Thesis

Collective modes in neutron stars

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Astrophysics

• Supervisor:

  Constança Providência

• Co-Supervisor:

• Institution:

  Universidade de Coimbra

• Host Institution:

  CFC

• Abstract:

  Collective modes in neutron stars Summary The transport properties of stellar matter depend on the modes which can be excited in this medium, either by its own free constituents (electron or neutron scattering) or by escaping neutrinos. Collective modes in neutron stars (NS) will be studied in order to calculate: a) under which conditions there could exist an instability driven by the onset of hyperons; b) how the strong magnetic fields existing inside magnetars may affect isospin and density waves in asymmetric nuclear matter, in particular, the crust-core transition density; c) the spin waves in nuclear matter which may be in part polarized due to the presence of the magnetic field; d) the low lying collective modes in slab-like clusters existing in the inner crust of NS. Motivation: Compact stars, such as neutron stars, strange stars or hybrid stars, are unique laboratories that allow us to probe the building blocks of matter and their interactions at regimes that terrestrial laboratories cannot explore. They are ideal “laboratories” to study matter under extreme conditions of temperature, density, magnetic field and spacetime curvature. The study of anomalous X-ray pulsars and of soft gamma-ray repeaters has brought more and more evidence for the existence of magnetars, young neutron stars with extremely high magnetic fields which power their emission. Also, the accurate measurements of neutron stars with masses around two solar masses (PSR J1614-2230 and PSR J0348+0432) has set important constraints on the nuclear equation of state, ruling out some theoretical models. Although the crust represents only a small fraction of a neutron star, its thermic and elastic properties are crucial for the physical interpretation of the astrophysical observations related to these objects: starquakes, X-ray bursts, glitches, cooling. It is thus an important issue to characterize the transport coefficients (heat conductivity, charge conductivity and viscosity) and elastic properties of clusterized matter, and how they depend on the different configurations that can be expected. For sub-saturation densities we expect that collective modes of the nuclear matter may influence in a drastic way the opacity of neutrinos formed during the supernova explosion. The presence of spin domains could give rise to coherent effects which would increase a lot the cross section of neutrinos with a typical energy of a few MeV. Another issue where the calculation of collective modes could bring interesting information is related with the possible existence of an instability driven by the onset of hyperons. The early conclusions ruling out hyperons from the NS core seem to be refuted by recent relativistic and non-relativistic mean-field models showing that a repulsive Sigma−N interaction is able to reconcile the two solar mass measurement with strangeness population. Since presently the information on hyperon interaction in hadronic matter is very scarce, different scenarios may be put forward depending on the choice of the hyperon couplings. In particular, it has been shown within a thermodynamical approach that a correct choice of the hyperon couplings could lead to the existence of instabilities inside a neutron star driven by the onset of strangeness, and still describe a 2 solar mass star. This could have important effects because in the vicinity of critical points the neutrino mean-free path is dramatically reduced so that a cooling slow down could be anticipated. Objectives The main objectives of the present project are: 1- imposing the few existing constraints on the hyperon interaction in hadronic matter, determine under which conditions collective modes in stellar matter due to density fluctuations will become unstable. 2- determine the density, isospin and spin collective modes in npe matter in the presence of a strong magnetic field. Determine the effect of the magnetic field on the crust-core transition by calculating the dynamical spinodal characterized by the surface where the frequency of the collective mode goes to zero. 3- calculate the low-lying modes for slab like-clusters in a Wigner-Seitz lattice. Method All calculations will be carried out within relativistic nuclear models. Collective modes will be determined within a relativistic Vlasov approach for stellar matter with neutron, protons and electrons (npe) only or including also hyperons (nphe). The realistic parametrizations that satisfy most of the laboratory constraints presently accepted will be used. Particular attention will be paid to the possible role of the density dependence of the symmetry energy. For the description of hyperonic matter mesons with hidden strangeness (sigma* and phi) will also be included. For the vector mesons we will go beyond the usual SU(6)-symmetry, and the couplings to the sigma* will be chosen so that an instability with the onset of hyperons arises. The description of the effect of ultra-strong magnetic fields will take into account the anomalous magnetic moment of the nucleons and the Landau quantization of the charged particle energy levels. For the calculations of collective modes in slab-like clusters we will consider as the ground state clusters determined within the simplified coexisting method with a zero thickness surface. References C. Providência, L. Brito, S. S. Avancini, D. P. Menezes, Ph. Chomaz, Phys. Rev. C 73 (2006) 025805. T. Maruyama, T. Tatsumi, D.N. Voskresensky, T. Tanigawa and S. Chiba, Phys. Rev. C 72, 015802 (2005) S. S. Avancini, L. Brito, J. R. Marinelli, D. P. Menezes, M. M. W. de Moraes, C. Providência, and A. M. Santos, Phys. Rev. C 79, 035804 (2009) S. S. Avancini, L. Brito, D. P. Menezes, and C. Providência Phys. Rev. C 71, 044323 (2005) Broderick, A.; Prakash, M.; Lattimer, J. M., Astrophysical Journal 537, 351 (2000) Broderick, A.; Prakash, M.; Lattimer, J. M. Phys.Lett. B531 (2002) 167-174 A Rabhi, C Providência and J Da Providência J. Phys. G: Nucl. Part. Phys. 35 (2008) 125201. R. Cavagnoli, D. P. Menezes, and C. Providˆencia, Phys. Rev. C 84, 065810 (2011). S. Weissenborn, D. Chatterjee, and J. Schaffner-Bielich, Phys. Rev. C 85 (2012) 065802 F. Gulminelli, Ad. R. Raduta, and M. Oertel Phys. Rev. C 86, 025805 (2012).