77 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
Kepler RR Lyrae stars: beyond period doubling
We examined the complete short cadence sample of Kepler RR Lyrae stars to
further investigate the recently discovered dynamical effects such as period
doubling and additional modes. Here we present the findings on four stars. V450
Lyr may be a non-classical double-mode RR Lyrae star pulsating in the
fundamental mode and the second overtone. In three cases we observe the
interaction of three different modes. Since the period ratios are close to
resonant values, we observe quasi-repetiting patterns in the pulsation cycles
in the stars. These findings support the mode-resonance explanations of the
Blazhko effect.Comment: 2 pages, 1 figure. Proceedings of the 301st IAU Symposium, Precision
Asteroseismology, August 2013, Wroc{\l}aw. V2: small corrections to the
wordin
Multiple and changing cycles of active stars I. Methods of analysis and application to the solar cycles
Long-term observational data have information on the magnetic cycles of
active stars and that of the Sun. The changes in the activity of our central
star have basic effects on Earth, like variations in the global climate.
Therefore understanding the nature of these variations is extremely important.
The observed variations related to magnetic activity cannot be treated as
stationary periodic variations, therefore methods like Fourier transform or
different versions of periodogramms give only partial information on the nature
of the light variability. We demonstrate that time-frequency distributions
provide useful tools for analyzing the observations of active stars. With test
data we demonstrate that the observational noise has practically no effect on
the determination in the the long-term changes of time-series observations of
active stars. The rotational signal may modify the determined cycles, therefore
it is advisable to remove it from the data. Wavelets are less powerful in
recovering complex long-term changes than other distributions which are
discussed. Applying our technique to the sunspot data we find a complicated,
multi-scale evolution in the solar activity.Comment: Accepted to Astronomy and Astrophysic
Nonlinear dynamical analysis of the Blazhko effect with the Kepler space telescope: the case of V783 Cyg
We present a detailed nonlinear dynamical investigation of the Blazhko
modulation of the Kepler RR Lyrae star V783 Cyg (KIC 5559631). We used
different techniques to produce modulation curves, including the determination
of amplitude maxima, the O-C diagram and the analytical function method. We
were able to fit the modulation curves with chaotic signals with the global
flow reconstruction method. However, when we investigated the effects of
instrumental and data processing artefacts, we found that the chaotic nature of
the modulation can not be proved because of the technical problems of data
stitching, detrending and sparse sampling. Moreover, we found that a
considerable part of the detected cycle-to-cycle variation of the modulation
may originate from these effects. According to our results, even the
four-year-long, unprecedented Kepler space photometry of V783 Cyg is too short
for a reliable nonlinear dynamical analysis aiming at the detection of chaos
from the Blazhko modulation. We estimate that two other stars could be suitable
for similar analysis in the Kepler sample and in the future TESS and PLATO may
provide additional candidates.Comment: 9 pages, 12 figures, accepted for publication in MNRA
Automated Nonlinear Stellar Pulsation Calculations: Applications to RR Lyrae stars. The Slope of the Fundamental Blue Edge and the First RRd Model Survey
We describe a methodology that allows us to follow the pulsational behavior
of an RR Lyrae model consistently and automatically along its evolutionary
track throughout the whole instability strip. It is based on the powerful
amplitude equation formalism, and resorts to a judicious combination of
numerical hydrodynamical simulations, the analytical signal time-series
analysis, and amplitude equations. A large-scale survey of the nonlinear
pulsations in RR Lyr instability strip is then presented, and the mode
selection mechanism is delineated throughout the relevant regions of parameter
space. We obtain and examine two regions with hysteresis, where the pulsational
state depends on the direction of the evolutionary tracks, namely a region with
either fundamental (RRab) or first overtone (RRc) pulsations and a region with
either fundamental (RRab) or double-mode (RRd) pulsations. The regions where
stable double-mode (DM, or RRd) pulsations occur are very narrow and hard to
find in astrophysical parameter (L, M, T_eff, X, Z) space with hydrodynamic
simulations, but our systematic and efficient methodology allows us to
investigate them with unprecedented detail. It is shown that by simultaneously
considering the effects of mode selection and of horizontal branch evolution we
can naturally solve one of the extant puzzles involving the topologies of the
theoretical and observed instability strips, namely the slope of the
fundamental blue edge. The importance of the interplay between mode selection
and stellar evolutionary effects is also demonstrated for the properties of
double-mode RR Lyr. Finally, the Petersen diagram of double-mode RR Lyr models
is discussed for the first time.Comment: 13 pages, 6 figures, accepted to be published in A&
Long time-scale behavior of the Blazhko effect from rectified Kepler data
In order to benefit from the 4-year unprecedented precision of the Kepler
data, we extracted light curves from the pixel photometric data of the Kepler
space telescope for 15 Blazhko RR Lyrae stars. For collecting all the flux from
a given target as accurately as possible, we defined tailor-made apertures for
each star and quarter. In some cases the aperture finding process yielded
sub-optimal result, because some flux have been lost even if the aperture
contains all available pixels around the star. This fact stresses the
importance of those methods that rely on the whole light curve instead of
focusing on the extrema (O-C diagrams and other amplitude independent methods).
We carried out detailed Fourier analysis of the light curves and the amplitude
independent O-C diagram. We found 12 (80%) multiperiodically modulated stars in
our sample. This ratio is much higher than previously found. Resonant coupling
between radial modes, a recent theory to explain of the Blazhko effect, allows
single, multiperiodic or even chaotic modulations. Among the stars with two
modulations we found three stars (V355 Lyr, V366 Lyr and V450 Lyr) where one of
the periods dominate in amplitude modulation, but the other period has larger
frequency modulation amplitude. The ratio between the primary and secondary
modulation periods is almost always very close to ratios of small integer
numbers. It may indicate the effect of undiscovered resonances. Furthermore, we
detected the excitation of the second radial overtone mode for three
stars where this feature was formerly unknown. Our data set comprises the
longest continuous, most precise observations of Blazhko RR Lyrae stars ever
published. These data which is made publicly available will be unprecedented
for years to come.Comment: 19 pages, 17 figures, accepted for publication in Astrophys. J.
Suppl. Rectified data are here: http://www.konkoly.hu/KIK/data.htm
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