Hydrodynamical simulations of dns systemsgravitational emission and equation of state

Resumen

Double neutron star systems (DNS hereafter) are of major interest due to their extreme physical characteristics. Because of neutron stars are extremely compact objects, with central densities ranging from 1014 to 1015 g/cm3 , very intense gravitational fields are governing the evolution of DNS systems. This evolution involves important general relativistic effects, being one of those the emission of gravitational waves (GW). These are an oscillating warpage of the spacetime that carries away energy from the system. In consequence the orbital distance of a DNS gradually decreases, shrinking the orbit and leading to an inevitable collision of the two neutron stars called merger, while the whole process is named coalescence. The most important observational evidence of this process is the orbital period decrease of the Hulse-Taylor binary pulsar (PSR1913+16), which has an excellent agreement with the theoretical prediction due to GW emission. GW have an important dependence on the equation of state (EOS) of neutron stars, which nowadays is still an important unknown. Therefore, GW have information regarding the actual EOS that describes the behavior of matter at nuclear densities. How to use GW to impose constraints in the set of theoretical nuclear EOS is the topic addressed in this thesis. To that purpose we have developed a fully lagrangian hydrocode based in the Smoothed Particle Hydrodynamics (SPH) technique. With this code we have performed 29 numerical simulations of DNS systems using eleven different EOS that are commonly used in literature. Eleven simulations have been done using a Newtonian quadrupolar approximation, while the remaining calculations have been done within the Post-Newtonian paradigm, including 1PN and 2.5PN corrections, following the BDS method. From the results we could extract which are the magnitudes that can impose significant constraints in the M-R diagram for neutron stars, and the precision up to which they should be obtained from future GW detections in ground interferometers like Advanced LIGO, VIRGO or GEO600. We also determined that the inclusion of relativistic corrections in fact helps to distinguish among different EOS and does not change the general results obtained from Newtonian simulations. This is a good prospect for future full relativistic simulations which will probably be able to determine strong constraints that help to determine which the actual EOS of nuclear matter is.