arXiv subjects

P. Haensel

Publications and source records attributed to P. Haensel.

At least 91 records · Page 5Linked to original sources

arXiv2002

Moments of inertia for neutron and strange stars: limits derived for the Crab pulsar

Recent estimates of the properties of the Crab nebula are used to derive constraints on the moment of inertia, mass and radius of the pulsar. To this purpose, we employ an approximate formula combining these three parameters. Our "empirical formula" I = a(x)MR^2, where x = (M/M_sun)(km/R), is based on numerical results obtained for thirty theoretical equations of state of dense matter. The functions a(x) for neutron stars and strange stars are qualitatively different. For neutron stars a_NS(x) = x/(0.1+2x) for x <= 0.1 (valid for M > 0.2 M_sun) and a_NS(x) = (2/9)(1+5x) for x > 0.1. For strange stars a_SS(x) = (2/5)(1+x) (not valid for strange stars with crust and M < 0.1 M_sun). We obtain also an approximate expression for the maximum moment of inertia I_max,45 = (-0.37 + 7.12x_max) (M_max/M_sun)(R_(M_max)/(10 km))^2, valid for both neutron stars and strange stars. Applying our formulae to the evaluated values of I_Crab, we derive constraints on the mass and radius of the pulsar. A very conservative evaluation of the expanding nebula mass, M_neb = 2 M_sun, yields M_Crab > 1.2 M_sun and R_Crab = 10-14 km. Setting the most recent evaluation ("central value") M_neb = 4.6 M_sun rules out most of the existing equations of state, leaving only the stiffest ones: M_Crab > 1.9 M_sun, R_Crab = 14-15 km.

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arXiv2002

Adiabatic Index of Dense Matter and Damping of Neutron Star Pulsations

The adiabatic index Gamma_1 for perturbations of dense matter is studied under various physical conditions which can prevail in neutron star cores. The dependence of Gamma_1 on the composition of matter (in particular, on the presence of hyperons), on the stellar pulsation amplitude, and on the baryon superfluidity is analyzed. Timescales of damping of stellar pulsations are estimated at different compositions, temperatures, and pulsation amplitudes. Damping of pulsations by bulk viscosity in the neutron-star cores can prevent the stars to pulsate with relative amplitudes > (1-15)% (depending on the composition of matter).

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arXiv2002

The Cooling Neutron Star in 3C 58

The upper limit of the effective surface temperature of the neutron star (NS) PSR J0205+6449 in the supernova remnant 3C 58 obtained recently by Slane et al. (2002) is analyzed using a modern theory of NS cooling (Kaminker et al. 2002). The observational limit can be explained by cooling of a superfluid NS with the core composed of neutrons, protons and electrons, where direct Urca process is forbidden. However, combined with the data on the surface temperatures of other isolated NSs, it gives evidence (emphasized by Slane et al.) that direct Urca process is open in the inner cores of massive NSs. This evidence turns out to be less stringent than the evidence provided by the well known observations of Vela and Geminga.

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arXiv2002

Equation of state of dense matter and the minimum mass of cold neutron stars

Equilibrium configurations of cold neutron stars near the minimum mass are studied, using the recent equation of state SLy, which describes in a unified, physically consistent manner, both the solid crust and the liquid core of neutron stars. Results are compared with those obtained using an older FPS equation of state of cold catalyzed matter. The value of M_min\simeq 0.09M_sun depends very weakly on the equation of state of cold catalyzed matter: it is 0.094 M_sun for the SLy model, and 0.088 M_sun for the FPS one. Central density at M_min is significantly lower than the normal nuclear density: for the SLy equation of state we get central density 1.7 10^{14} g/cm^3, to be compared with 2.3 10^{14} g/cm^3 obtained for the FPS one. Even at M_min, neutron stars have a small liquid core of radius of about 4 km, containing some 2-3% of the stellar mass. Neutron stars with 0.09 M_sun <M<0.17 M_sun are bound with respect to dispersed configuration of the hydrogen gas, but are unbound with respect to dispersed Fe^56. The effect of uniform rotation on the minimum-mass configuration of cold neutron stars is studied. Rotation increases the value of M_min; at rotation period of 10 ms the minimum mass of neutron stars increases to 0.13 M_sun, and corresponds to the mass-shedding (Keplerian) configuration. In the case of the shortest observed rotation period of radio pulsars 1.56 ms, minimum mass of uniformly rotating cold neutron stars corresponds to the mass-shedding limit, and is found at 0.61 M_sun for the SLy EOS and 0.54 M_sun for the FPS EOS.

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arXiv2001

Bulk viscosity in superfluid neutron star cores. III. Effects of $Σ^-$ hyperons

Bulk viscosity of neutron star cores containing hyperons is studied taking into account non-equilibrium weak process $n+n \rightleftharpoons p+Σ^-$. Rapid growth of the bulk viscosity within the neutron star core associated with switching on new reactions (modified Urca process, direct Urca process, hyperon reactions) is analyzed. The suppression of the bulk viscosity by superfluidity of baryons is considered and found out to be very important.

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arXiv2001

Apparent radii of neutron stars and equation of state of dense matter

Apparent (radiation) radius of neutron star,R_\infty, depends on the star gravitational mass in quite a different way than the standard coordinate radius in the Schwarzschild metric, R. We show that, for a broad set of equations of state of dense matter, R_\infty(M_max) for the configurations with maximum allowable masses is very close to the absolute lower bound on R_\infty at fixed M, resulting from the very definition of R_\infty. Also, the value of R_infty at given M, corresponding to the maximum compactness (minimum R) of neutron star consistent with general relativity and condition v_sound<c, is only 0.6% higher than this absolute lower bound. Theoretical predictions for R_\infty are compared with existing observational estimates of the apparent radii of neutron stars.

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arXiv2001

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 effective 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 first-order phase transition, with relative density jump of about one percent. The EOS of the liquid core is calculated assuming (minimal) n-p-e-mu 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_sun and 2.05 M_sun, respectively. Effects of rotation on the minimum and the maximum mass of neutron stars are briefly discussed.

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arXiv2001

Recycling strange stars to millisecond periods

Recycling strange stars to millisecond periods is studied within the framework of general relativity. We employ equations of state of strange quark matter based on the MIT Bag Model, with massive strange quarks and lowest order QCD interactions. The presence of the crust of normal matter is taken into account, with a bottom density assumed to be equal to the neutron-drip one. The calculations are performed by solving the exact 2-D equations for rigidly rotating stationary configurations in general relativity. Evolutionary tracks of accreting strange stars are computed, and their dependence on the initial strange star mass and on the fraction of the angular momentum transferred to the star from the infalling matter, is studied. The differences between recycling strange stars and neutron stars are pointed out.

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arXiv2001

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 profiles of superfluid critical temperatures $T_{cn}(ρ)$ and $T_{cp}(ρ)$ of neutrons and protons. Taking a suitable profile of $T_{cp}(ρ)$ with maximum $\sim 5 \times 10^9$ K one can obtain smooth transition from slow to rapid cooling with increasing stellar mass. Adopting the same profile one can explain the majority of observations of thermal emission from isolated middle--aged NSs by cooling of NSs with different masses either with no neutron superfluidity in the cores or with a weak superfluidity, $T_{cn} < 10^8$ K. The required masses range from $\sim 1.2 M_\odot$ for (young and hot) RX J0822-43 and (old and warm) PSR 1055-52 and RX J1856-3754 to $\approx 1.45 M_\odot$ for the (colder) Geminga and Vela pulsars. Observations constrain the $T_{cn}(ρ)$ and $T_{cp}(ρ)$ profiles with respect to the threshold density of direct Urca process and maximum central density of NSs.

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arXiv2001

Thermal Conductivity of Neutrons in Neutron Star Cores

The diffusive thermal conductivity of neutrons in dense matter [$ρ\sim (1 - 8) \times 10^{14}$ g cm$^{-3}$] of neutron star cores is calculated. The contribution from neutron--neutron and neutron--proton collisions is taken into account. We use the transition probabilities calculated for symmetric dense nucleon matter on the basis of the Dirac--Brueckner approach to the in-medium effects and the Bonn model of bare nucleon--nucleon interaction. The diffusive thermal conductivity of neutrons in the presence of neutron and proton superfluidities is analyzed in a microscopic manner; the effects of superfluidity are shown to be significant. The low temperature behavior of the thermal conductivity appears to be extremely sensitive to the relation between critical temperatures of neutrons and protons. The results are fitted by simple analytic expressions. In combination with the formulae for the electron and muon thermal conductivities, obtained earlier, the present expressions provide a realistic description of the full diffusive thermal conductivity in the neutron star cores for normal and various superfluid phases.

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arXiv2001

The crust of rotating strange quark stars

Calculations of the properties of rotating strange stars with crusts are performed within the framework of general relativity. We employ an equation of state (EOS) of strange quark matter based on the MIT Bag Model with massive strange quarks and lowest order QCD interactions. The crust is described by the BPS equation of state. A significant increase of the stellar radius is found close to the Keplerian (mass-shedding limit) configuration. This leads to the disappearance of the gap between the stellar surface and the innermost stable circular orbit (ISCO) at very high rotation rates, for a rather broad range of stellar masses. The Keplerian configuration for the strange star with crust corresponds to values of J, T/W which are about 20% smaller than in the case of bare strange stars. Because the Keplerian configuration is achieved due to the increase of the stellar oblateness, the Keplerian frequency (of the rotation) remains almost unaltered. The lack of the gap close to the Keplerian rotation could imply a more stringent limit on the frequency of the ISCO, if the existence of such a gap is supported by observations, as in the source 4U 1820-30 with the upper QPO frequency 1.07 kHz. During the spin-down of a magnetized strange quark star with crust, the crust matter is absorbed in the equatorial region by the strange matter core. The deconfinement of absorbed crust matter is a strongly exothermic process, which would influence the cooling curve of this compact object.

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arXiv2001

Bulk viscosity in superfluid neutron star cores. II. Modified Urca processes in $npeμ$ matter

We study the bulk viscosity in neutron star cores due to modified Urca processes involving nucleons, electrons and muons and analize its reduction by singlet-state or triplet-state superfluidity of nucleons. In combination with the results of our previous paper on the bulk viscosity due to direct Urca processes, a realistic description of the bulk viscosity in superfluid neutron star cores is obtained. Switching off direct Urca processes with decreasing density in a nonsuperfluid matter lowers the bulk viscosity by 3-5 orders of magnitude. The presense of muons opens additional source of bulk viscosity due to muon Urca processes and lowers the threshold density of the electron direct Urca process. The superfluidity may strongly reduce the bulk viscosity and affect thus damping of neutron star vibrations.

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arXiv2000

Protoneutron stars and constraints on maximum and minimum mass of neutron stars

Constraints on minimum and maximum mass of ordinary neutron stars are imposed by the consideration of their early evolution (protoneutron star stage). Calculations are performed for a realistic standard model of hot, dense matter (Lattimer & Swesty 1991) valid for both supranuclear and subnuclear densities. Various assumptions concerning protoneutron star interior (large trapped lepton number, no trapped lepton number, isentropic, isothermal) are taken into account.

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arXiv2000

Protoneutron stars and neutron stars

We find constraints on minimum and maximum mass of ordinary neutron stars imposed by their early evolution (protoneutron star stage). We calculate models of protoneutron stars using a realistic standard equation of state of hot, dense matter valid for both supranuclear and subnuclear densities. Results for different values of the nuclear incompressibility are presented.

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arXiv2000

Neutrino Emission from Neutron Stars

We review the main neutrino emission mechanisms in neutron star crusts and cores. Among them are the well-known reactions such as the electron-positron annihilation, plasmon decay, neutrino bremsstrahlung of electrons colliding with atomic nuclei in the crust, as well as the Urca processes and neutrino bremsstrahlung in nucleon-nucleon collisions in the core. We emphasize recent theoretical achievements, for instance, band structure effects in neutrino emission due to scattering of electrons in Coulomb crystals of atomic nuclei. We consider the standard composition of matter (neutrons, protons, electrons, muons, hyperons) in the core, and also the case of exotic constituents such as the pion or kaon condensates and quark matter. We discuss the reduction of the neutrino emissivities by nucleon superfluidity, as well as the specific neutrino emission produced by Cooper pairing of the superfluid particles. We also analyze the effects of strong magnetic fields on some reactions, such as the direct Urca process and the neutrino synchrotron emission of electrons. The results are presented in the form convenient for practical use. We illustrate the effects of various neutrino reactions on the cooling of neutron stars. In particular, the neutrino emission in the crust is critical in setting the initial thermal relaxation between the core and the crust. Finally, we discuss the prospects of exploring the properties of supernuclear matter by confronting cooling simulations with observations of the thermal radiation from isolated neutron stars.

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arXiv2000

Rapidly rotating strange stars

We study effects of the strange quark mass and of the QCD coupling constant in the MIT bag model of quark matter with lowest order quark-gluon interactions on the rapid rotation of strange stars. The influence of rotation on global parameters of strange stars is greater than in the case of the neutron star models. We show that independently of the strange quark mass and the coupling constans the ratio of the rotational kinetic energy to the absolute value of the gravitational potential energy for rotating strange stars is significantly higher than for ordinary neutron stars. This might indicate that rapidly rotating strange stars could be important sources of gravitational waves.

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arXiv2000

Inner edge of neutron-star crust with SLy effective nucleon-nucleon interactions

The boundary between the solid crust, assumed to be in complete thermodynamic equilibrium (cold catalyzed matter), and the liquid core of a neutron star is studied using Skyrme SLy effective N-N interactions. An approximate value of the density at the inner edge of the crust is obtained from the threshold for the instability of homogeneous $npe$ matter with respect to small periodic density perturbations. Calculations of the bottom layer of the crust are performed within the Compressible Liquid Drop Model. Spherical nuclei are energetically preferred over exotic ones (cylinders, plates, tubes, bubbles), down to the inner edge of the crust, found at the density 0.08 fm^{-3}.

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arXiv2000

On the mass of moderately rotating strange stars in the MIT bag model and LMXBs

We compute the maximum mass of moderately rotating strange stars as a function of the strange quark mass, of the QCD coupling constant, $α_c$, and of the bag constant (vacuum energy density), $B$, in the MIT bag model of quark matter with lowest order quark-gluon interactions. For a fixed value of $B$, the maximum stellar mass depends only weakly on $α_c$, and is independent of this coupling in the limit of massless quarks. However, if it is the value of the chemical potential of quark matter at zero pressure which is held constant, for example at the value corresponding to the stability limit of nucleons against conversion to quark matter, the maximum mass of the strange star is higher by up to 25% for $α_c=0.6$, than for non-interacting quarks, and this may be relevant in the discussion of kHz QPO sources. The maximum mass of a non-rotating strange star could be sufficiently high to allow an orbital frequency as low as 1.0 kHz in the marginally stable orbit. However, for all $α_c<0.6$, the stellar mass cannot exceed $2.6M_\odot$ at any rotational period $\ge1.6 $ms.

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