253 research outputs found
More concerning the anelastic and subseismic approximations for low-frequency modes in stars
Two approximations, namely the subseismic approximation and the anelastic
approximation, are presently used to filter out the acoustic modes when
computing low frequency modes of a star (gravity modes or inertial modes). In a
precedent paper (Dintrans & Rieutord 2001), we observed that the anelastic
approximation gave eigenfrequencies much closer to the exact ones than the
subseismic approximation. Here, we try to clarify this behaviour and show that
it is due to the different physical approach taken by each approximation: On
the one hand, the subseismic approximation considers the low frequency part of
the spectrum of (say) gravity modes and turns out to be valid only in the
central region of a star; on the other hand, the anelastic approximation
considers the Brunt-Vaisala frequency as asymptotically small and makes no
assumption on the order of the modes. Both approximations fail to describe the
modes in the surface layers but eigenmodes issued from the anelastic
approximation are closer to those including acoustic effects than their
subseismic equivalent.
We conclude that, as far as stellar eigenvalue problems are concerned, the
anelastic approximation is better suited for simplifying the eigenvalue problem
when low-frequency modes of a star are considered, while the subseismic
approximation is a useful concept when analytic solutions of high order
low-frequency modes are needed in the central region of a star.Comment: 5 pages 3 fig, to appear in MNRA
Identification of gravity waves in hydrodynamical simulations
The excitation of internal gravity waves by an entropy bubble oscillating in
an isothermal atmosphere is investigated using direct two-dimensional numerical
simulations. The oscillation field is measured by a projection of the simulated
velocity field onto the anelastic solutions of the linear eigenvalue problem
for the perturbations. This facilitates a quantitative study of both the
spectrum and the amplitudes of excited g-modes.Comment: 12 pages, 11 figures, Appendices only available onlin
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&
Convective quenching of stellar pulsations
Context: we study the convection-pulsation coupling that occurs in cold
Cepheids close to the red edge of the classical instability strip. In these
stars, the surface convective zone is supposed to stabilise the radial
oscillations excited by the kappa-mechanism.
Aims: we study the influence of the convective motions onto the amplitude and
the nonlinear saturation of acoustic modes excited by kappa-mechanism. We are
interested in determining the physical conditions needed to lead to a quenching
of oscillations by convection.
Methods: we compute two-dimensional nonlinear simulations (DNS) of the
convection-pulsation coupling, in which the oscillations are sustained by a
continuous physical process: the kappa-mechanism. Thanks to both a frequential
analysis and a projection of the physical fields onto an acoustic subspace, we
study how the convective motions affect the unstable radial oscillations.
Results: depending on the initial physical conditions, two main behaviours
are obtained: (i) either the unstable fundamental acoustic mode has a large
amplitude, carries the bulk of the kinetic energy and shows a nonlinear
saturation similar to the purely radiative case; (ii) or the convective motions
affect significantly the mode amplitude that remains very weak. In this second
case, convection is quenching the acoustic oscillations. We interpret these
discrepancies in terms of the difference in density contrast: larger
stratification leads to smaller convective plumes that do not affect much the
purely radial modes, while large-scale vortices may quench the oscillations.Comment: 15 pages, 17 figures, 3 tables, accepted for publication in A&
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