Superradiant amplification by stars and black holes
  In this thesis we study the phenomenon of superradiance and its implications to the stability of black-holes (BH) and perfect-fluid stars.   Superradiance is a radiation enhancement process that involves rotating dissipative systems. In BH spacetimes, superradiance is due to dissipation at the event horizon, with interesting associated phenomena, namely floating orbits and BH-bombs. BH superradiance is a very interdisciplinary topic, and its study allows us to obtain important results in the area of particle physics. The scattering of a scalar field by a rotating BH leads to the formation of quasi-boundstates. In rotational systems, these states can give rise to superradiant instabilities. These results were recently used to impose constraints to the mass of fundamental particles and darkmatter candidates.   In this work, it is shown that, when dissipation is properly included, similar results are achievable in self-gravitating systems other than black-holes, such as perfect fluid stars. It is also demonstrated that the relativistic effects related to the phenomenon of frame-dragging are neglectable in this calculation. 
Jo\~ao Lu\'is de Figueiredo Rosa
gr-qc