846 research outputs found
Nonlinear Properties of the Semiregular Variable Stars
We demonstrate how, with a purely empirical analysis of the irregular
lightcurve data, one can extract a great deal of information about the stellar
pulsation mechanism. An application to R Sct thus shows that the irregular
lightcurve is the result of the nonlinear interaction of two highly
nonadiabatic pulsation modes, namely a linearly unstable, low frequency mode,
and the second mode that, although linearly stable, gets entrained through a
2:1 resonance. In the parlance of nonlinear dynamics the pulsation is the
result of a 4 dimensional chaotic dynamics.Comment: 8 pages to appear in "Mass-Losing Pulsating Stars and Their
Circumstellar Matter", Eds. Y. Nakada & M.Honma, ASSL Ser. (in press). a
version with better quality figures is available from
http://www.phys.ufl.edu/~buchler
Period doubling bifurcation and high-order resonances in RR Lyrae hydrodynamical models
We investigated period doubling, a well-known phenomenon in dynamical
systems, for the first time in RR Lyrae models. These studies provide
theoretical background for the recent discovery of period doubling in some
Blazhko RR Lyrae stars with the Kepler space telescope. Since period doubling
was observed only in Blazhko-modulated stars so far, the phenomenon can help in
the understanding of the modulation as well. Utilising the Florida-Budapest
turbulent convective hydrodynamical code, we identified the phenomenon in
radiative and convective models as well. A period-doubling cascade was also
followed up to an eight-period solution confirming that the destabilisation of
the limit cycle is indeed the underlying phenomenon. Floquet stability roots
were calculated to investigate the possible causes and occurrences of the
phenomenon. A two-dimensional diagnostic diagram was constructed to display the
various resonances between the fundamental mode and the different overtones.
Combining the two tools, we confirmed that the period-doubling instability is
caused by a 9:2 resonance between the 9th overtone and the fundamental mode.
Destabilisation of the limit cycle by a resonance of a high-order mode is
possible because the overtone is a strange mode. The resonance is found to be
sufficiently strong enough to shift the period of overtone with up to 10
percent. Our investigations suggest that a more complex interplay of radial
(and presumably non-radial) modes could happen in RR Lyrae stars that might
have connections with the Blazhko effect as well.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Double-Mode Stellar Pulsations
The status of the hydrodynamical modelling of nonlinear multi-mode stellar
pulsations is discussed. The hydrodynamical modelling of steady double-mode
(DM) pulsations has been a long-standing quest that is finally being concluded.
Recent progress has been made thanks to the introduction of turbulent
convection in the numerical hydrodynamical codes which provide detailed results
for individual models. An overview of the modal selection problem in the HR
diagram can be obtained in the form of bifurcation diagrams with the help of
simple nonresonant amplitude equations that capture the DM phenomenon.Comment: 34 pages, to appear as a chapter in Nonlinear Stellar Pulsation in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselov (prints double column with pstops
'2:[email protected](22.0cm,-2cm)[email protected](22.0cm,11.0cm)' in.ps out.ps
Nonlinear Analysis of Irregular Variables
The Fourier spectral techniques that are common in Astronomy for analyzing
periodic or multi-periodic light-curves lose their usefulness when they are
applied to unsteady light-curves. We review some of the novel techniques that
have been developed for analyzing irregular stellar light or radial velocity
variations, and we describe what useful physical and astronomical information
can be gained from their use.Comment: 31 pages, to appear as a chapter in `Nonlinear Stellar Pulsation' in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselo
Evidence for Low-Dimensional Chaos in Semiregular Variable Stars
An analysis of the photometric observations of the light curves of the five
large amplitude, irregularly pulsating stars R UMi, RS Cyg, V CVn, UX Dra and
SX Her is presented. First, multi-periodicity is eliminated for these
pulsations, i.e. they are not caused by the excitation of a small number of
pulsation modes with constant amplitudes. Next, on the basis of energetics we
also eliminate stochasticity as a cause, leaving low dimensional chaos as the
only alternative. We then use a global flow reconstruction technique in an
attempt to extract quantitative information from the light curves, and to
uncover common physical features in this class of irregular variable stars that
straddle the RV Tau to the Mira variables. Evidence is presented that the
pulsational behavior of R UMi, RS Cyg, V CVn and UX Dra takes place in a
4-dimensional dynamical phase space, suggesting that two vibrational modes are
involved in the pulsation. A linear stability analysis of the fixed points of
the maps further indicates the existence of a two-mode resonance, similar to
the one we had uncovered earlier in R Sct: The irregular pulsations are the
result of a continual energy exchange between two strongly nonadiabatic modes,
a lower frequency pulsation mode and an overtone that are in a close 2:1
resonance. The evidence is particularly convincing for R UMi, RS Cyg and V CVn,
but much weaker for UX Dra. In contrast, the pulsations of SX Her appear to be
more complex and may require a 6D space.Comment: 20 pages, 14 figures, accepted in ApJ - paper with clearer figures is
available at http://www.phys.ufl.edu/~buchler/publications/u12.ps.gz (1Mb
A mirrorless spinwave resonator
Optical resonance is central to a wide range of optical devices and
techniques. In an optical cavity, the round-trip length and mirror reflectivity
can be chosen to optimize the circulating optical power, linewidth, and
free-spectral range (FSR) for a given application. In this paper we show how an
atomic spinwave system, with no physical mirrors, can behave in a manner that
is analogous to an optical cavity. We demonstrate this similarity by
characterising the build-up and decay of the resonance in the time domain, and
measuring the effective optical linewidth and FSR in the frequency domain. Our
spinwave is generated in a 20 cm long Rb gas cell, yet it facilitates an
effective FSR of 83 kHz, which would require a round-trip path of 3.6 km in a
free-space optical cavity. Furthermore, the spinwave coupling is controllable
enabling dynamic tuning of the effective cavity parameters.Comment: 13 pages, 4 figure
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
Feedback cooling of a cantilever's fundamental mode below 5 mK
We cool the fundamental mechanical mode of an ultrasoft silicon cantilever
from a base temperature of 2.2 K to 2.9 +/- 0.3 mK using active optomechanical
feedback. The lowest observed mode temperature is consistent with limits
determined by the properties of the cantilever and by the measurement noise.
For high feedback gain, the driven cantilever motion is found to suppress or
"squash" the optical interferometer intensity noise below the shot noise level.Comment: 4 pages, 6 figure
Honey bee genotypes and the environment
Although knowledge about honey bee geographic and genetic diversity has increased tremendously in recent decades (Meixner et al., 2013), the adaptation of honey bees to their local environment has not been well studied. The current demand for high economic performance of bee colonies with desirable behavioural characteristics contributes to changing the natural diversity via mass importations and an increasing practice of queen trade and colony movement. At the same time, there is also a growing movement in opposition to this trend, aimed at
conserving the natural heritage of local populations, with on-going projects in several countries (Strange et al., 2008; DallâOlio et al., 2008, De la RĂșa et al., 2009)
- âŠ