168 research outputs found
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&
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