Equilibrium and stability of supermassive stars in binary systems - art. no. 024004

We investigate the equilibrium and stability of supermassive stars of mass M greater than or similar to 10(5)M(.) in binary systems. We find that coro tating binaries are secularly unstable for close, circular orbits with r le ss than or similar to 4R(M/10(6)M(.))(1/6) where r is the orbital separatio n and R the stellar radius. We also show that corotation cannot be achieved for distant orbits with r greater than or similar to 12R(M/10(6)M(.))(-11/ 24), since the time scale for viscous angular momentum transfer associated with tidal torques is longer than the evolution time scale due to emission of thermal radiation. These facts suggest that the allowed mass range and o rbital separation for corotating supermassive binary stars is severely rest ricted. In particular, for supermassive binary stars of large mass M greate r than or similar to 6x10(6)M(.), corotation cannot be achieved, as viscosi ty is not adequate to mediate the transfer between orbital and spin angular momentum. One possible outcome for binary supermassive stars is the onset of quasi-radial, relativistic instability which drives each star to collaps e prior to merger: We discuss alternative outcomes of collapse and possible spin states of the resulting black holes. We estimate the frequency and am plitude of gravitational waves emitted during several inspiral and collapse scenarios.