102 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
Linear dynamics of weakly viscous accretion disks: A disk analog of Tollmien-Schlichting waves
This paper discusses new perspectives and approaches to the problem of disk
dynamics where, in this study, we focus on the effects of viscous instabilities
influenced by boundary effects. The Boussinesq approximation of the viscous
large shearing box equations is analyzed in which the azimuthal length scale of
the disturbance is much larger than the radial and vertical scales. We examine
the stability of a non-axisymmetric potential vorticity mode, i.e. a
PV-anomaly. in a configuration in which buoyant convection and the
strato-rotational instability do not to operate. We consider a series of
boundary conditions which show the PV-anomaly to be unstable both on a finite
and semi-infinite radial domains. We find these conditions leading to an
instability which is the disk analog of Tollmien-Schlichting waves. When the
viscosity is weak, evidence of the instability is most pronounced by the
emergence of a vortex sheet at the critical layer located away from the
boundary where the instability is generated. For some boundary conditions a
necessary criterion for the onset of instability for vertical wavelengths that
are a sizable fraction of the layer's thickness and when the viscosity is small
is that the appropriate Froude number of the flow be greater than one. This
instability persists if more realistic boundary conditions are applied,
although the criterion on the Froude number is more complicated. The unstable
waves studied here share qualitative features to the instability seen in
rotating Blasius boundary layers. The implications of these results are
discussed. An overall new strategy for exploring and interpreting disk
instability mechanisms is also suggested.Comment: Accepted for publication in Astronomy and Astrophysics. 18 pages.
This version 3 with corrected style fil
A test of time-dependent theories of stellar convection
Context: In Cepheids close to the red edge of the classical instability
strip, a coupling occurs between the acoustic oscillations and the convective
motions close to the surface.The best topical models that account for this
coupling rely on 1-D time-dependent convection (TDC) formulations. However,
their intrinsic weakness comes from the large number of unconstrained free
parameters entering in the description of turbulent convection. Aims: We
compare two widely used TDC models with the first two-dimensional nonlinear
direct numerical simulations (DNS) of the convection-pulsation coupling in
which the acoustic oscillations are self-sustained by the kappa-mechanism.
Methods: The free parameters appearing in the Stellingwerf and Kuhfuss TDC
recipes are constrained using a chi2-test with the time-dependent convective
flux that evolves in nonlinear simulations of highly-compressible convection
with kappa-mechanism. Results: This work emphasises some inherent limits of TDC
models, that is, the temporal variability and non-universality of their free
parameters. More importantly, within these limits, Stellingwerf's formalism is
found to give better spatial and temporal agreements with the nonlinear
simulation than Kuhfuss's one. It may therefore be preferred in 1-D TDC
hydrocodes or stellar evolution codes.Comment: 7 pages, 5 figures, 2 tables, accepted for publication in A&
Nonlinear Beat Cepheid Models
The numerical hydrodynamic modelling of beat Cepheid behavior has been a
longstanding quest in which purely radiative models have failed miserably. We
find that beat pulsations occur naturally when turbulent convection is
accounted for in our hydrodynamics codes.
The development of a relaxation code and of a Floquet stability analysis
greatly facilitates the search for and analysis of beat Cepheid models.
The conditions for the occurrence of beat behavior can be understood easily
and at a fundamental level with the help of amplitude equations. Here a
discriminant D arises whose sign decides whether single mode or double mode
pulsations can occur in a model, and this D depends only on the values of the
nonlinear coupling coefficients between the fundamental and the first overtone
modes. For radiative models D is always found to be negative, but with
sufficiently strong turbulent convection its sign reverses.Comment: 5 pages, incl. 4 figs - apj lett, accepted may 18, 199
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
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
- …