arXiv subjects

D. G. Yakovlev

Publications and source records attributed to D. G. Yakovlev.

At least 91 records · Page 5Linked to original sources

arXiv2001

Three Types of Cooling Superfluid Neutron Stars: Theory and Observations

Cooling of neutron stars (NSs) with the cores composed of neutrons, protons, and electrons is simulated assuming $^1$S$_0$ pairing of neutrons in the NS crust, and also $^1$S$_0$ pairing of protons and weak $^3$P$_2$ pairing of neutrons in the NS core, and using realistic density profiles of the superfluid critical temperatures $T_{\rm c}(ρ)$. The theoretical cooling models of isolated middle-aged NSs can be divided into three main types. (I) {\it Low-mass}, {\it slowly cooling} NSs where the direct Urca process of neutrino emission is either forbidden or almost fully suppressed by the proton superfluidity. (II) {\it Medium-mass} NSs which show {\it moderate} cooling via the direct Urca process suppressed by the proton superfluidity. (III) {\it Massive} NSs which show {\it fast} cooling via the direct Urca process weakly suppressed by superfluidity. Confronting the theory with observations we treat RX J0822--43, PSR 1055--52 and RX J1856--3754 as slowly cooling NSs. To explain these sufficiently warm sources we need a density profile $T_{\rm c}(ρ)$ in the crust with a rather high and flat maximum and sharp wings. We treat 1E 1207--52, RX J0002+62, PSR 0656+14, Vela, and Geminga as moderately cooling NSs. We can determine their masses for a given model of proton superfluidity, $T_{\rm cp}(ρ)$, and the equation of state in the NS core. No rapidly cooling NS has been observed so far.

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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

$^1$S$_0$ Neutron Pairing vs Observations of Cooling Neutron Stars

As shown recently by Kaminker et al. (2001), current observations of thermal emission of isolated middle-aged neutron stars (NSs) can be explained by cooling of NSs of different masses with the cores composed of neutrons, protons and electrons, assuming rather strong superfluidity (SF) of protons, weak triplet-state SF of neutrons and neglecting singlet-state SF of neutrons. We show that this explanation remains correct in the presence of singlet-state SF of neutrons in the NS crust and outermost core but under stringent constraints on the density profile of the SF critical temperature $T_{\rm cns}(ρ)$. In order to explain observations of (young and hot) RX J0822--43 and (old and warm) PSR 1055--52 and RX J1856--3754 as cooling not too massive NSs, the maximum $T_{\rm cns}^{\rm max}$ should be rather high ($\ga 5 \times 10^9$ K) and the decrease of $T_{\rm cns}(ρ)$ outside the maximum should be sharp. These results place important constraints on the models of nucleon SF in dense matter.

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arXiv2001

Thermodynamic functions of harmonic Coulomb crystals

Phonon frequency moments and thermodynamic functions (electrostatic and vibrational parts of the free energy, internal energy, and heat capacity) are calculated for bcc and fcc Coulomb crystals in the harmonic approximation with a fractional accuracy within one part on 10^5. Temperature dependence of thermodynamic functions is fitted by analytic formulas with an accuracy of a few parts in 10^5. The static-lattice (Madelung) part of the free energy is calculated with an accuracy about 10^{-12}. The Madelung constant and frequency moments of hcp crystals are also computed.

physics.plasm-ph↗

arXiv2001

Thermal structure and cooling of neutron stars with magnetized envelopes

The thermal structure of neutron stars with magnetized envelopes is studied using modern physics input. The relation between the internal (T_i) and local surface temperatures is calculated and fitted by analytic expressions for magnetic field strengths B from 0 to 10^{16} G and arbitrary inclination of the field lines to the surface. The luminosity of a neutron star with dipole magnetic field is calculated and fitted as a function of B, T_i, stellar mass and radius. In addition, we simulate cooling of neutron stars with magnetized envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and also the effects of the magnetic field of the Vela pulsar on the determination of critical temperatures of neutron and proton superfluids in its core.

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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

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

Strongly coupled Coulomb and nuclear plasma in inner crusts of neutron stars

Matter of subnuclear density in the inner crusts of neutron stars consists of neutron-rich atomic nuclei immersed in strongly degenerate relativistic gas of electrons and strongly nonideal liquid of neutrons. We give a brief account of thermodynamic and kinetic properties of this matter which are greatly affected by Coulomb and nuclear interactions (a companion paper on the outer envelopes is astro-ph/0012316) and show how they can be studied from observations of thermal radiation of young (age < 100 yr) neutron stars (for a discussion of the latter possibility, see astro-ph/0012306).

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arXiv2000

Coulomb plasmas in outer envelopes of neutron stars

Outer envelopes of neutron stars consist mostly of fully ionized, strongly coupled Coulomb plasmas characterized by typical densities about 10^4-10^{11} g/cc and temperatures about 10^4-10^9 K. Many neutron stars possess magnetic fields about 10^{11}-10^{14} G. Here we briefly review recent theoretical advances which allow one to calculate thermodynamic functions and electron transport coefficients for such plasmas with an accuracy required for theoretical interpretation of observations.

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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

Bulk viscosity in superfluid neutron star cores. I. Direct Urca processes in npeμmatter

The bulk viscosity of the neutron star matter due to the direct Urca processes involving nucleons, electrons and muons is studied taking into account possible superfluidity of nucleons in the neutron star cores. The cases of singlet-state pairing or triplet-state pairing (without and with nodes of the superfluid gap at the Fermi surface) of nucleons are considered. It is shown that the superfluidity may strongly reduce the bulk viscosity. The practical expressions for the superfluid reduction factors are obtained. For illustration, the bulk viscosity is calculated for two models of dense matter composed of neutrons, protons,electrons and muons. The presence of muons affects the bulk viscosity due to the direct Urca reactions involving electrons and produces additional comparable contribution due to the direct Urca reactions involving muons. The results can be useful for studying damping of vibrations of neutron stars with superfluid cores.

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arXiv1999

Coulomb crystals in the harmonic lattice approximation

The dynamic structure factor ${\tilde S}({\bf k},ω)$ and the two-particle distribution function $g({\bf r},t)$ of ions in a Coulomb crystal are obtained in a closed analytic form using the harmonic lattice (HL) approximation which takes into account all processes of multi-phonon excitation and absorption. The static radial two-particle distribution function $g(r)$ is calculated for classical ($T \gtrsim \hbar ω_p$, where $ω_p$ is the ion plasma frequency) and quantum ($T \ll \hbar ω_p$) body-centered cubic (bcc) crystals. The results for the classical crystal are in a very good agreement with extensive Monte Carlo (MC) calculations at $1.5 \lesssim r/a \lesssim 7$, where $a$ is the ion-sphere radius. The HL Coulomb energy is calculated for classical and quantum bcc and face-centered cubic crystals, and anharmonic corrections are discussed. The inelastic part of the HL static structure factor $S''(k)$, averaged over orientations of wave-vector {\bf k}, is shown to contain pronounced singularities at Bragg diffraction positions. The type of the singularities is different in classical and quantum cases. The HL method can serve as a useful tool complementary to MC and other numerical methods.

physics.plasm-ph↗

arXiv1999

Cooling neutron stars and superfluidity in their interiors

We study the heat capacity and neutrino emission reactions (direct and modified Urca processes, nucleon-nucleon bremsstrahlung, Cooper pairing of nucleons) in matter of supranuclear density of the neutron star cores with superfluid neutrons and protons. Various superfluidity types are analysed (singlet-state pairing and two types of triplet-state pairing, without and with nodes of the gap at a nucleon Fermi surface). The results are used for cooling simulations of isolated neutron stars. Both, the standard cooling and the cooling enhanced by the direct Urca process, are strongly affected by nucleon superfluidity. Comparison of cooling theory of isolated neutron stars with observations of their thermal radiation may give stringent constraints on the critical temperatures of the neutron and proton superfluidities in the neutron star cores.

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arXiv1999

Transport properties of degenerate electrons in neutron star envelopes and white dwarf cores

New calculations of the thermal and electrical electron conductivities are performed for a broad range of physical parameters typical for envelopes of neutron stars and cores of white dwarfs. We consider stellar matter composed of astrophysically important chemical elements from H to Fe in the density range from 10^2-10^4 g/ccm up to 10^7-10^{10} g/ccm, where atoms are fully ionized and electrons are strongly degenerate. We have used modified ion structure factors suggested in physics/9811052. In the ion liquid, these modifications take into account, in an approximate way, instantaneous electron-band structures that reduce the electron-ion scattering rate. In crystallized matter, the new structure factors include multi-phonon processes important at temperatures not very much lower than the melting temperature T_m. The transport coefficients obtained differ significantly from those derived earlier in the important temperature range T_m/5 < T < 5 T_m. The results of our numerical calculations are fitted by analytical expressions convenient for astrophysical applications.

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arXiv1999

Neutrino emission due to proton pairing in neutron stars

We calculate the neutrino energy emission rate due to singlet-state pairing of protons in the neutron star cores taking into account the relativistic correction to the non-relativistic rate. The non-relativistic rate is numerically small, and the relativistic correction appears to be about 10 -- 50 times larger. It plays thus the leading role, reducing great difference between the neutrino emissions due to pairing of protons and neutrons. The results are important for simulations of neutron star cooling.

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arXiv1999

A simple model of cooling neutron stars with superfluid cores: comparison with observations

Cooling of neutron stars (NSs) with superfluid cores is simulated taking into account neutrino emission produced by Cooper pairing of nucleons. The critical temperatures of neutron and proton superfluidities, $T_{cn}$ and $T_{cp}$, are assumed to be constant over the NS core, and treated as free parameters. They are constrained using the surface temperatures $T_s$ of isolated NSs (RX J0822-43, PSR 1055-52, 1E 1207-52, Vela, Geminga, PSR 0656+14, RX J0002+62), obtained by interpretation of observed thermal radiation either with black body spectrum or with hydrogen atmosphere models.

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arXiv1998

Neutrino-pair bremsstrahlung by electrons in neutron star crusts

Neutrino-pair bremsstrahlung by relativistic degenerate electrons in a neutron-star crust at densities (10^9 - 1.5x10^{14}) g/cm^3 is analyzed. The processes taken into account are neutrino emission due to Coulomb scattering of electrons by atomic nuclei in a Coulomb liquid, and electron-phonon scattering and Bragg diffraction (the static-lattice contribution) in a Coulomb crystal. The static-lattice contribution is calculated including the electron band-structure effects for cubic Coulomb crystals of different types and also for the liquid crystal phases composed of rod- and plate-like nuclei in the neutron-star mantle (at 10^{14} - 1.5x10^{14} g/cm^3). The phonon contribution is evaluated with proper treatment of the multi-phonon processes which removes a jump in the neutrino bremsstrahlung emissivity at the melting point obtained in previous works. Below 10^{13} g/cm^3, the results are rather insensitive to the nuclear form factor, but results for the solid state near the melting point are affected significantly by the Debye-Waller factor and multi-phonon processes. At higher densities, the nuclear form factor becomes more significant. A comparison of the various neutrino generation mechanisms in neutron star crusts shows that electron bremsstrahlung is among the most important ones.

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