GRAVITATIONAL-WAVES FROM ULTRACOMPACT STARS - THE OPTICAL GEOMETRY VIEW OF TRAPPED MODES

Ultracompact stars-if they exist-have a unique signature: a discrete s pectrum of slowly damped gravitational-wave quasinormal modes. It is k nown that this feature is due to a potential well present for stars wi th R < 3M. In this letter we show that the appearance of this well is essentially due to the non-Newtonian behaviour of the centrifugal pote ntial: here we explain this behaviour adopting the optical geometry po int of view. This insight leads us to an approximation, valid for high ly compact stars, by which the real part of the spectrum is easily com puted with good accuracy.