Evolution and instabilities of adolescent neutron stars

01/10/2016 to 30/09/2019

Several gravitational wave detectors are currently being built. Some of them are in the latest phases of their construction and expected to start operating in the next two years. Therefore the need to identify and extensively explore the astrophysical phenomena that might lead to the emission of strong gravitational wave signal is increasing. In the current project we concentrate on one of the promising sources of gravitational waves – the oscillations of neutron stars and more precisely on the oscillation modes which are unstable due to the Chandrasekhar-Friedman-Schutz (CFS) instability. Such unstable modes can reach very large amplitudes and eventually become emitters of copious amount of gravitational radiation. The CFS instability is better pronounced for rapidly rotating massive stars, so our studies will be concentrated mainly on the newly formed neutron stars. Such objects can be created after the merger of two neutron stars and also after the core-collapse of a massive star. The CFS instability is a secular instability which develops on larger timescales. Therefore, in order to track the emitted gravitational radiation, we plan to build a code which follows the evolution of rapidly rotating neutron stars as they mature, including the magnetic field and the detailed thermal profile. Concerning the gravitational wave emission, we will take into account various classes of modes, such as f-, r- and g-modes. For computing the oscillation modes of the star and determining which ones are CFS unstable, we will use a linear perturbation code. Such a code was already developed by the Theoretical Astrophysics group at the University of Tübingen in the Cowling approximation, and we plan to extend it to the case of hot neutron stars. A major step will be to drop the Cowling approximation and to consider the full general relativistic case. Having all these tools in hand we will investigate the evolution of various rapidly rotating neutron star configurations and determine the most promising sources of gravitational radiation. In the last part of the project we will be concentrated on the evolution of neutron stars in generalized theories of gravity. Our goal is to determine the differences compared to the pure general realistic case and make predictions which would allow us to test the strong field regime of these theories using the forthcoming observations of gravitational waves.