1,047 research outputs found
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
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&
Spin-Down of Neutron Stars and Compositional Transitions in the Cold Crustal Matter
Transitions of nuclear compositions in the crust of a neutron star induced by
stellar spin-down are evaluated at zero temperature. We construct a
compressible liquid-drop model for the energy of nuclei immersed in a neutron
gas, including pairing and shell correction terms, in reference to the known
properties of the ground state of matter above neutron drip density, . Recent experimental values and extrapolations of
nuclear masses are used for a description of matter at densities below neutron
drip. Changes in the pressure of matter in the crust due to the stellar
spin-down are calculated by taking into account the structure of the crust of a
slowly and uniformly rotating relativistic neutron star. If the initial
rotation period is ms, these changes cause nuclei, initially being in
the ground-state matter above a mass density of about , to absorb neutrons in the equatorial region where the matter
undergoes compression, and to emit them in the vicinity of the rotation axis
where the matter undergoes decompression. Heat generation by these processes is
found to have significant effects on the thermal evolution of old neutron stars
with low magnetic fields; the surface emission predicted from this heating is
compared with the observations of X-ray emission from millisecond
pulsars and is shown to be insufficient to explain the observed X-ray
luminosities.Comment: 32 pages, LaTeX, 11 Postscript figures. Accepted for publication in
Ap
Superbursts from Strange Stars
Recent models of carbon ignition on accreting neutron stars predict
superburst ignition depths that are an order of magnitude larger than observed.
We explore a possible solution to this problem, that the compact stars in low
mass X-ray binaries that have shown superbursts are in fact strange stars with
a crust of normal matter. We calculate the properties of superbursts on strange
stars, and the resulting constraints on the properties of strange quark matter.
We show that the observed ignition conditions exclude fast neutrino emission in
the quark core, for example by the direct Urca process, which implies that
strange quark matter at stellar densities should be in a color superconducting
state. For slow neutrino emission in the quark matter core, we find that
reproducing superburst properties requires a definite relation between three
poorly constrained properties of strange quark matter: its thermal
conductivity, its slow neutrino emissivity and the energy released by
converting a nucleon into strange quark matter.Comment: 4 pages, submitted to Ap. J. Let
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&
Dynamical stability of strange quark stars
We show that the mass-radius
relation corresponding to the MIT bag models of strange quark matter
(SQM) and the models obtained by Day et al (1998) do not provide the necessary
and sufficient condition for dynamical stability for the equilibrium
configurations, since such configurations can not even fulfill the necessary
condition of hydrostatic equilibrium provided by the exterior Schwarzschild
solution. These findings will remain unaltered and can be extended to any other
sequence of pure SQM. This study explicitly show that although the strange
quark matter might exist in the state of zero pressure and temperature, but the
models of pure strange quark `stars' can not exist in the state of hydrostatic
equilibrium on the basis of General Relativity Theory. This study can affect
the results which are claiming that various objects like - RX J1856.5-3754, SAX
J1808.4-3658, 4U 1728-34, PSR 0943+10 etc. might be strange stars.Comment: 7 pages (including 6 tables and 1 figure) in MNRAS styl
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