Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-04-02
Astronomy and Astrophysics
Astrophysics
11 pages, 9 figures, 1 table, accepted for publication in A&A
Scientific paper
10.1051/0004-6361:200809891
Context: We study the kappa-mechanism that excites radial oscillations in Cepheid variables. Aims: We address the mode couplings that manages the nonlinear saturation of the instability in direct numerical simulations (DNS). Methods: We project the DNS fields onto an acoustic subspace built from the regular and adjoint eigenvectors that are solutions to the linear-oscillations equations. Results: We determine the time evolution of both the amplitude and kinetic energy of each mode that propagates in the DNS. More than 98% of the total kinetic energy is contained in two modes that correspond to the linearly-unstable fundamental mode and the linearly-stable second overtone. Because the eigenperiods ratio is close to 1/2, we discover that the nonlinear saturation is due to a 2:1 resonance between these two modes. An interesting application of this result concerns the reproduction of Hertzsprung's progression observed in Bump Cepheids.
Dintrans Boris
Gastine T.
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