arXiv subjects

D. G. Yakovlev

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

105 records · Page 6Linked to original sources

arXiv1998

Neutrino emission due to Cooper pairing of nucleons in cooling neutron stars

The neutrino energy emission rate due to formation of Cooper pairs of neutrons and protons in the superfluid cores of neutron stars is studied. The cases of singlet-state pairing with isotropic superfluid gap and triplet-state pairing with anisotropic gap are analysed. The neutrino emission due to singlet-state pairing of protons is found to be greatly suppressed with respect to the cases of singlet- and triplet-state pairings of neutrons. The neutrino emission due to pairing of neutrons is shown to be very important in the superfluid neutron-star cores with the standard neutrino luminosity and with the luminosity enhanced by the direct Urca process. It can greatly accelerate both, standard and enhanced, cooling of neutron stars with superfluid cores. This enables one to interpret the data on surface temperatures of six neutron stars, obtained by fitting the observed spectra with the hydrogen atmosphere models, by the standard cooling with moderate nucleon superfluidity.

astro-ph↗

arXiv1998

Direct Urca process in strong magnetic fields and neutron star cooling

The effect of the magnetic field on the energy loss rate in the direct Urca reactions is studied. The general expression for the neutrino emissivity at arbitrary magnetic field B is derived. The main emphasis is laid on a case, in which the field is not superstrong, and charged reacting particles (e and p) populate many Landau levels. The magnetic field keeps the process operative if Delta k / k_{Fn} < N_{Fp}^{-2/3} (N_{Fp} is the number of the Landau levels populated by protons and Delta k = k_{Fn}-k_{Fp}-k_{Fe}), that is beyond the well-known switch-on limit in the absence of the field, Delta k < 0. Cooling of magnetized neutron stars with strong neutron superfluid in the outer cores and nonsuperfluid inner cores is simulated. The magnetic field near the stellar center speeds up the cooling if the stellar mass M is slightly less than the minimum mass M_c at which the direct Urca reaction becomes allowed for B=0. If B=3x10^{16} G the affected mass range is M_c-M < 0.1M_c, while for B=3x10^{15} G the range is M_c-M < 0.015M_c. This may influence a theoretical interpretation of the observed thermal radiation as illustrated for the Geminga pulsar. The case of superstrong magnetic fields (B>10^{18} G), such that e and p populate only the lowest Landau levels is briefly outlined.

astro-ph↗

arXiv1998

Ion structure factors and electron transport in dense Coulomb plasmas

The dynamical structure factor of a Coulomb crystal of ions is calculated at arbitrary temperature below the melting point taking into account multi-phonon processes in the harmonic approximation. In a strongly coupled Coulomb ion liquid, the static structure factor is split into two parts, a Bragg-diffraction-like one, describing incipient long-range order structures, and an inelastic part corresponding to thermal ion density fluctuations. It is assumed that the diffractionlike scattering does not lead to the electron relaxation in the liquid phase. This assumption, together with the inclusion of multi-phonon processes in the crystalline phase, eliminates large discontinuities of the transport coefficients (jumps of the thermal and electric conductivities, as well as shear viscosity, reported previously) at a melting point.

physics.plasm-ph↗

arXiv1997

Internal temperatures and cooling of neutron stars with accreted envelopes

The relationships between the effective surface (T_e) and internal temperatures of neutron stars (NSs) with and without accreted envelopes are calculated for T_e > 5\times10^4 K using new data on the equation of state and opacities in the outer NS layers. We examine various models of accreted layers (H, He, C, O shells produced by nuclear transformations in accreted matter). We employ new Opacity Library (OPAL) radiative opacities for H, He, and Fe. In the outermost NS layers, we implement the modern OPAL equation of state for Fe, and the Saumon-Chabrier equation of state for H and He. The updated thermal conductivities of degenerate electrons include the Debye-Waller factor for the electron-phonon scattering in solidified matter, while in liquid matter they include the contributions from electron-ion collisions (evaluated with non-Born corrections and with the ion structure factors in responsive electron background) and from the electron-electron collisions. For T_e < 10^{5.5} K, the electron conduction in non-degenerate layers of the envelope becomes important, reducing noticeably the temperature gradient. The accreted matter further decreases this gradient at T_e > 10^5 K. Even a small amount of accreted matter (with mass > 10^{-16} Msun) affects appreciably the NS cooling, leading to higher T_e at the neutrino cooling stage and to lower T_e at the subsequent photon stage.

astro-ph↗

arXiv1997

Neutrino synchrotron emission from dense magnetized electron gas of neutron stars

We study the synchrotron emission of neutrino pairs by relativistic, degenerate electrons in strong magnetic fields. Particular attention is paid to the case in which the dominant contribution comes from one or several lowest cyclotron harmonics. Calculations are performed using the exact quantum formalism and the quasiclassical approach. Simple analytic fits to the neutrino synchrotron emissivity are obtained in the domain of magnetized, degenerate and relativistic electron gas provided the electrons populate either many Landau levels or the ground level alone. The significance of the neutrino synchrotron energy losses in the interiors of cooling neutron stars is discussed.

astro-ph↗

arXiv1997

Electron conduction along quantizing magnetic fields in neutron star crusts. II. Practical formulae

Practical expressions are derived for a rapid and accurate evaluation of electric and thermal conductivities and thermopower of degenerate relativistic electrons along quantizing magnetic fields in outer neutron star crusts. The electron Coulomb scattering on ions is considered in liquid matter, and on high-temperature phonons or charged impurities in solid matter. A semi-quantitative treatment of low-temperature phonons is also proposed. The transport properties are expressed through the energy dependent effective electron relaxation time, which is calculated using the theoretical formalism of the previous work (astro-ph/9603133). Accurate fitting expressions for the relaxation time are obtained, and an efficient energy averaging procedure proposed. A Fortran code, which calculates the longitudinal transport properties of degenerate electrons in strong magnetic fields for any parameters of dense stellar matter of practical interest, is available from the authors upon request.

astro-ph↗

arXiv1997

Neutrino pair emission due to scattering of electrons off fluxoids in superfluid neutron star cores

We study the emission of neutrinos, resulting from the scattering of electrons off magnetic flux tubes (fluxoids) in the neutron star cores with superfluid (superconducting) protons. In the absence of proton superfluidity (T> T_{cp}), this process transforms into the well known electron synchrotron emission of neutrino pairs in a locally uniform magnetic field B, with the neutrino energy loss rate Q proportional to B^2 T^5. For temperatures T not much below T_{cp}, the synchrotron regime (Q \propto T^5) persists and the emissivity Q can be amplified by several orders of magnitude due to the appearance of the fluxoids and associated enhancement of the field within them. For lower T, the synchrotron regime transforms into the bremsstrahlung regime (Q \propto T^6) similar to the ordinary neutrino-pair bremsstrahlung of electrons which scatter off atomic nuclei. We calculate Q numerically and represent our results through a suitable analytic fit. In addition, we estimate the emissivities of two other neutrino-production mechanisms which are usually neglected -- neutrino-pair bremsstrahlung processes due to electron-proton and electron-electron collisions. We show that the electron-fluxoid and electron-electron scattering can provide the main neutrino production mechanisms in the neutron star cores with highly superfluid protons and neutrons at T < 5 10^8 K. The electron-fluxoid scattering is significant if the initial, locally uniform magnetic field B > 10^{13} G.

astro-ph↗

arXiv1996

Comment on a paper by M. A. Bautista ``Atomic data from the Iron Project. XVI. Photoionization cross sections and oscillator strengths for Fe V''

Recently, Bautista (1996, A&AS, 119, 105) reported new calculations of photoionization cross sections for Fe V, and compared them with the earlier results of Reilman & Manson (1979, ApJS, 40, 815) and Verner et al. (1993, Atomic Data Nucl. Data Tables, 55, 233). Bautista claimed that beyond 10 Ry the new cross sections ``converge well toward the results by Reilman and Manson'', whereas ``the calculations by Verner et al. still underestimate the cross section by almost factor of two''. We show that Bautista erroneously compared the total (summed over shells) cross sections from Reilman & Manson with the partial 3d-shell cross sections from Verner et al. Actually, the total cross sections from Verner et al. and Reilman & Manson agree within 3% at all energies.

astro-ph↗

arXiv1996

Standard and Enhanced Cooling of Neutron Stars with Superfluid Cores

Calculations are performed of the cooling of neutron stars with standard and enhanced neutrino energy losses in the presence of neutron and proton superfluidities in the stellar cores. The effects of superfluidity on the heat capacity and the neutrino luminosity produced by the direct and modified Urca processes and by the neutrino bremsstrahlung emission in nucleon-nucleon collisions are taken into account. The constraints are analyzed on the critical temperatures $\tn$ and $\tp$ of the transition of neutrons and protons to the superfluid state, which can be obtained from a comparison of observational data on thermal radiation from neutron stars with theoretical cooling curves for the standard and enhanced neutrino energy losses. Possible ranges of $\tn$ and $\tp$ for the pulsar Geminga are discussed.

astro-ph↗

arXiv1996

Thermal and Electric Conductivities of Coulomb Crystals in Neutron Stars and White Dwarfs

Thermal and electric conductivities are calculated for degenerate electrons scattered by phonons in a crystal made of atomic nuclei. The exact phonon spectrum and the Debye--Waller factor are taken into account. Monte Carlo calculations are performed for body-centered cubic (bcc) crystals made of C, O, Ne, Mg, Si, S, Ca, and Fe nuclei in the density range from $10^3$ to $10^{11}$ g cm$^{-3}$ at temperatures lower than the melting temperature but higher than the temperature at which the Umklapp processes begin to be "frozen out". A simplified method of calculation is proposed, which makes it possible to describe the results in terms of simple analytic expressions, to extend these expressions to any species of nucleus, and to consider face-centered cubic (fcc) crystals. The kinetic coefficients are shown to depend tangibly on the lattice type. The results are applicable to studies of heat transfer and evolution of the magnetic field in the cores of white dwarfs and in the crusts of neutron stars. The thermal drift of the magnetic field in the crust of a neutron star is discussed.

astro-ph↗

arXiv1996

Thermal and Electric Conductivities of Coulomb Crystals in the Inner Crust of a Neutron Star

Thermal and electric conductivities of relativistic degenerate electrons are calculated for the case when electrons scatter by phonons in Coulomb crystals made of spherical finite--size nuclei at densities $10^{11}$~g/cm$^3 < ρ< 10^{14}$~g/cm$^3$, corresponding to the inner crust of a neutron star. In combination with the results of the previous article (for lower $ρ$), simple unified fits are obtained which describe the kinetic coefficients in the range $10^3$~g/cm$^3 < ρ< 10^{14}$~g/cm$^3$, for matter with arbitrary nuclear composition. The results are valid for studying thermal evolution of neutron stars and evolution of their magnetic fields. The difference between the kinetic coefficients in the neutron star crust composed of ground state and accreted matters is analyzed. Thermal drift of the magnetic field in the neutron star crust is discussed.

astro-ph↗

arXiv1996

On Cooling of Magnetized Neutron Stars

Cooling of neutron stars with dipole magnetic fields is simulated using a realistic model of the anisotropic surface temperature distribution produced by magnetic fields. Suppression of the electron thermal conductivity of outer stellar layers across the field increases thermal isolation of these layers near the magnetic equator. Enhancement of the radiative and longitudinal electron thermal conductivities in quantizing magnetic fields reduces thermal isolation near the magnetic poles. The equatorial increase of the isolation is pronounced for $B \ga 10^{10}$ G, while the polar decrease -- for $B \ga 10^{12}$ G. The effects compensate partly each other, and the actual influence of the magnetic fields on the cooling is weaker than predicted by the traditional theories where the equatorial effects have been neglected.

astro-ph↗

arXiv1996

Neutrino-Pair Emission due to Electron-Phonon Scattering in a Neutron Star Crust

Neutrino-pair bremsstrahlung radiation is considered due to electron--phonon scattering of degenerate, relativistic electrons in a lattice of spherical atomic nuclei in a neutron star crust. The neutrino energy generation rate is calculated taking into account exact spectrum of phonons, the Debye--Waller factor, and the nuclear form--factor in the density range from $10^7$~g~cm$^{-3}$ to $10^{14}$~g~cm$^{-3}$ at arbitrary nuclear composition for body-centered-cubic and face-centered-cubic Coulomb crystals. The results are fitted by a unified analytic expression. A comparison is given of the neutrino bremsstrahlung energy losses in a neutron star crust composed of ground state and accreted matter, in the solid and liquid phases.

astro-ph↗

arXiv1996

Electron $ν\barν$ Bremsstrahlung in a Liquid Phase of Neutron Star Crusts

Neutrino emissivity from the electron $ν\barν$ bremsstrahlung in the liquid layers of the neutron star crusts is studied. Nuclear composition of matter in neutron star crusts is considered for various scenarios of neutron star evolution. For the deep layers of the crust, the compositions of cold catalyzed matter, accreted matter and hot matter ($T \ga 5 \times 10^9$~K) are shown to be very different, and this implies differences in the neutrino emissivity at given density and temperature. Neutrino-pair bremsstrahlung, due to collisions of relativistic degenerate electrons in a Coulomb liquid of atomic nuclei, is considered. The neutrino energy loss rate is expressed in a simple form through a Coulomb logarithm $L$ -- a slowly varying function of density, temperature, and nuclear composition. Non-Born corrections, thermal width of electron Fermi sphere, and finite sizes of nuclei are taken into account. Implications for cooling of neutron stars are discussed.

astro-ph↗

arXiv1996

Atomic Data for Astrophysics. II. New Analytic Fits for Photoionization Cross Sections of Atoms and Ions

We present a complete set of analytic fits to the non-relativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.

astro-ph↗