A new magnetic field generation mechanism in radiation-heated plasma is considered. This mechanism is based on the Nernst effect and operates in plasma with oppositely directed temperature and density gradients. The efficiency of the mechanism is determined by the values of these gradients.

It is generally believed that the first stars were hot and massive because of the lack of efficient coolants in the metal-free primordial gas. This paper considers the thermal generation of the magnetic field in such stars. The mechanism operates in the surface layers of hot stars where departures from the local thermodynamic equilibrium form a region with the inverse temperature gradient (it occurs in atmospheric layers with the optical depth <= 0.01-0.001). The thermal generation is efficient in stars with a low mass-loss rate. A growth rate of the magnetic field can be such high that even young stars with the age similar to 10(4)-10(5) yr possess the magnetic field comparable to that detected in massive stars of the present-day Universe.

Diffusion caused by a combined influence of the electric current and Hall effect is considered, and it is argued that such diffusion can form inhomogeneities of a chemical composition in plasma. The considered mechanism can be responsible for the formation of element spots in laboratory and astrophysical plasmas. This current-driven diffusion can be accompanied by propagation of a particular type of waves in which the impurity number density oscillates alone. These compositional waves exist if the magnetic pressure in plasma is much greater than the gas pressure.

A new magnetic field-generation mechanism based on the Nernst effect is considered in hot massive stars. This mechanism can operate in the upper atmospheres of O and B stars where departures from the LTE form a region with the inverse temperature gradient.

The stars of the middle main sequence often have spot-like chemical structures at their surfaces. We consider diffusion caused by electric currents and argue that such current-driven diffusion can form chemical inhomogeneities in a plasma. The considered mechanism can contribute to a formation of element spots in Hg-Mn and Ap-stars. Due to the Hall effect, diffusion in the presence of electric currents can be accompanied by the propagation of a particular type of magnetohydrodynamic modes in which only the impurity number density oscillates. Such modes exist if the magnetic pressure is much greater than the gas pressure and can be the reason for variations of the abundance peculiarities in stars. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The stars of the middle main sequence often have relatively quiescent outer layers and spot-like chemical structures may develop in their atmospheres. Recent observations show that abundance peculiarities can change as stars evolve on the main sequence and the timescale of these changes lies in a wide range from million years to months. These observations imply that, perhaps, our understanding of diffusion processes at work in magnetic stars is incomplete and a more detailed analysis of these processes is required. In the present paper, we consider diffusion caused by a combined influence of the electric current and the Hall effect. Such diffusion has a number of very particular properties and, generally, can change the surface chemistry of stars in combination with other diffusion processes. For instant, current-driven diffusion is accompanied by a propagation of the special type of waves in which only the impurity number density oscillates. Propagation of such waves changes the shape and size of spots as well as chemical abundances within them. The period of compositional waves depends on the parameters of plasma (magnetic field, electric current, temperature, etc.) and can be different for the waves of different elements. Compositional waves exist in the regions where the magnetic pressure is greater than the gas pressure. These waves can be the reason of variations with different timescales in the abundance peculiarities of magnetic stars.

The stars of the middle main sequence often have spot-like chemical structures at their surfaces. We consider the diffusion process caused by electric currents that can lead to the formation of such chemical spots. Diffusion is considered using the partial momentum equations derived by the Chapman-Enskog method. We argue that diffusion caused by electric currents can substantially change the surface chemistry of stars and form spotted chemical structures even in a relatively weak magnetic field. The considered mechanism can be responsible for a formation of element spots in Hg-Mn and Ap-stars. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Context. The recent discovery of long-period, low magnetic field pulsars in low-mass X-ray binaries (LMXBs) represents a challenge for the standard evolutionary scenario. These pulsars have a magnetic field strength comparable to that of millisecond pulsars (similar to 10(8)-10(9) G), but their period is at least an order of magnitude longer.Aims. We discuss the origin of this new class of pulsars within the standard picture of LMXBs formation and apply our results to the case of IGR J17480-2446.Methods. The magnetothermal evolution of the binary system is studied numerically by taking into account the effect of different accretion rates during the Roche-lobe overflow in the framework of the minimal cooling scenario.Results. We show that, in addition to standard millisecond pulsars, long-period low magnetic field pulsars should also be expected as a possible outcome of the binary evolution, depending on the strength of the accretion rate during the Roche-lobe overflow. In particular, we argue that IGR J17480-2446 belongs to this new class of objects.

We consider combined rotational, magnetic, and thermal evolution of the neutron star during the accretion phase in a binary system. A rapid accretion-driven decay of the magnetic field decreases substantially the efficiency of angular momentum transfer. As a result, the neutron star cannot spin-up to periods shorter than some limiting value even if accretion is very long and accretion rate is high. The proposed mechanism can explain a discrepancy between the shortest detected period and minimal possible spin period of neutron stars.

The thermomagnetic evolution of the young neutron star in Cassiopea A is studied by considering fast neutrino emission processes. In particular, we consider neutron star models obtained from the equation of state computed in the framework of the Brueckner-Bethe-Goldstone many-body theory and variational methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of state. It is shown that it is possible to explain a fast cooling regime as the one observed in the neutron star in Cassiopea A if the Joule heating produced by dissipation of the small-scale magnetic field in the crust is taken into account. We thus argue that it is difficult to put severe constraints on the superfluid gap if the Joule heating is considered.

Context. Magnetohydrodynamic (MHD) instabilities can play an important role in the dynamics of the pulsar magnetosphere and can be responsible for the formation of various structures. Aims. We consider the instability caused by a gradient of the magnetic pressure, which can occur in a non-neutral magnetospheric plasma of the pulsars. Methods. Stability is discussed by means of a linear analysis of the force-free MHD equations. Results. We argue that the pulsar magnetospheres are always unstable. The unstable disturbances have a form of filaments directed along the magnetic field lines with plasma motions being almost parallel (or anti-parallel) to the magnetic field. The growth rate of instability is high and can reach a fraction of ck ,w herek is the wavevector of unstable disturbances. The instability can be responsible for fluctuations of plasma and the short-term variability of pulsar emission.

The study of the magnetic field in stellar radiation zones is an important topic in modern astrophysics because the magnetic field can play an important role in several transport phenomena such as mixing and angular momentum transport. We consider the influence of rotation on stability of a predominantly toroidal magnetic field in the radiation zone. We find that the effect of rotation on the stability depends on the magnetic configuration of the basic state. If the toroidal field increases sufficiently rapidly with the spherical radius, the instability cannot be suppressed entirely even by a very fast rotation although the strength of the instability can be significantly reduced. On the other hand, if the field increases slowly enough with the radius or decreases, the instability has a threshold and can be completely suppressed in rapidly rotating stars. We find that in the regions where the instability is entirely suppressed a particular type of magnetohydrodynamic waves may exist which are marginally stable.

The magnetic field in stellar radiation zones can play an important role in phenomena such as mixing, angular momentum transport, etc. We study the effect of rotation on the stability of a predominantly toroidal magnetic field in the radiation zone. In particular we considered the stability in spherical geometry by means of a linear analysis in the Boussinesq approximation. It is found that the effect of rotation on the stability depends on a magnetic configuration. If the toroidal field increases with the spherical radius, the instability cannot be suppressed entirely even by a very fast rotation. Rotation can only decrease the growth rate of instability. If the field decreases with the radius, the instability has a threshold and can be completey suppressed.

Context. Magnetohydrodynamic (MHD) instabilities can play an important role in the structure and dynamics of the pulsar magnetosphere. Aims. We consider the instability caused by differential rotation that is suggested by many theoretical models. Methods. Stability is considered by means of a linear analysis within the frame of the force-free MHD. Results. We argue that differentially rotating magnetospheres are unstable for any particular geometry of the magnetic field and rotation law. The characteristic growth time of instability is of the order of the rotation period. The instability can lead to fluctuations of the emission and enhancement of diffusion in the magnetosphere.

We perform 3D numerical simulation of jets containing both the azimuthal and axial field of subthermal strength. We argue that in azimuthal and axial magnetic fields the jet is always unstable to non-axisymmetric perturbations. Stabilization does not occur even if the strengths of these field components are comparable. If the axial field is weaker than the azimuthal one, instability occurs for perturbations with any azimuthal wave number in, and the growth rate reaches a saturation value for low values of m. If the axial field is stronger than the toroidal one, the instability shows up for perturbations with relatively high m.

Context. Magnetohydrodynamic instabilities can be responsible for the formation of structures with various scales in astrophysical jets.Aims. We consider the stability properties of jets containing both the azimuthal and axial field of subthermal strength. A magnetic field with complex topology in jets is suggested by theoretical models and is consistent with recent observations.Methods. Stability is discussed by means of a linear analysis of the ideal magnetohydrodynamic equations.Results. We argue that in azimuthal and axial magnetic fields the jet is always unstable to non-axisymmetric perturbations. Stabilization does not occur even if the strengths of these field components are comparable. If the axial field is weaker than the azimuthal one, instability occurs for perturbations with any azimuthal wave number m, and the growth rate reaches a saturation value for low values of m. If the axial field is stronger than the toroidal one, the instability shows for perturbations with relatively high m.

MHD waves can be responsible for plasma fluctuations and short-term variations of the pulsar emission. We consider the properties of plane and cylindrical waves that can exist in the force-free magnetosphere. Waves are considered by means of a linear analysis of the force-free MHD equations. We argue that these particular types of waves can exist in the magnetosphere of pulsars. These waves are closely related to the Alfven waves of the standard magnetohydrodynamics but are modified by the force-free condition and non-zero charge density. We derive the dispersion relation for magnetospheric waves and show that the wave periods are likely within the range $\sim 10^{-2}-10^(-4}$ s depending on the magnetospheric parameters.

Context. MHD instabilities can be responsible for the complex morphology of astrophysical jets. Aims. We consider the stability properties of jets containing bot h the azimuthal and axial field of subthermal strength. The pr esence of the magnetic field with complex topology in jets is suggested by t heoretical models and it is consistent with recent observat ions. Methods. Stability is discussed by means of a linear analysis of the id eal MHD equations. Results. We argue that, in the presence of azimuthal and axial magnetic fields, the jet is always unstable to non-axisymmetric pert urbations. Stabilization does not occur even if the strengths of these fi eld components are comparable. If the axial field is weaker th an the azimuthal one, instability occurs for perturbations with any azimuthal wave number m, and the growth rate reach a saturation value for small values of m. If the axial field is stronger than the toroidal one, the instabi lity shows off for perturbations with relatively large m.

Magnetorotational instability (MRI) has been suggested to lead a rapid growth of the magnetic field in core collapse supernovae and produce departures from spherical syymmetry that can be important in determining the explosion mechanism. We address the problem of stability in differentially rotating magnetized proto-neutron stars at the beginning of their evolution. Criteria for MRI in proto-neutron stars are derived without simplying assumptions about a weak magnetic field and are substantially different from the standard condition. If the magnetic field is strong, MRI can occur only in the neighbourhood of the region where the spherical radial component of the magnetic field vanishes. The growth rate of MRI is relatively low except for perturbations with very small scales which usually are not detected in numerical simulations. We find that MRI in proto-neutron stars grows more slowly than than the double diffusive instability analogous the Goldreich-Schubert-Fricke instability in ordinary stars.