3,679 research outputs found
On the Blazhko Effect in RR Lyrae Stars
The Blazhko effect is a long term, generally irregular modulation of the
light curves that occurs in a sizeable number of RR Lyrae stars. The physical
origin of the effect has been a puzzle ever since its discovery over a hundred
years ago. We build here upon the recent observational and theoretical work of
Szabo et al. on RRab stars who found with hydrodynamical simulations that the
fundamental pulsation mode can get destabilized by a 9:2 resonant interaction
with the 9th overtone. Alternating pulsation cycles arise, although these
remain periodic, i.e. not modulated as in the observations.
Here we use the amplitude equation formalism to study this nonlinear,
resonant interaction between the two modes. We show that not only does the
fundamental pulsation mode break up into a period two cycle through the
nonlinear, resonant interaction with the overtone, but that the amplitudes are
modulated, and that in a broad range of parameters the modulations are
irregular as in the observations. This irregular behavior is in fact chaotic
and arises from a strange attractor in the dynamics.Comment: 5 pages, 3 figure
Ultra-low Amplitude Variables in the LMC -- Classical Cepheids, Pop. II Cepheids, RV Tau Stars and Binary Variables
A search for variable stars with ultra-low amplitudes (ULA), in the millimag
range, has been made in the combined MACHO and OGLE data bases in the broad
vicinity of the Cepheid instability strip in the HR diagram. A total of 25
singly periodic and 4 multiply periodic ULA objects has been uncovered. Our
analysis does not allow us to distinguish between pulsational and ellipsoidal
(binary) variability, nor between LMC and foreground objects. However, the
objects are strongly clustered and appear to be associated with the pulsational
instability strips of LMC Pop. I and II variables. When combined with the ULA
variables of Buchler et al (2005) a total of 20 objects fall close to the
classical Cepheid instability strip. However, they appear to fall on parallel
period-magnitude relations that are shifted to slightly higher magnitude which
would confer them a different evolutionary status. Low amplitude RV Tauri and
Pop. II Cepheids have been uncovered that do not appear in the MACHO or OGLE
catalogs. Interestingly, a set of binaries seem to lie on a PM relation that is
essentially parallel to that of the RV Tauri/Pop. II Cepheids.Comment: 13 pages, 13 (color) figures. Astrophysical Journal (accepted for
publlication
Design research and visual analysis
Design research has been built on the method and contextual paradigms that have been the traditions of a range of other research areas. This situation creates distinct issues and challenges for the researcher and the research community. Particularly in methods available for the visual analysis of product shape, it was sensed that the existing ones were not enough to provide information that was significant to design concerns. For the analysis of artefacts, there is a plethora of methods readily available. However it is our experience that, as borrowed investigative tools, these do not supply design-relevant information on consumer products. We discuss the need for a new way that effectively considers products as design artefacts while focusing exclusively on their outward appearance.Peer reviewe
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
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