Native protein folds often have a high degree of symmetry. We study the
relationship between the symmetries of native proteins, and their
designabilities -- how many different sequences encode a given native
structure. Using a two-dimensional lattice protein model based on
hydrophobicity, we find that those native structures that are encoded by the
largest number of different sequences have high symmetry. However only certain
symmetries are enhanced, e.g. x/y-mirror symmetry and rotation, while
others are suppressed. If it takes a large number of mutations to destabilize
the native state of a protein, then, by definition, the state is highly
designable. Hence, our findings imply that insensitivity to mutation implies
high symmetry. It appears that the relationship between designability and
symmetry results because protein substructures are also designable. Native
protein folds may therefore be symmetric because they are composed of repeated
designable substructures.Comment: 13 pages, 10 figure
Investigation of Lunar Surface Chemical Contamination by LEM Descent Engine and Associated Equipment
Lunar surface and atmospheric contamination study caused by LEM rocket exhaust and inorganic, organic, and microbiological contaminant
Quantum Non-demolition Measurements on Qubits
We discuss the characterization and properties of quantum non-demolition
(QND) measurements on qubit systems. We introduce figures of merit which can be
applied to systems of any Hilbert space dimension thus providing universal
criteria for characterizing QND measurements. We discuss the controlled-NOT
gate and an optical implementation as examples of QND devices for qubits. We
also discuss the QND measurement of weak values
Period and light curve fluctuations of the Kepler Cepheid V1154 Cyg
We present a detailed period analysis of the bright Cepheid-type variable
star V1154 Cygni (V =9.1 mag, P~4.9 d) based on almost 600 days of continuous
observations by the Kepler space telescope. The data reveal significant
cycle-to-cycle fluctuations in the pulsation period, indicating that classical
Cepheids may not be as accurate astrophysical clocks as commonly believed:
regardless of the specific points used to determine the O-C values, the cycle
lengths show a scatter of 0.015-0.02 days over the 120 cycles covered by the
observations. A very slight correlation between the individual Fourier
parameters and the O-C values was found, suggesting that the O - C variations
might be due to the instability of the light curve shape. Random fluctuation
tests revealed a linear trend up to a cycle difference 15, but for long term,
the period remains around the mean value. We compare the measurements with
simulated light curves that were constructed to mimic V1154 Cyg as a perfect
pulsator modulated only by the light travel time effect caused by low-mass
companions. We show that the observed period jitter in V1154 Cyg represents a
serious limitation in the search for binary companions. While the Kepler data
are accurate enough to allow the detection of planetary bodies in close orbits
around a Cepheid, the astrophysical noise can easily hide the signal of the
light-time effect.Comment: published in MNRAS: 8 pages, 7 figure
Abell 43: Longest period Planetary Nebula Nucleus variable
Based on 24h high speed photometry of the hybrid PG 1159 star Abell 43, we
have detected 6 sighificant pulsations with periods between 2380 s and 6075 s.
A short (4h) run on the almost spectroscopic twin NGC 7094 central star
resulted in detection of 3 low amplitude pulsations with periods between 2000 s
and 5000 s. The results are close to predictions for g-mode pulsations driven
by the kappa-mechanism induced by the partial ionization of carbon and oxygen.Comment: 5 pages, 5 figures, to be published in Astronomy and Astrophysic
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