Spin-up of the hyperon-softened accreting neutron stars

We study the spin-up of the accreting neutron stars with a realistic hyperon-softened equation of state. Using precise 2-D calculations we study the evolutionary tracks of accreting neutron stars in the angular-momentum - frequency plane. In contrast to the case of spinning-down solitary radio-pulsars, where a strong back-bending behavior has been observed, we do not see back-bending phenomenon in the accretion-powered spinning-up case. We conclude that in the case of accretion-driven spin-up the back-bending is strongly suppressed by the mass-increase effect accompanying the angular-momentum increase.Comment: 5 pages, 5 figures, accepted by Astronomy & Astrophysic

Maximum mass of neutron stars and strange neutron-star cores

Recent measurement of mass of PSR J1614-2230 rules out most of existing models of equation of state (EOS) of dense matter with high-density softening due to hyperonization, based on the recent hyperon-nucleon and hyperon-hyperon interactions, leading to a "hyperon puzzle". We study a specific solution of "hyperon puzzle", consisting in replacing a too soft hyperon core by a sufficiently stiff quark core. We construct an analytic approximation fitting very well modern EOSs of 2SC and CFL color superconducting phases of quark matter. This allows us for simulating continua of sequences of first-order phase transitions from hadronic matter to the 2SC, and then to the CFL state of color superconducting quark matter. We obtain constraints in the parameter space of the EOS of superconducting quark cores, resulting from M_max> 2 M_sol. We also derive constraints that would result from significantly higher measured masses. For 2.4 M_sol required stiffness of the CFL quark core should have been close to the causality limit, the density jump at the phase transition being very small. Condition M_max > 2 M_sol puts strong constraints on the EOSs of the 2SC and CFL phases of quark matter. Density jumps at the phase transitions have to be sufficiently small and sound speeds in quark matter - sufficiently large. A strict condition of thermodynamic stability of quark phase results in the maximum mass of hybrid stars similar to that of purely baryon stars. Therefore, to get M_max>2 M_sol for stable hybrid stars, both sufficiently strong additional hyperon repulsion at high density baryon matter and a sufficiently stiff EOS of quark matter would be needed. However, it is likely that the high density instability of quark matter (reconfinement) indicates actually the inadequacy of the point-particle model of baryons in dense matter at very high densities.Comment: 8 pages, 10 figures, submitted to A&

Energy release associated with a first-order phase transition in a rotating neutron star core

We calculate energy release associated with a first order phase transition at the center of a rotating neutron star. The results are based on precise numerical 2-D calculations, in which both the polytropic equations of state (EOS) as well as realistic EOS of the normal phase are used. Presented results are obtained for a broad range of metastability of initial configuration and size of the new superdense phase core in the final configuration. For small radii of the superdense phase core analytical expressions for the energy release are obtained. For a fixed "overpressure" dP (the relative excess of central pressure of collapsing metastable star over the pressure of equilibrium first-order phase transition) the energy release remarkably does not depend on the stellar angular momentum and coincides with that for nonrotating stars with the same dP. The energy release is proportional to dP^2.5 for small dPs, when sufficiently precise brute force 2-D numerical calculations are out of question. For higher dPs, results of 1-D calculations of energy release for non-rotating stars are shown to reproduce, with very high precision, the exact 2-D results for rotating stars.Comment: 8 pages, 8 figures, submitted to A&

Formation scenarios and mass-radius relation for neutron stars

Neutron star crust, formed via accretion of matter from a companion in a low-mass X-ray binary (LMXB), has an equation of state (EOS) stiffer than that of catalyzed matter. At a given neutron star mass, M, the radius of a star with an accreted crust is therefore larger, by DR(M), than for usually considered star built of catalyzed matter. Using a compressible liquid drop model of nuclei, we calculate, within the one-component plasma approximation, the EOSs corresponding to different nuclear compositions of ashes of X-ray bursts in LMXB. These EOSs are then applied for studying the effect of different formation scenarios on the neutron-star mass-radius relation. Assuming the SLy EOS for neutron star's liquid core, derived by Douchin & Haensel (2001), we find that at M=1.4 M_sun the star with accreted crust has a radius more than 100 m larger that for the crust of catalyzed matter. Using smallness of the crust mass compared to M, we derive a formula that relates DR(M) to the difference in the crust EOS. This very precise formula gives also analytic dependence of DR on M and R of the reference star built of catalyzed matter. The formula is valid for any EOS of the liquid core. Rotation of neutron star makes DR(M) larger. We derive an approximate but very precise formula that gives difference in equatorial radii, DR_eq(M), as a function of stellar rotation frequency.Comment: 6 pages, 4 figures. Accepted for publication in Astronomy and Astrophysic

On the minimum radius of strange stars with crust

The minimum value of the radius of strange star covered by the crust of nuclear matter is determined. The results for the maximum possible thickness of the crust (up to the neutron drip) as well as the possibility of thinner crust postulated by some authors are discussed. The minimum radius of the strange star with maximal crust is 5.5 km. The useful scaling formulae with respect to the main parameters describing strange matter and the density at the bottom of the crust are presented.Comment: accepted for publication in A&