1,217 research outputs found

### Analytical representations of unified equations of state of neutron-star matter

Analytical representations are derived for two equations of state (EOSs) of
neutron-star matter: FPS and SLy. Each of these EOSs is unified, that is, it
describes the crust and the core of a neutron star using the same physical
model. Two versions of the EOS parametrization are considered. In the first
one, pressure and mass density are given as functions of the baryon density. In
the second version, pressure, mass density, and baryon density are given as
functions of the pseudo-enthalpy, which makes this representation particularly
useful for 2-D calculations of stationary rotating configurations of neutron
stars.Comment: 7 pages, 5 figures, 3 tables, accepted by A&A. In v.2, auxiliary fits
(15) and (16) are correcte

### 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, $4.3
\times 10^{11} g cm^{-3}$. 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 $\sim$ ms, these changes cause nuclei, initially being in
the ground-state matter above a mass density of about $3 \times 10^{13} g
cm^{-3}$, 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 $ROSAT$ 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

### Accelerated expansion of the Crab Nebula and evaluation of its neutron-star parameters

A model of an accelerated expansion of the Crab Nebula powered by the
spinning-down Crab pulsar is proposed, in which time dependence of the
acceleration is connected with evolution of pulsar luminosity. Using recent
observational data, we derive estimates of the Crab neutron-star moment of
inertia. Correlations between the neutron star moment of inertia and its mass
and radius allow for rough estimates of the Crab neutron-star radius and mass.
In contrast to the previously used constant-acceleration approximation, even
for the expanding nebula mass ~7 M_sun results obtained within our model do not
stay in conflict with the modern stiff equations of state of dense matter.Comment: to be submitted to Astronomy & Astrophysic

### 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

### Dynamical stability of strange quark stars

We show that the mass-radius
$(M-R)$ 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

### The outer crust of non-accreting cold neutron stars

The properties of the outer crust of non-accreting cold neutron stars are
studied by using modern nuclear data and theoretical mass tables updating in
particular the classic work of Baym, Pethick and Sutherland. Experimental data
from the atomic mass table from Audi, Wapstra, and Thibault of 2003 is used and
a thorough comparison of many modern theoretical nuclear models, relativistic
and non-relativistic ones, is performed for the first time. In addition, the
influences of pairing and deformation are investigated. State-of-the-art
theoretical nuclear mass tables are compared in order to check their
differences concerning the neutron dripline, magic neutron numbers, the
equation of state, and the sequence of neutron-rich nuclei up to the dripline
in the outer crust of non-accreting cold neutron stars.Comment: 20 pages, 10 figures, accepted for publication in Phys. Rev.

### 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

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