Symmetry energy: nuclear masses and neutron stars

We describe the main features of our most recent Hartree-Fock-Bogoliubov nuclear mass models, based on 16-parameter generalized Skyrme forces. They have been fitted to the data of the 2012 Atomic Mass Evaluation, and favour a value of 30 MeV for the symmetry coefficient J, the corresponding root-mean square deviation being 0.549 MeV. We find that this conclusion is compatible with measurements of neutron-skin thickness. By constraining the underlying interactions to fit various equations of state of neutron matter calculated {\it ab initio} our models are well adapted to a realistic and unified treatment of all regions of neutron stars. We use our models to calculate the composition, the equation of state, the mass-radius relation and the maximum mass. Comparison with observations of neutron stars again favours a value of J = 30 MeV.Comment: 10 pages, 9 figures, to appear in EPJA special volume on symmetry energ

Structure of neutron stars with unified equations of state

We present a set of three unified equations of states (EoSs) based on the nuclear energy-density functional (EDF) theory.These EoSs are based on generalized Skyrme forces fitted to essentially all experimental atomic mass data and constrained to reproduce various properties of infinite nuclear matter as obtained from many-body calculations using realistic two- and three-body interactions. The structure of cold isolated neutron stars is discussed in connection with some astrophysical observations.Comment: 4 pages, to appear in the proceedings of the ERPM conference, Zielona Gora, Poland, April 201

Neutron drip transition in accreting and nonaccreting neutron star crusts

The neutron-drip transition in the dense matter constituting the interior of neutron stars generally refers to the appearance of unbound neutrons as the matter density reaches some threshold density $\rho_\textrm{drip}$. This transition has been mainly studied under the cold catalyzed matter hypothesis. However, this assumption is unrealistic for accreting neutron stars. After examining the physical processes that are thought to be allowed in both accreting and nonaccreting neutron stars, suitable conditions for the onset of neutron drip are derived and general analytical expressions for the neutron drip density and pressure are obtained. Moreover, we show that the neutron-drip transition occurs at lower density and pressure than those predicted within the mean-nucleus approximation. This transition is studied numerically for various initial composition of the ashes from X-ray bursts and superbursts using microscopic nuclear mass models.Comment: 24 pages, accepted for publication in Physical Review

Giant Pulsar Glitches and the Inertia of Neutron-Star Crusts

Giant pulsar frequency glitches as detected in the emblematic Vela pulsar have long been thought to be the manifestation of a neutron superfluid permeating the inner crust of a neutron star. However, this superfluid has been recently found to be entrained by the crust, and as a consequence it does not carry enough angular momentum to explain giant glitches. The extent to which pulsar-timing observations can be reconciled with the standard vortex-mediated glitch theory is studied considering the current uncertainties on dense-matter properties. To this end, the crustal moment of inertia of glitching pulsars is calculated employing a series of different unified dense-matter equations of state.Comment: 11 pages, 6 figures, submitted to PR

Light clusters in the liquid proto-neutron star inner crust

Being born hot from core-collapse supernova, the crust of the proto-neutron star is expected to be made of a Coulomb liquid and composed of an ensemble of different nuclear species. In this work, we study the beta-equilibrated proto-neutron-star crust in the liquid phase in a self-consistent multi-component approach, employing a compressible liquid-drop description of the ions including the ion centre-of-mass motion. Particular care is also devoted to the calculation of the rearrangement term, thus ensuring thermodynamic consistency. We compare the results of the multi-component plasma calculations with those obtained within a one-component (single-nucleus) approach, showing that important differences arise between the predictions of the two treatments. In particular, the abundances of helium clusters become important using a complete multi-component plasma approach, and eventually dominate the whole distribution at higher temperature in the crust.Comment: Submitted to the European Physical Journal A (EPJA) for the Topical Collection "The Nuclear Many-Body Problem

The proto-neutron star inner crust in a multi-component plasma approach

Proto-neutron stars (PNS) are born hot, with temperatures exceeding a few times $10^{10}$ K. In these conditions, the PNS crust is expected to be made of a Coulomb liquid composed of an ensemble of different nuclear species. We perform a study of the beta-equilibrated PNS crust in the liquid phase in a self-consistent multi-component plasma (MCP) approach, thus allowing us to consistently calculate the impurity parameter, often taken as a free parameter in cooling simulations. We developed a self-consistent MCP approach at finite temperature using a compressible liquid-drop description of the ions, with surface parameters adjusted to reproduce experimental masses. The treatment of the ion centre-of-mass motion was included through a translational free-energy term accounting for in-medium effects. The results of self-consistent MCP calculations are systematically compared with those performed in a perturbative and in the one-component plasma treatment. We show that the inclusion of non-linear mixing terms arising from the ion centre-of-mass motion leads to a breakdown of the ensemble equivalence between the one-component and MCP approach. Our findings illustrate that the abundance of light nuclei becomes important, eventually dominating the distribution at higher density and temperature. This is reflected in the impurity parameter, which, in turn, may have a potential impact on NS cooling. For practical applications, we also provide a fitting formula for the impurity parameter in the PNS inner crust. Our results obtained within a self-consistent MCP approach show important differences in the prediction of the PNS composition with respect to those obtained with a one-component or a perturbative MCP approximation, particularly in the deeper region of the crust. This highlights the importance of a full, self-consistent MCP calculation for reliable predictions of the PNS crust composition.Comment: 16 pages, 15 figures, accepted for publication in Astronomy and Astrophysic

Masses of neutron stars and nuclei

We calculate the maximum mass of neutron stars for three different equations of state (EOS) based on generalized Skyrme functionals that are simultaneously fitted to essentially all the 2003 nuclear mass data (the rms deviation is 0.58 MeV in all three cases) and to one or other of three different equations of state of pure neutron matter, each determined by a different many-body calculation using realistic two- and three-body interactions but leading to significantly different degrees of stiffness at the high densities prevailing in neutron-star interiors. The observation of a neutron star with mass 1.97 $\pm$ 0.04 $\mathcal{M}_{\odot}$ eliminates the softest of our models (BSk19), but does not discriminate between BSk20 and BSk21. However, nuclear-mass measurements that have been made since our models were constructed strongly favor BSk21, our stiffest functional.Comment: 11 pages, 4 figures; Physical Review C in pres

Symmetry energy and composition of the outer crust of neutron stars

In this paper, we study the role of the symmetry energy on the composition of the outer crust of a neutron star. Although some correlations can be observed at the neutron-drip transition, the composition of the outer crust is mainly sensitive to the details of the nuclear structure far from the valley of stability rather than to the symmetry energy only