Stellar pulsations

White dwarf stars are  g-mode pulsationally unstable in three temperature regimes, with typical periods in the range 100-1000 seconds. Over the last past decade, the study of pulsational pattern of variable white dwarfs through asteroseismological techniques has become a very powerful tool for probing the internal structure and evolution of these stars. In particular, asteroseismology of massive white dwarfs has  recently drawn the attention of researchers in view of  the possibility it offers to place constraints on the crystallization process in the  interior of white dwarfs.  This has been motivated by the discovery of pulsation in the star BPM  37093, a  massive white dwarf which should be largely crystallized.

Our group has developed a pulsational code that compute the linear, adiabatic, non-radial stellar pulsations (Córsico, 2003, PhD., University of La Plata).  This code is fully coupled to the LPCODE evolutionary code, which has enabled us to study the pulsations of  variable white dwarfs. One of our main results concerns the mode trapping properties of white dwarfs.  We find that element  diffusion strongly  smoothes out  the chemical  profiles,  making the mode trapping caused by the outer chemical  interfaces notably less important (Córsico et  al., 2001, A&A, 380, L17).  In collaboration with Michael Montgomery of the University of Texas we started a joint project aimed at exploring the pulsational properties of massive white dwarfs on the basis of  new and improved evolutionary models for these  stars that take into account time-dependent element diffusion, nuclear  burning and  the history of the white dwarf progenitor.  Our first results suggest that the pulsational properties become very sensitive to the occurrence of core overshooting during the evolutionary stages prior to the white dwarf formation  (Althaus L.G., Serenelli A. M., Córsico  A. H. & Montgomery M. H., 2003, A&A, 404, 593). In this connection, we are currently investigating the effect of a solid core on the pulsational pattern of crystallized white dwarfs (see Córsico A. H., Althaus L.G.,  Montgomery M. H., García-Berro E. & Isern J., 2005, A&A, 429, 277)