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2 Acta Astronomica

2 2008

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Double-Mode Classical Cepheid Models - Revisited

Smolec R., Moskalik P.

Acta Astronomica

2008

Vol. 58, No. 3

233--261

For many years modeling of double-mode pulsation of classical pulsators was a challenging problem. Inclusion of turbulent convection into pulsation hydrocodes finally led to stable double-mode models. However, it was never analyzed, which factor of turbulent convection is crucial. We show that the double-mode behavior displayed in the computed models results from incorrect assumptions adopted in some of the pulsation hydrocodes, namely from the neglect of buoyant forces in convectively stable layers. This leads to significant turbulent energies and consequently to strong eddy-viscous damping in deep, convectively stable layers of the model. Resulting differential reduction of fundamental and first overtone amplitudes favors the occurrence of double-mode pulsation. Once buoyant forces in convectively stable regions are taken into account (as they should), no stable double-mode behavior is found. The problem of modeling double-mode behavior of classical pulsators remains open.

Convective Hydrocodes for Radial Stellar Pulsation. Physical and Numerical Formulation

Smolec R., Moskalik P.

Acta Astronomica

2008

Vol. 58, No. 3

193--232

In this paper we describe our convective hydrocodes for radial stellar pulsation. We adopt the Kuhfuß (1986) model of convection, reformulated for the use in stellar pulsation hydrocodes. Physical as well as numerical assumptions of the code are described in detail. Described tests show, that our models are numerically robust and reproduce basic observational constraints. We discuss the effects of different treatment of some quantities in other pulsation hydrocodes. Our most important finding concerns the treatment of the turbulent source function in convectively stable regions. In our code we allow for negative values of source function in convectively stable zones, which reflects negative buoyancy. However, some authors restrict the source term to non-negative values. We show that this assumption leads to very high turbulent energies in convectively stable regions. The effect looks like overshooting, but it is not, because turbulence is generated by pulsations. Also, turbulent elements do not carry kinetic nor thermal energy into convectively stable layers. The range of this artificial overshooting (as we shall call it) is as large as six local pressure scale heights, leading to unphysical internal damping through the eddy-viscous forces, in deep, convectively stable parts of the star.