31 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
The Cepheid Phase Lag Revisited
We compute the phase lags between the radial velocity curves and the light
curves for classical Cepheid model
sequences both in the linear and the nonlinear regimes. The nonlinear phase
lags generally fall below the linear ones except for high period models where
they lie above, and of course for low pulsation amplitudes where the two merge.
The calculated phase lags show good agreement with the available observational
data of normal amplitude Galactic Cepheids. The metallicity has but a moderate
effect on the phase lag, while the mass-luminosity relation and the parameters
of the turbulent convective model (time-dependent mixing length) mainly
influence the modal selection and the period, which is then reflected in the
period -- diagram. We discuss the potential application of this
observable as a discriminant for pulsation modes and as a test for ultra-low
amplitudes (ULA) pulsation.Comment: 11 pages, 8 figures, accepted for publication in ApJ, minor revisions
in the text and figures, (black and white version available from 2nd author's
website
Beat Cepheids as Probes of Stellar and Galactic Metallicity
The mere location of a Beat Cepheid model in a Period Ratio vs. Period
diagram (Petersen diagram) puts very tight constraints on its metallicity Z.
The Beat Cepheid Peterson diagrams are revisited with linear nonadiabatic
turbulent convective models, and their accuracy as a probe for stellar
metallicity is evaluated. They are shown to be largely independent of the
helium content Y, and they are also only weakly dependent on the
mass-luminosity relation that is used in their construction. However, they are
found to show sensitivity to the relative abundances of the elements that are
lumped into the metallicity parameter Z. Rotation is estimated to have but a
small effect on the 'pulsation metallicities'. A composite Petersen diagram is
presented that allows one to read off upper and lower limits on the metallicity
Z from the measured period P0 and period ratio P1/P0.Comment: 9 pages, 12 color figures (black and white version available from 1st
author's website). With minor revisions. to appear in Ap
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
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
Hydrodynamic stability and mode coupling in Keplerian flows: local strato-rotational analysis
Aims. Qualitative analysis of key (but yet unappreciated) linear phenomena in
stratified hydrodynamic Keplerian flows: (i) the occurrence of a vortex mode,
as a consequence of strato-rotational balance, with its transient dynamics;
(ii) the generation of spiral-density waves (also called inertia-gravity or
waves) by the vortex mode through linear mode coupling in shear
flows. Methods. Non-modal analysis of linearized Boussinesq equations written
in the shearing sheet approximation of accretion disk flows. Results. It is
shown that the combined action of rotation and stratification introduces a new
degree of freedom -- vortex mode perturbation -- which is linearly coupled with
the spiral-density waves. These two modes are jointly able to extract energy
from the background flow and they govern the disk dynamics in the small-scale
range. The transient behavior of these modes is determined by the non-normality
of the Keplerian shear flow. Tightly leading vortex mode perturbations undergo
substantial transient growth, then, becoming trailing, inevitably generate
trailing spiral-density waves by linear mode coupling. This course of events --
transient growth plus coupling -- is particularly pronounced for perturbation
harmonics with comparable azimuthal and vertical scales and it renders the
energy dynamics similar to the 3D unbounded plane Couette flow case.
Conclusions. Our investigation strongly suggests that the so-called bypass
concept of turbulence, which has been recently developed by the hydrodynamic
community for spectrally stable shear flows, can also be applied to Keplerian
disks. This conjecture may be confirmed by appropriate numerical simulations
that take in account the vertical stratification and consequent mode coupling
in the high Reynolds number regime.Comment: A&A (accepted
Limit-cycle behavior in one-zone convective models
We present the results of a detailed set of one-zone models that account for
the coupling between pulsation and convection following the original
prescriptions of Stellingwerf (1986). Motivated by the arbitrary nature of the
input parameters adopted in this theoretical framework, we computed several
sequences of models that cover a substantial fraction of the parameter space
and a longer integration time. We also included a turbulent pressure term and
found that this physical mechanism plays a crucial role in the pulsation
characteristics of the models by removing the sharp discontinuities along the
light and the velocity curves showed by models that do not account for
turbulent pressure. Finally, we investigated the vibrational and the
pulsational stability of completely convective models. We consider as the most
important finding of the present work the identification of a well-defined
region in the parameter space where they approach limit-cycle stability.
Several numerical experiments performed by adopting different values of the
adiabatic exponent and of the shell thickness indicate that the coupling
between pulsation and convection is the key driving mechanism for LPVs, a
finding supported by recent theoretical predictions.Comment: 11 pages, 10 figures; accepted for publication in Astrophysical
Journa
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
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