Spacetime modes of relativistic stars

The problem of relativistic stellar pulsations is studied in a somewhat ad hoc approximation that ignores all fluid motions. This Inverse Cowling Approximation (ICA) is motivated by two observations: 1) For highly damped (w-mode) oscillations the fluid plays very little role. 2) If the fluid motion is neglected the problem for polar oscillation modes becomes similar to that for axial modes. Using the ICA we find a polar mode spectrum that has all features of the w-mode spectrum of the full problem. Moreover, in the limit of superdense stars, we find the ICA spectrum to be qualitatively similar to that of the axial modes. These results clearly show the importance of general relativity for the pulsation modes of compact stars, and that there are modes whose existence do not depend on motions of the fluid at all; pure “spacetime” modes.