Neutron Stars: matter under extreme conditions

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 are enabling new observations and breakthrough discoveries (kHz quasi- periodic oscillations, bursting millisecond pulsars, half-day long X-ray superbursts). The thermal emission from isolated neutron stars has provided important information on their radii and cooling history. At the same time, improvements in radio telescopes and interferometric techniques have increased the number of known binary pulsars, allowing for extremely precise neutron star mass measurements and tests of general relativity. 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 these new facilities will allow 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. There is growing evidence that hyperons are the first exotic component to appear at supranuclear densities in the core of neutron stars, and play a decisive role for several properties of such objects. The best existing models of the hyperon-nucleon and hyperon-hyperon interactions lead to a very soft equation of state, and to a drastic reduction of the maximum neutron star mass. Three-body forces for hyperons (hyperon-nucleon-nucleon, hyperon-hyperon-nucleon and hyperon-hyperon-hyperon) will presumably solve this problem since they are expected to be repulsive. Hyperonic three-body forces have been almost not considered in the literature, although, as it has just been said, they can make the equation of state of dense matter stiffer solving the problem of the maximum mass of neutron stars. One of the main tasks of this project will be the phenomenological and microscopic construction of such three-body hyperon forces consistent with the known two-body hyperon-nucleon and hyperon-hyperon interactions, and constrained by the scattering data and binding energies of light hypernuclei. Many-body techniques based on the Brueckner-Hartree-Fock method will then be used to study the effect of these three-body forces on the structure and properties of hypernuclei. The effect of hyperon three body forces in the equation of state and structure of neutron stars will also be investigated. 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. Part of the present project will be devoted to the study of the effect of hyperons (with and without the inclusion of hyperonic three body forces) on the cooling properties of neutron star. The aim will be to include hyperonic interaction effects in the dense hadronic medium on the most important processes generating the emission of neutrinos from neutron star cores. The effect of hyperons on the bulk viscosityof hot and dense hadronic matter will also be considered in connection with the r-mode stability. Part of this project will also be devoted to the study of the neutrino mean free path inside dense hot isospin asymmetric nuclear matter. This study will be extended in the next years to beta-stable matter including both nucleonic and hyperonic degrees of freedom 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: 01-04-2011

End date: 30-06-2012

Financing: 18000 Euros

Financing entity: FCT

Project ID: CERN/FP/116366/2010

Person*month: 30.6

Group person*month: 30.6