Dynamical approaches for secular instabilities in rotating stars

We propose a new method of investigating nonlinear dynamics focusing on the physical phenomena of rotating stars in Newtonian gravity which appear in secu- lar timescale. Since acoustic waves normally interact with the surrounding material in dynamical timescale, we need a special technique to achieve secular timescale in dynamical approach. We particularly focus on r-mode instability and construct a nu- merical scheme of killing the effect of acoustic wave by imposing an anelastic condition at every timestep. Although we only explain the rotating star in Newtonian gravity here, this technique can be also applied to relativistic gravitation (conformally at approximation in general relativity). Boundary condition for solving pressure Poisson equation and future extension from the anelastic conditions are also discussed.