10 research outputs found
Strange Cepheids and RR Lyrae
Strange modes can occur in radiative classical Cepheids and RR Lyrae models.
These are vibrational modes that are trapped near the surface as a result of a
'potential barrier' caused by the sharp hydrogen partial ionization region.
Typically the modal number of the strange mode falls between the 7th and 12th
overtone, depending on the astrophysical parameters of the equilibrium stellar
models (L, M, \Teff, X, Z). Interestingly these modes can be linearly unstable
outside the usual instability strip, in which case they should be observable as
new kinds of variable stars, 'strange Cepheids' or 'strange RR Lyrae' stars.
The present paper reexamines the linear stability properties of the strange
modes by taking into account the effects of an isothermal atmosphere, and of
turbulent convection. It is found that the linear vibrational instability of
the strange modes is resistant to both of these effects. Nonlinear hydrodynamic
calculations indicate that the pulsation amplitude of these modes is likely to
saturate at the millimagnitude level. These modes should therefore be
detectable albeit not without effort.Comment: 6 pages, 7 figures, submitted to Ap
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
RR Lyrae - Theory vs Observation
The luminosities, effective temperatures and metallicities that are derived
empirically by Kovacs and Jurcsik from the light curves of a large number of
globular cluster and field RRab and RRc stars are compared to theoretical RR
Lyrae models. The strong luminosity dependence of the empirical blue and red
edges (Log L vs Log Teff diagram) is in disagreement with that of both
radiative and convective models. A reexamination of the theoretical
uncertainties in the modelling leads us to conclude that the disagreement is
irreconcilable.Comment: 6 pages, 5 figures (revised april 2000, revisions relatively minor
Mode Switching Time Scales in the Classical Variable Stars
Near the edges of the instability strip the rate of stellar evolution is
larger than the growth-rate of the pulsation amplitude, and the same holds
whenever the star is engaged in pulsational mode switching. Stellar evolution
therefore controls both the onset of pulsation at the edges of the instability
strip and of mode switching inside it. Two types of switchings (bifurcations)
occur. In a soft bifurcation the switching time scale is the inverse harmonic
mean of the pulsational modal growth-rate and of the stellar evolution rate. In
a hard bifurcation the switching times can be substantially longer than the
thermal time scale which is typically of the order of a hundred periods for
Cepheids and RR Lyrae. We discuss some of the observational consequences, in
particular the paucity of low amplitude pulsators at the edges of the
instability strip.Comment: 5 pages, 3 figures, ApJ (in press
Hydrodynamical Survey of First Overtone Cepheids
A hydrodynamical survey of the pulsational properties of first overtone
Galactic Cepheids is presented. The goal of this study is to reproduce their
observed light- and radial velocity curves. The comparison between the models
and the observations is made in a quantitative manner on the level of the
Fourier coefficients. Purely radiative models fail to reproduce the observed
features, but convective models give good agreement.
It is found that the sharp features in the Fourier coefficients are indeed
caused by the P1/P4 = 2 resonance, despite the very large damping of the 4th
overtone. For the adopted mass-luminosity relation the resonance center lies
near a period of 4.2d +/- 0.2 as indicated by the observed radial velocity
data, rather than near 3.2d as the light-curves suggest.Comment: ApJ, 12 pages, (slightly) revise