Compact Stars: Laboratories for nuclear and particle physics

One of the most fascinating enigmas in modern astrophysics concerns the true nature of the ultradense compact objects called neutron stars. These objects are an excellent observatory to test our understanding of nuclear matter at extreme conditions of density and temperature. The new generation of space X-ray and gamma-ray observatories and improvements in radio telescopes and interferometric techniques are enabling new observations and breakthrough discoveries. A large multinational effort has taken place in the last decade to build detectors, offering the exciting prospect of the detection of gravitational waves. Finally, the new generation of exotic beam facilities in France (SPIRAL), Germany (FAIR), Japan (RIKEN, J-PARC), USA (RIA) or the EU (EURISOL) will allow experimental studies of very exotic nuclei which will have a direct impact on the modeling of compact stars. In particular, a major effort is being invested in to determine the density dependence of the nuclear symmetry energy, a quantity that determines to a large extent the composition of beta-stable matter and therefore the structure and mass of a neutron star. These new facilities will allow also a efficient production of hypernuclei thus offering promising perspectives for studying the interactions between hyperons and nucleons. This is imperative for determining the interior composition of neutron stars at densities above 2-3 times the normal saturation density of nuclear matter. A deeper understanding of the density dependence of the nuclear symmetry energy is of crucial importance not only for the surface structure of exotic nuclei with large neutron excess, but also for the structure and composition of neutron stars from the ultradense core to the crust at subsaturation densities, and for the core collapse supernovae, where the at high densities the symmetry energy determines the energy of the shock and at low densities it affects the nuclear composition, neutrino interactions and aspects of nucleosyntesis. One of the tasks of this project will be a systematic study of the density denpendence of the symmetry energy paying special attention to the contribution of the different partial waves and to the operatorial structure of the nucleon-nucleon interaction, in order to identify the dominant contribution. Hyperonic three-body forces have been almost not considered in the literature, although they can make the equation of state of dense matter stiffer solving the problem of the maximum mass of neutron stars. The phenomenological and microscopic construction of such three-body hyperon forces consistently with the known two-body hyperon-nucleon and hyperon-hyperon interactions, and constrained by the scattering data and binding energies of light hypernuclei was one of the main task of the project CERN/FP/116366/2010. This task will be continued and extended during the present project. Hyperons can also deeply affect the thermal evolution of neutron stars by allowing fast URCA reactions since they may modify neutrino emissivities. Moreover, the damping of neutron star pulsations (hence the emission of gravitational waves) can be strongly influenced by the presence of hyperons, because they may also alter the bulk viscosity. The study of the effect of hyperons (with and without the inclusion of hyperonic three body forces) on the cooling properties of neutron star, including hyperonic interaction effects in the dense hadronic medium on the most important processes generating the emission of neutrinos from neutron star cores, was other of the tasks of the project CERN/FP/116366/2010. This task will also be continued and extended during the present project. Pulsar glitches are sudden spin-ups in the otherwise steadily decreasing frequency of rotating magnetized neutron stars. According to the vortex-model, glitches may represent direct evidence for the existence of a macroscopic superfluid inside such stars. In this project we plan also to study the vortices induced in nonuniform nuclear matter formed by nuclear clusters immersed in a neutron superfluid gas. Among the properties we intend to investigate are the threshold angular velocity for the rotation of a superfluid gas in a rotating vessel and the dependence of the vortex-nucleus pinning energy on the intensity of the pairing correlations. The study will be done in a self-consistent Hartre-Fock-Bogoliubov (HFB) approach. Finally, the effect of cluster formation on the latent heat of the liquid-gas phase transition in nuclear matter and on the density dependence of ths symmetry energy will also be studied in this project. The team of the present project is integrated in a RNP (Research Network Program) of the European Science Foundation called "The Physics of Compact Stars", which aims at linking the best European scientist on the field to reach a better understanding of the physics of compact stars.

Status: Concluded

Starting date: 1/Jul/2012

End date: 30/Jun/2014

Financing: 40000 Euros

Financing entity: FCT

Project ID: CERN/FP/123608/2011

Person*month: 68.1

Group person*month: 68.1