Spectropolarimetry and IR photometry of magnetic white dwarfs: Vacuum polarization or rydberg states in their magnetic fields?

We suggest two mechanisms to explain IR photometric and spectropolarimetric observations of magnetic white dwarfs: vacuum polarization and the existence of Rydberg atomic states with large dipole moments arising due to atomic collisions in the strong magnetic field of the white dwarf (so-called magnetic collision-induced absorption, or magnetic CIA). Both mechanisms can explain the observed rotations of the polarization ellipses and the depression of the IR spectral energy distribution. We present the results of spectropolarimetric observations of several magnetic white dwarfs with the Special Astrophysical Observatory 6-m telescope, together with photometric observations in the near-IR obtained with the Russian-Italian AZT-24 Telescope at Campo Imperatore.     

New measurements of the circular polarization of the radio emission of Active Galactic Nuclei on parsec scales

We describe and compare methods for reducing radio circular-polarization data for Active Galactic Nuclei on parsec (VLBI) scales, and present results derived by applying these methods to 15 GHz observations of 29 AGN obtained on the Very Long Baseline Array. Circular polarization was detected in six of these objects, with characteristic degrees of several tenths of a percent. According to results obtained in other studies and the new results presented here, the most probable mechanism for the formation of the circular polarization is Faraday conversion of linear polarization when the radio emission passes through a magnetized plasma. For the objects OJ 287, 1334-127, and 3C 279, which were observed at several epochs, we find a constant degree and sign of the circular polarization on time scales of several years, testifying to the presence of stable ordered magnetic fields in the corresponding regions.     

Polarization effects in the radiation of magnetized envelopes and extended accretion structures

We have calculated the degree and position angle of the polarization of radiation scattered in a magnetized, optically thin or optically thick envelope around a central source, taking into account Faraday rotation of the plane of polarization during the propagation of the scattered radiation and the finite size of the radiation source. The wavelength dependence of the degree of polarization can be used to estimate the magnetic field of the source (a star, the region around a neutron star, or a black hole), and we have used our calculations to estimate the magnetic fields in a number of individual objects: several hot O and Wolf-Rayet stars, compact objects in X-ray close binaries with black holes (SS 433, Cyg X-1), and supernovae. The spectrum of the linear polarization can be used to determine the magnetic field in the vicinity of a central supermassive black hole, where the polarized optical radiation is generated. In a real physical model, this value can be extrapolated to the region of the last stable orbit. In the future, the proposed technique will make it possible to directly estimate the magnetic field in the region of the last stable orbit of a supermassive black hole using X-ray polarimetry.     

Thermal cyclotron radiation by isolated magnetic white dwarfs and constraints on the parameters of their coronas

An efficient method for the detection and estimation of the parameters of the coronas of isolated white dwarfs possessing magnetic fields of about 10⁷ G is tested. This method is based on the detection of thermal radiation of the coronal plasma at harmonics of the electron gyrofrequency, which is manifest as a polarized infrared excess. The Stokes parameters for the thermal cyclotron radiation from the hot corona of a white dwarf with a dipolar magnetic field are calculated. A new upper limit for the electron density, 10¹⁰ cm^?3, in a corona with a temperature of ?10⁶ K is found for the white dwarf G99-47 (WD 0553+053). This limit is a factor of 40 lower than the value derived earlier from ROSAT X-ray observations. Recommendations for subsequent infrared observations of isolated magnetic white dwarfs aimed at detecting their coronas or deriving better constraints on their parameters are presented.     

Merging of components in close binaries: Type Ia supernovae,massive white dwarfs,and Ap stars

The “Scenario Machine” (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes. We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance must be taken into account at estimating the parameters of the Universe expansion acceleration. According to theoretical estimates, the most massive—as a rule, magnetic—white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main-sequence stars (M ? 1.5 M _⊙) with convective envelopes.     

Linear polarization of the radiation from active galactic nuclei and the redshift dependence of their main parameters

We consider the observed continuum linear polarization of extragalactic objects with various redshifts z, most of which have degrees of polarization p ≤ 10%. We propose that this polarization is due to multiple scattering of the radiation in magnetized accretion disks around the Active Galactic Nuclei (AGN; the Milne problem in an optically thick atmosphere). The structure of the accretion disks and the polarization of the emergent radiation depend on the main parameters of the AGN—the mass of the central body M _BH , accretion rate $ \dot M $ \dot M , magnetic field at the black-hole event horizon B _H , angular momentum a _*, and the explicit form of the magnetic-field distribution in the accretion disk. Theoretical expressions for the degree of polarization are averaged over all angles of the disks to the line of sight, and the resulting formula compared with the mean observed polarizations in redshift intervals Δz = 0.25. The dependence of the observed degree of polarization and the main parameters on the redshift z is derived. The degrees of polarization of 305 objects from the catalog of Hutsemekers et al. with redshifts from zero to z = 2.25 are used for the analysis.     

Stellar activity observed by the Kepler Space Telescope. The M dwarf of the Kepler-32 system with five orbiting planets

The activity of the central star of the Kepler-32 planetary system is studied using continuous 1141-day observations with the Kepler Space Telescope. The Kepler-32 system includes a slowly rotating Mdwarf (rotational period of 37.8 d) with a mass of 0.54M _⊙ and five planets. One of the unique properties of the system is its compactness: the orbits of all five planets are less than a third of the size of the orbit of Mercury; the planet closest to the star is separated from it by only 4.3 stellar radii. Surface-temperature inhomogeneities of the central star are studied using precise photometric observations of Kepler-32, and their evolution traced. In total, 42 624 individual brightness measurements in the 1141-day (3.1-year) observing interval were selected for the analysis. The calculated amplitude power spectra for the first and second halves of the interval of the Kepler-32 observations indicate appreciable variability of the photometric period, corresponding to the evolution of active regions at various latitudes on the stellar surface. Evidence for the existence of two active regions on the stellar surface separated in phase by 0.42 has been found. Time intervals in which the longitudes of the active regions changed (“flip-flops”) with durations of the order of 200–300 days have been established. The spotted area of the star was, on average, about 1% of the total visible surface, and varied from 0.3 to 1.7%. The results for the dwarf Kepler-32 are compared with those from a spectropolarimetric survey of 23 M dwarfs, including both fully convective stars and stars with weakly radiative cores. For a more detailed comparison, temperature inhomogeneities on the surface of one of the survey stars, DS Leo, was reconstructed using the ground-based observations (316 individual measurements of the V-band brightness of the star during seven observing seasons in an all-sky automated survey). The general properties and evolution of the active regions on DS Leo and Kepler-32 are considered. The positions of the active regions on the surface of Kepler-32 yields no evidence for differential rotation of this star. The possibility of detecting the magnetic field of Kepler-32 is proposed. The analysis of the photometric data for Kepler-32 are also compared to the previous results for the fully convective, low-mass M dwarfs GJ 1243 and LHS 6351. This demonstrates that the observed manifestations of activity on Kepler-32 correspond to those for active G-K stars and to M dwarfs with masses of the order of 0.5M _⊙, rather than Mdwarfs with masses from 0.2 to 0.5M _⊙.     

Full 3D MHD calculations of accretion flow structure in magnetic cataclysmic variables with strong,complex magnetic fields

We have performed three-dimensional magnetohydrodynamical calculations of stream accretion in cataclysmic variable stars for which the white dwarf primary possesses a strong, complex magnetic field. These calculations were motivated by observations of polars: cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation of an accretion disk. In this case, an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white dwarfs in some binaries possess complex (non-dipolar) magnetic fields. We performed simulations of ten polars, with the only variable being the azimuthal angle of the secondary with respect to the white dwarf. These calculations are also applicable to asynchronous polars, where the spin period of the white dwarf differs by a few percent from the orbital period. Our results are equivalent to calculating the structure of one asynchronous polar at ten different spin-orbit beat phases. Our models have an aligned dipolar plus quadrupolar magnetic field centered on the whitedwarf primary. We find that, with a sufficiently strong quadrupolar component, an accretion spot arises near the magnetic equator for slightly less than half our simulations, while a polar accretion zone is active for most of the remaining simulations. For two configurations, accretion at a dominant polar region and in an equatorial zone occurs simultaneously. Most polar studies assume that the magnetic field is dipolar, especially for single-pole accretors. We demonstrate that, with the orbital parameters and magnetic-field strengths typical of polars, the accretion flow patterns can vary widely in the case of a complex magnetic field. This may make it difficult formany polars to determine observationally whether the field is pure dipolar or is more complex, but there shoulid be indications for some systems. In particular, a complex magnetic field should be suspected if there is an accretion zone near the white dwarf’s equator (assumed to be in the orbital plane) or if there are two or more accretion regions that cannot be fitted by dipolar magnetic field. Magnetic-field constraints are expected to be substantially stronger for asynchronous polars, with clearer signs of complex field geometry due to changes in the accretion flow structure as a function of azimuthal angle. These indications become clearer in asynchronous polars because each azimuthal angle corresponds to a different spin-orbit beat phase.     

Formation of the white-dwarf population of the Galactic halo

Microlensing events detected in the MACHO project are sometimes interpreted as evidence for the presence of a rich population of white dwarfs in the halo of the Galaxy, containing up to 10¹² objects. In this connection, we study the possible formation of this population in the Galaxy via the accretion of gaseous or gaseous-stellar systems. The accumulation of a large number of white dwarfs in the Galactic halo is possible only if several specific conditions are simultaneously satisfied. We conclude that the presence of a rich white-dwarf population in the halo is unlikely.     

Supersoft X-ray sources: Basic parameters

The parameters of ten supersoft X-ray sources observed by ROSAT obtained using blanketing approximations and LTE model atmospheres are analyzed. The consistency of the resulting parameters with a model with stable/recurrent burning on the surface of the white dwarf is studied. The luminosity and sizes of seven of the sources are in good agreement with this model. The masses of the white dwarfs in these sources are estimated. A formula that can be used to estimate the masses of white dwarfs in classical supersoft sources based on their effective temperatures is presented.     

Polarimetry of the protoplanetary nebula AFGL 2688

Our spectroscopic (with resolution R=75 000) and spectropolarimetric (R=15 000) observations with the 6-m telescope of the Special Astrophysical Observatory have enabled us to distinguish photospheric and circumstellar features in the optical spectrum of the bipolar protoplanetary nebula AFGL 2688 for the first time. The linear polarization of the radiation was measured at 5000–6600 Å. The emission in the lines of the sodium doublet and the Swan bands is formed in the envelope, and the mechanism exciting the transitions is resonance fluorescence. We conclude that the circumstellar envelope has a low density. Features of the structure of the nebula are discussed based on published high-angular-resolution photometric and polarimetric (HST NICMOS) data.     

Magnetic fields in methanol maser condensations based on data for related regions. Seven sources: Observational parameters

Results of polarization observations of gas-dust condensations obtained on the Nançay radio telescope in the 1665 and 1667 MHz OH lines in all four Stokes parameters are reported. Seven OH maser sources associated with methanol masers were selected for this study. The goal was to estimate the magnetic fields in methanol condensations from the Zeeman splitting of OH maser lines associated with the methanol masers. The Gaussian parameters of features in the OH spectra are presented, and their polarization parameters are estimated: the degree of circular polarization m _C , flux density in linear polarization p, and degree of linear polarization m _L . The magnetic field intensity B has been estimated from the Zeeman splitting of the OH lines and approximation of the Stokes parameter V from the derivative of Stokes parameter I. B varies from ≤0.5 to 1.4 mG for different sources. The association of OH masers with methanol emission has been analyzed; the magnetic fields of OH masers in interstellar condensations associated with Class I methanol masers can be determined more reliably than the fields in interstellar condensations with OH masers associated with Class II methanol emission, and have higher values. The sizes of the studied regions suggest they may be bound structures such as Bok globules, small IRDC clouds, or protoplanetary disks.     

Polarization of the solar corona in the green line: Contradiction between theory and observations

The main results of polarization observations in the 530.3-nm line and their role in studying the physical conditions, structure, and magnetic field in the solar corona are discussed. A serious discrepancy between the observations and widely-accepted theoretical concepts was revealed: the theory predicts that the orientation of the polarization electric vector should be nearly radial, in contradiction with the observational results. In particular, the polarization vectors for both the green line and white-light corona in high-latitude streamers were tangential during the eclipse of July 11, 1991. The dependence of the degree of polarization on the angle between the radial direction and the magnetic-field vector was calculated without any a priori assumptions about the configuration of coronal fields. This theoretical analysis of the polarization-vector orientation for magnetic-dipole emission in the green line are in agreement with results obtained previously in other studies. Some ways to resolve the observed discrepancies are discussed.     

Measurement of the integrated Faraday rotations of BL Lac objects

We present the results of multi-frequency polarization VLA observations of radio sources from the complete sample of northern, radio-bright BL Lac objects compiled by H. Kühr and G. Schmidt. These were used to determine the integrated rotation measures of 18 sources, 15 of which had never been measured previously, which hindered analysis of the intrinsic polarization properties of objects in the complete sample. These measurements make it possible to correct the observed orientations of the linear polarizations of these sources for the effect of Faraday rotation. The most probable origin for Faraday rotation in these objects is the Galactic interstellar medium. The results presented complete measurements of the integrated rotation measures for all 34 sources in the complete sample of BL Lac objects.     

Axially symmetrical models of the products of a conservative merger of a binary white dwarf with similar-mass components

The possibility of a conservative merger of a binary white dwarf whose components have similar masses is studied. Axially symmetrical models for single, rapidly rotating white dwarfs that are possible products of such mergers are constructed and their physical characteristics investigated. The merger products must be turbulent, and the viscosity of the electron gas is not sufficient to support the observed luminosities of massive, bright white dwarfs. The amount of dissipative energy and the timescale for its release are estimated.     

Solution of the radiative-transfer equations with an azimuth-averaged Hanle phase matrix

The system of radiative-transfer equations for polarized radiation is solved analytically using an azimuth-averaged Hanle phase matrix. A Milne-Eddington model with a constant ratio of the line and continuum absorption coefficients and a linear dependence of the source function on optical depth is adopted for the atmosphere. The vector magnetic field is taken to be constant with optical depth. The dependence of the linear polarization obtained during observations at the solar limb on the magnitude and inclination of the magnetic field to the normal of the atmosphere is presented as an illustration of the theoretical computations. The measured polarization corresponds to two magnetic-field values and several possible field inclinations. The measured polarization is directly proportional to the quantum coefficient W ₂ determining the resonance polarization in lines.     

Equilibrium Configurations of Rotating White Dwarfs at Finite Temperatures

In this work, cold and hot, static and rotating white dwarf stars are investigated within the framework of classical physics, employing the Chandrasekhar equation of state. The main parameters of white dwarfs such as the central density, pressure, total mass and radius are calculated fulfilling the stability criteria for hot rotating stars. To construct rotating configurations the Hartle approach is involved. It is shown that the effects of finite temperatures become crucial in low-mass white dwarfs, whereas rotation is relevant in all mass range. The simultaneous accounting for temperature and rotation is critical in the calculation of the radii of white dwarfs. The results obtained in this work can be applied to explain a variety of observational data for white dwarfs from the Sloan Digital Sky Survey Data Releases.     

Population synthesis of rapidly rotating main-sequence stars with companions

Usingthe “Scenario Machine” (a specialized numerical code formodeling the evolution of large ensembles of binary systems), we have studied the physical properties of rapidly rotating main-sequence binary stars (Be stars) with white-dwarf companions and their abundance in the Galaxy. The calculations are the first to take into account the cooling of the compact object and the effect of synchronization of the rotation on the evolution of Be stars in close binaries. The synchronization time scale can be shorter than the main-sequence lifetime of a Be star formed during the first mass transfer. This strongly influences the distribution of orbital periods for binary Be stars. In particular, it can explain the observed deficit of short-period Be binaries. According to our computations, the number of binary systems in the Galaxy containing a Be star and white dwarf is large: 70–80% of all Be stars in binaries should have degenerate dwarf companions. Based on our calculations, we conclude that the compact components in these systems have high surface temperatures. Despite their high surface temperatures, the detection of white dwarfs in such systems is hampered by the fact that the entire orbit of the white dwarf is embedded in the dense circumstellar envelope of the primary, and all the extreme-UV and soft X-ray emission of the compact object is absorbed by the Be star’s envelope. It may be possible to detect the white dwarfs via observations of helium emission lines of Be stars of not very early spectral types. The ultraviolet continuum energies of these stars are not sufficient to produce helium line emission. We also discuss numerical results for Be stars with other evolved companions, such as helium stars and neutron stars, and suggest an explanation for the absence of Be-black-hole binaries.