The Zeeman effect for weak magnetic fields

The polarization of a normal Zeeman triplet is discussed for the case in which the lifetime of the excited state of the atom is comparable to or shorter than the period of Larmor precession.     

Some problems concerning gas flows in close binary systems of dwarf stars

The influence of the stream flowing from the secondary on the disk-like envelope of the main star is briefly considered. It is found that the lifetime of the envelope after the flow ceases must be less than 10⁶ sec.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

The effect of rotation on the luminosity of magnetic stars

The effect of rotation and a general magnetic field on the luminosity, radius, and effective temperature of the upper Main-Sequence stars has been investigated using a perturbation analysis. The magnetic field profile prevailing inside the star is assumed to have both poloidal and toroidal components. The case of constant as well as differential rotation is admitted. Model calculations indicate that these stellar parameters modify considerably as a result of coupling between rotation and the magnetic field.     

Some limits on cosmic-ray heating ofHI clouds by magnetic stars

Estimates are made of the low-energy, cosmic-ray power generated by the rotational braking of magnetic Ap stars through interaction with the interstellar medium. The resulting cosmic-ray power density (1.4×10^–29 J s^–1 m^–3) in the galactic plane is at least three orders too small to maintain the observed kinetic temperatures (100 K) ofHi clouds. About 0.4% ofHi clouds contain an Ap star; in such cases 10²⁶ J s^–1 may be generated by the star during the early part of its Main Sequence life-time, but this makes only a small contribution to the heating of the cloud.     

Tunnel effect in molecules in strong magnetic fields of neutron stars

It is shown that, in the strong magnetic field of the neutron starB=10¹²–10¹³ G, the probability of the tunnel effect in the molecules increases significantly. It is quite probable that this effect can catalyze nuclear reactions at the neutron star surface.     

Some observational results on long-time variability of stars with magnetic fields

Ziel dieser Arbeit ist die Zusammenfassung von langzeitigen Phänomenen magnetischer Sterne, die am Potsdamer Observatorium erhalten wurden. Die betrachteten magnetischen Sterne sind γ Equ, HD 9996, 53 Cam, 52 Her und v Cep. Viele Ergebnisse wurden schon publiziert; hier sollen die Resultate zusammengefaßt und durch neue Details vervollständigt werden.     

Convective stability in magnetic stars

Dynamical stability of a static axisymmetrical magnetic star with respect to high-order modes of oscillation is investigated by means of the energy method, neglecting the Eulerian perturbation of gravity. The magnetic field is assumed to be continuous across the surface of the star and its first-order spatial derivatives, but it may have both toroidal and poloidal components.The second variation of the potential energy is written in a way which, in the case of apurely toroidal field, and for axisymmetrical and non-axisymmetrical modes, yields Tayler's local stability criteria which are necessary and sufficient conditions for convective stability, and in the case of ageneral field yields a single local stability criterion, which is a sufficient condition for convective stability.     

Theory of magnetic stars

The principal observational facts on the early-type magnetic stars are summarized, and interpreted in terms of the oblique rotator model. In the theoretical discussion of the interaction between a large-scale magnetic field and the star's thermal, gravitational and rotation fields, emphasis is laid on effects that are relevant to both fossil and dynamo theories of the origin of the magnetic field.     

Superheavy elements in the atmospheres of magnetic stars

The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly.     

Magnetic braking in magnetic binary stars

The role of an external magnetic field in the magnetic braking of a star with a dipolar field is investigated. In a magnetic cataclysmic variable system (i.e. the primary compact star has a strong magnetic field), the field external to the braking star (a late-type main-sequence star with a dynamo-generated field) originates from the compact star. A closed field region — the system dead zone — is formed within the binary system, and it does not take part in magnetic braking. The overall braking rate depends on the extent of this region and of the open flux, and is dependent on centrifugal effects. In the case of two interacting dipoles, the dipole orientations relative to the spin axes and to each other are found to be important, leading to different amounts of open flux and therefore of magnetic braking, owing to different centrifugal effects on closed field regions. However, in circumstances consistent with observations and dynamo theory, the white dwarf's field reduces the magnetic braking of the secondary significantly, a finding qualitatively similar to the results previously obtained for two anti-aligned dipoles perpendicular to the orbital plane. In the cases where the two dipole axes are not perpendicular to the orbital plane, but are inclined in the plane that links them, the 'cut-off' in magnetic braking is less abrupt, and this effect is more obvious as the inclinations increase. Only in the extreme cases when the two dipole axes are aligned in the orbital plane does the braking increase with white dwarf field strength. We conclude that detailed evolutionary modelling of AM Herculis systems needs to take account of the inclination effect.     

The search for the elusive Zeeman effect in HI

The paper describes the early endeavours by radioastronomers to detect the weak signature of the Zeeman effect in interstellar neutral hydrogen clouds in an effort to measure the Galactic magnetic field strength. The search is set in the context of the neutral hydrogen programme at Jodrell Bank. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On non-axisymmetric magnetic equilibria in stars

In previous work, stable approximately axisymmetric equilibrium configurations for magnetic stars were found by numerical simulation. Here, I investigate the conditions under which more complex, non-axisymmetric configurations can form. I present numerical simulations of the formation of stable equilibria from turbulent initial conditions and demonstrate the existence of non-axisymmetric equilibria consisting of twisted flux tubes lying horizontally below the surface of the star, meandering around the star in random patterns. Whether such a non-axisymmetric equilibrium or a simple axisymmetric equilibrium forms depends on the radial profile of the strength of the initial magnetic field. The results could explain observations of non-dipolar fields on stars such as the B0.2 main-sequence star τ Sco or the pulsar 1E 1207.4-5209. The secular evolution of these equilibria due to Ohmic and buoyancy processes is also examined.     

The effect of magnetic fields on the formation of circumstellar discs around young stars

We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation.     

Microwave radiation from magnetic stars

Cyclotron microwave emission from magnetic stars is considered, assuming that they have coronae with the temperatureT10⁷ K and the emission measureEM10⁵⁴ cm^–3. It has been shown that the cyclotron radiation from a star with a dipole magnetic field has a specific spectrum with a maximum in the frequency rangesv _o/2 >v >sv _o/2 (s being the number of cyclotron harmonic, andv _o the gyrofrequency corresponding to the polar magnetic field) and radiation flux decreasing towards lower frequencies asv _4/3. The frequency of the spectrum maximum depends on the angle between the line-of-sight and the magnetic axis of the star. The observed radiation from a rotating magnetic star can be modulated with a modulation depth of about 0.2 at frequencies near maximum. The radiation is partially circularly-polarized in the sense of an extraordinary mode. The degree of polarization is almost constant at frequenciesv >sv _o/2 and increases with frequency atv >sv _o/2. The estimation of cyclotron radio fluxes of the nearest magnetic stars shows that they are observable in microwaves by means of modern radio astronomy.     

New magnetic chemically peculiar stars

Our observations with the 6-m telescope revealed six new magnetic chemically peculiar (CP) stars among objects with large depressions in the continuum: HDE 293764, BD+17°3622, HD 169887, HDE 231054, HDE 338226, and HDE 343872. The presence of a magnetic field is suspected in several other CP stars. The maximum longitudinal field component B _e exceeds 1.5 kG for all six stars; in HDE 293764 and HDE 343872, it reaches 3.8 kG. For each object, we present our magnetic-field measurements and published data on previous studies. The method of searching for magnetic stars based on an analysis of spectrophotometry shows its efficiency.     

General properties of magnetic CP stars

We present the review of our previous studies related to observational evidence of the fossil field hypothesis of formation and evolution of magnetic and non-magnetic chemically peculiar stars. Analysis of the observed data shows that these stars acquire their main properties in the process of gravitational collapse. In the non-stationary Hayashi phase, a magnetic field becomes weakened and its configuration complicated, but the fossil field global orientation remains. After a non-stationary phase, relaxation of young star’s tangled field takes place and by the time of joining ZAMS (Zero Age Main Sequence) it is generally restored to a dipole structure. Stability of dipole structures allows them to remain unchanged up to the end of their life on the Main Sequence which is 10⁹ years at most.