P. Haensel

A unified equation of state of dense matter and neutron star structure

An equation of state (EOS) of neutron star matter, describing both the neutron star crust and the liquid core, is calculated. It is based on the eective nuclear interaction SLy of the Skyrme type, which is particularly suitable for the application to the calculation of the properties of very neutron rich matter (Chabanat et al. 1997, 1998). The structure of the crust, and its EOS, is calculated in the T = 0 approximation, and under the assumption of the ground state composition. The crust-core transition is a very weakly rst-order phase transition, with relative density jump of about one percent. The EOS of the liquid core is calculated assuming (minimal) npe composition. Parameters of static neutron stars are calculated and compared with existing observational data on neutron stars. The minimum and maximum masses of static neutron stars are 0:094 M and 2:05 M, respectively. Eects of rotation on the minimum and the maximum mass of neutron stars are briefly discussed.

Hyperons in neutron-star cores and a 2 M⊙ pulsar

Context. A recent measurement of the mass of PSR J1614-2230 rules out most existing models of the equation of state (EOS) of dense matter that is subjected to the high-density softening caused by either hyperonization or a phase transition to either quark matter or a boson condensate. Aims. We attempt to resolve the apparent differences between the predictions derived from up-to-date hypernuclear data, which in- clude the appearance of hyperons at about three nuclear densities and the existence of a M = 2.0 Mneutron star. Methods. We consider a non-linear relativistic mean field (RMF) model involving the baryon octet coupled to meson fields. An effective Lagrangian includes quartic terms in the meson fields. The values of the model parameters are obtained by fitting the semi- empirical parameters of nuclear matter at the saturation point, as well as potential wells for hyperons in nuclear matter and the strength of the Λ − Λ attraction in double-Λ hypernuclei. Results. We propose a non-linear RMF model that is consistent with up-to-date semi-empirical nuclear and hypernuclear data and allows for neutron stars with hyperon cores and M > 2 M� . The model involves hidden-strangeness scalar and vector mesons, coupled only to hyperons, and quartic terms involving vector meson fields. Conclusions. Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS (in which hyperons are artificially sup- pressed) above five nuclear densities. The required stiffening is generated by the quartic terms involving the hidden-strangeness vector meson.

Neutron star cooling after deep crustal heating in the X-ray transient KS 1731-260

We simulate the cooling of the neutron star in the X-ray transient KS 1731−260 after the source returned to quiescence in 2001 from a long (≳12.5 yr) outburst state. We show that the cooling can be explained assuming that the crust underwent deep heating during the outburst stage. In our best theoretical scenario the neutron star has no enhanced neutrino emission in the core, and its crust is thin, superfluid, and has the normal thermal conductivity. The thermal afterburst crust–core relaxation in the star may not be over.

Maximum mass of neutron stars and strange neutron-star cores

Context. The recent measurement of mass of PSR J1614-2230 rules out most existing models of the equation of state (EOS) of dense matter with high-density softening due to hyperonization that were based on the recent hyperon-nucleon and hyperon-hyperon interactions, which leads to a “hyperon puzzle”. Aims. We study a specific solution of this hyperon puzzle that consists of replacing a too soft hyperon core by a sufficiently stiff quark core. In terms of the quark structure of the matter, one replaces a strangeness-carrying baryon phase of confined quark triplets, some of them involving s quarks, by a quark plasma of deconfined u, d, and s quarks. Methods. We constructed an analytic approximation that fits modern EOSs of the two flavor (2SC) and the color-flavor-locked (CFL) color-superconducting phases of quark matter very well. Then, we used it to generate a continuum of EOSs of quark matter. This allowed us to simulate continua of sequences of first-order phase transitions at prescribed pressures, from hadronic matter to the 2SC and then to the CFL state of color-superconducting quark matter. Results. We obtain constraints in the parameter space of the EOS of superconducting quark cores, EOS.Q, resulting from Mmax > 2 M� . These constraints depend on the assumed EOS of baryon phase, EOS.B. We also derive constraints that would

Nucleon superfluidity vs. observations of cooling neutron stars

Cooling simulations of neutron stars (NSs) are performed assuming that stellar cores consist of neutrons, protons and electrons and using realistic density proles of superfluid critical temperatures Tcn( )a ndTcp( )o f neutrons and protons. Taking a suitable prole of Tcp( )w ith maximum5 10 9 K one can obtain smooth transition from slow to rapid cooling with increasing stellar mass. Adopting the same prole one can explain the majority of observations of thermal emission from isolated middle{aged NSs by cooling of NSs with dierent masses either with no neutron superfluidity in the cores or with a weak superfluidity, Tcn < 10 8 K. The required masses range from1:2 M for (young and hot) RX J0822{43 and (old and warm) PSR 1055{52 and RX J1856- 3754 to1:45 M for the (rather cold) Geminga and Vela pulsars. Observations constrain the Tcn( )a ndTcp() proles with respect to the threshold density of direct Urca process and maximum central density of NSs.

Thermal state of transiently accreting neutron stars

We study thermal states of transiently accreting neutron stars (with mean accretion rates u M ∼ 10 −14 −10 −9 Myr −1 ) determined by the deep crustal heating of accreted matter sinking into stellar interiors. We formalize a direct correspondence of this problem to the problem of cooling neutron stars. Using a simple toy model we analyze the most important factors which affect the thermal states of accreting stars: a strong superfluidity in the cores of low-mass stars and a fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We briefly compare the results with the observations of soft X-ray transients in quiescence. If the upper limit on the quiescent thermal luminosity of the neutron star in SAX J1808.4-3658 (Campana et al. 2002) is associated with deep crustal heating, it favors the model of nucleon neutron-star cores with switched-on direct Urca process.

Thermal conductivity of neutrons in neutron star cores

The diffusive thermal conductivity of neutrons in dense matter [

The cooling neutron star in 3C 58

\rho \sim (1 - 8) \times 10^{14}

Neutron stars with hyperon cores: stellar radii and equation of state near nuclear density

g cm

Bulk viscosity in superfluid neutron star cores - III. Effects of

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