Wave beams in the magnetoelastic crust of pulsars

The propagation of axially symmetric magnetoelastic waves near the equatorial plane of the crust of a neutron star embedded in a transverse magnetic field is examined. The crust is treated as a solid-state plasma and waves are excited in it in the form of a transverse magnetic field applied to the inner boundary of the star’s crust. The time dependent equation is solved in a linear approximation assuming that the perturbing magnetic field is small compared to the unperturbed field. A simple, exact solution in the form of linear gaussian beams is obtained without additional conditions being imposed on the dissipation, dispersion, and narrowness of the beam, provided only that the velocity c_n of these waves depends weakly on position. This last condition is satisfied for the plasma in the crust of a neutron star. As it propagates to the star’s surface, the radius of the beam remains constant. The electric currents generated by the wave beam on the star’s surface are also calculated. __________ Translated from Astrofizika, Vol. 50, No. 4, pp. 547–556 (November 2007).     

Complicated magnetic field structure of the star HD 45583

Preliminary data on the magnetic field structure of the unique magnetic star HD 45583 are obtained. The observational data are well described by a configuration of two magnetic dipoles located on opposite sides relative to the star’s center, with their axes directed roughly in a radial direction. The positive monopoles are closer to the surface and the negative, closer to the star’s center. For this reason, there appear to be two positive magnetic poles on the star’s surface but no negative poles. The need for further observations of this unique object is pointed out. Translated from Astrofizika, Vol. 52, No. 1, pp. 127–133 (February 2009).     

Butterfly diagrams of strongly spotted late type stars

At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region, φ₀, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφ_low/dt are (− 1)–(− 2) deg year⁻¹ in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year⁻¹ for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory.     

Equilibrium configuration of the magnetosphere of a star loaded with accreted magnetized mass

Equilibrium configuration of the magnetosphere of a star loaded by the gravitationally accreted plasma having its own magnetic field is investigated. Axisymmetry around the star’s magnetic axis is assumed for simplicity. It is seen that two distinct configurations appear for the cases of parallel and antiparallel magnetic field of the accreted plasma with respect to the star’s magnetic moment. If the external field is antiparallel to the star’s magnetic moment, the stellar magnetosphere is confined within a spherical region surrounded by the external field with a separatric surface between them. This is an extension of the case of the spherical accretion of non-magnetic plasma dealt with thus far in connection with the mass accretion by the degenerate stars in X-ray binaries. It is noticed that the mass slides down along the field lines to the point closest to the star and is stratified hydrostatically in equilibrium to form a disk in the equatorial plane. The mass loading compresses the sphere as a whole in this case. If, on the other hand, the external field is parallel to the star’s magnetic moment, there appears a ring of magnetic neutral point in the equatorial plane. Polar field is open and extends to infinity while the low-latitude field is closed and faces the external field of opposite polarity across the neutral point. The increase of the loaded mass in this case causes a shrink of the closed field region, and the open polar flux is increased. Therefore, the transition between equilibria with small and large amount of the loaded mass requires the reconnection of magnetic lines of force, and the reconnection of the flux through the magnetic neutral ring is proposed as the mechanism of the steady or the intermittent mass leakage like the ones postulated for some X-ray bursters. Visiting Scientist supported by the Japan Society for the Promotion of Sciences.     

Long-Term Starspot Activity of the Eclipsing Variable System CG Cyg

Photometric observations of the eclipsing variable system CG Cyg (G9+K3, P=0 ^d .63), which belongs to a group of short-period, chromospherically active RS CVn type stars with the same type of activity as the sun, have been made over two seasons in 2003 and 2004. The spotting of the star’s surface is modelled using the original observations together with photometric data published since 1965. In every season the spots were concentrated near the equator and covered up to 18% of the star’s entire surface, while the temperature difference between the quiescent photosphere and the spotted regions was 2100 K. Cyclical variations in the total area and average latitude of the spots, as well as flip-flop of the active longitudes, are suspected. Latitudinal drift of the spots during a cycle and differential rotation of the star are discovered. The following orbital elements are obtained: M₁ = 0.93 M⊙, M₂ = 0.81 M⊙, R₁ = 1.01 R⊙, and R₂ = 0.82 R⊙.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 349–363 (August 2005).     

Pulsar radio emission

The theory of pulsar radio emission has been developed in a series of our papers since 1992. It was shown that pulsar radio emission is produced in the lower part of a channel of open magnetic field lines, in a region with a height h ≈ 1.1-10⁷ μ ₃₀ ^1/3 /P^4/21 cm above a magnetic cap of the neutron star (P is the pulsar’s period and μ is the star’s magnetic moment). Here, owing to vigorously occurring processes (the production of photons of curvature radiation and their annihilation into e⁺e^- pairs), two ultrarelativistic particle fluxes are formed: an electron flux moving upward and a positron flux falling onto the star’s magnetic cap. These main fluxes are accompanied by narrow strips of positron and electron fluxes of relatively low energy, the curvature emission from which is a strong coherent radio source. The present paper is a review of earlier papers, and important additions and refinements are also made. Equations are offered for the radio luminosity of a pulsar, the solid angle of the radio beam, and the magnetic moment and moment of inertia of the pulsar’s neutron star. Translated from Astrofizika, Vol. 43, No. 1, pp. 147-169, January–March, 2000.     

Spectroscopic Observations of the Ee Cep Eclipse in 2003

High-resolution spectroscopy during the eclipse of EE Cep was obtained and presented for the first time. The star’s spectroscopic behaviour can be roughly interpreted as a partial eclipse of the high luminosity Be primary and its emitting gaseous ring by the semi-transparent gaseous envelope around an invisible, opaque secondary, most probably a dark disk. This paper is based on spectroscopic observations from Asiago (Italy), DDO (Canada), Rozhen (Bulgaria), SPM (Mexico), Terskol (Russia) and Torun (Poland) observatories.     

Statistical Assessment of Photospheric Magnetic Features in Imminent Solar Flare Predictions

In this study we use the ordinal logistic regression method to establish a prediction model, which estimates the probability for each solar active region to produce X-, M-, or C-class flares during the next 1-day time period. The three predictive parameters are (1) the total unsigned magnetic flux T _flux, which is a measure of an active region’s size, (2) the length of the strong-gradient neutral line L _gnl, which describes the global nonpotentiality of an active region, and (3) the total magnetic dissipation E _diss, which is another proxy of an active region’s nonpotentiality. These parameters are all derived from SOHO MDI magnetograms. The ordinal response variable is the different level of solar flare magnitude. By analyzing 174 active regions, L _gnl is proven to be the most powerful predictor, if only one predictor is chosen. Compared with the current prediction methods used by the Solar Monitor at the Solar Data Analysis Center (SDAC) and NOAA’s Space Weather Prediction Center (SWPC), the ordinal logistic model using L _gnl, T _flux, and E _diss as predictors demonstrated its automatic functionality, simplicity, and fairly high prediction accuracy. To our knowledge, this is the first time the ordinal logistic regression model has been used in solar physics to predict solar flares.     

Variations in brightness and polarization of UX Draconis

Photoelectric observations of the brightness and polarization of the carbon star UX Dra in 1989–1993 are presented and discussed. The strong variations in this star’s period and the impossibility of determining it from our observations forced us to use an extrapolation of Vetešnik’s periodvariation curve. As in Vetesnik’s observations, our determinations of UX Dra’s brightness are satisfied far better by a period twice as long. The variations in UX Dra’s brightness and polarization parameters resemble most closely those of an RV Tau star, in our opinion. Translated from Astrofizika, Vol. 41. No. 4, pp. 561–568, October–December, 1998.     

A magnetic mechanism for halting inward protoplanet migration: I. Necessary conditions and angular momentum transfer timescales

A magnetic torque associated with the magnetic field linking a giant, gaseous protoplanet to its host pre-main-sequence star can halt inward protoplanet migration. This torque results from a toroidal magnetic field generated from the star’s poloidal (dipole) field by the twisting differential motion between the star’s rotation and the protoplanet’s revolution. Outside the corotation radius, where a protoplanet orbits slower than its host star spins, this torque transfers angular momentum from the star to the protoplanet, halting inward migration. Necessary conditions for angular momentum transfer include the requirement that the Alfvén speed v _A in the region magnetically linking a protoplanet to its host star exceeds the protoplanet’s orbital speed v _K . In addition, the timescale for Ohmic dissipation τ _D must exceed the protoplanet’s orbital period P to ensure that the protoplanet is magnetically coupled to its host star. For a Jupiter-mass protoplanet orbiting a solar-mass pre-main-sequence star, v _A >v _K and τ _D >P only when the migrating protoplanet approaches within about 0.1 AU of its host star, primarily because of the rapid drop in the strength of the magnetic field with increasing distance from the central star. Because of this restricted reach, inwardly migrating gaseous protoplanets can be expected to “pile up” very close to their central stars, as is indeed observed for extrasolar planets. The characteristic timescale required for a magnetic torque to transfer angular momentum outward from a more rapidly spinning central star to a magnetically coupled protoplanet is found to be comparable to planet-forming disk lifetimes and protoplanet migration timescales.     

UBVR photometry of gas-darkened variable systems. EM Cep

Photoelectric UBVR light curves of the gas-darkened variable EM Cep are given. A flare was detected on November 15–16, 1991: while a brightness increase was observed in the R band, a decrease in the star’s U luminosity was observed. Translated from Astrofizika, Vol. 42, No. 4, pp. 531–535, October–December, 1999.     

Rotation of a two-component model neutron star in the GTR

Equations are obtained for the dynamics of the rotation of a two-component model neutron star within the framework of the generai theory of relativity. It is shown that for steady rotation of the star’s normal component, Ω_c = const, the angular velocity Ω_s of the superfluid component depends on the coordinates and is Ω_c + ω, where ω is the nondiagonal component of the metric tensor. Translated from Astrofizika, Vol. 40, No. 3, pp. 403–412, August, 1997.     

The reclassification of the suspected R coronae borealis star SY hyi as a semiregular variable

SY Hyi is classified as a suspected R Coronae Borealis-type variable star. Photometric and spectroscopic observations of SY Hyi lead to the star’s reclassification as a semiregular variable of spectral type M5-6.     

Polarimetric and photometric observations of the star m cephei

The results of photometric and polarimetric observations of the star Μ Cep at Byurakan Observatory are presented. Some interesting correlations between the parameters of the star’s brightness variation and the degree of polarization of the light are obtained. It is suggested that the recorded rapid changes in the degree of polarization may result from Μ Cep being a double star. Translated from Astrofizika, Vol. 43, No. 2, pp. 219-228, April–June, 2000.     

Are Sakurai’s object and Nova Aquilae 1919 recurrent outbursts of classical novae?

It is proposed to interpret the outbursts of Nova Aquilae 1919 (V605 Aql) and the nova-like outburst of Sakurai’s object in 1996 as recurrent outbursts of classical novae of the DQ Her and RR Pic types with recurrence periods of about 1100 and 50,000 years, respectively, and some characteristics of these stars are derived on that basis. Only now has V605 Aql returned to the quiet state, and it has an absolute visual magnitude no fainter than 8^m−9^m. The orbital inclination of the prenova’s binary system is about 90°. The absolute visual magnitude of the prenova of V4334 Sgr is no brighter than 3^m. It is quite possible that the orbital inclination of the prenova’s binary system is close to 0°. The expansion velocities of the shells are 30–40 km/sec in both cases. The structure of the ejection is conserved in the recurrent outbursts. Translated from Astrofizika, Vol. 42, No. 4, pp. 563–570, October–December, 1999.     

Accretion magnetar in the close binary system 4U 2206+54

The magneto-rotational evolution of a neutron star in the massive binary system 4U 2206+54 is discussed in light of the recent discovery of its 5555 s rotational period and its average rate of spin-down. We show that this behavior of the neutron star means that its magnetic field exceeds the quantum mechanical critical limit and it is an accretion magnetar. The system’s evolution is explained by wind driven mass transfer without formation of an accretion disk. The constant character of the x-ray source indicates a steady rate of accretion and raises anew the question of the stability of the boundary of the magnetosphere of a star undergoing spherical accretion. A solution to this problem is also a key to determining the mechanism for the slowing down of the star’s rotation.     

Spin-driven changes in neutron star magnetic fields

Interactions among a neutron star’s superfluid neutrons, superconducting protons, and solid crust cause predictable spin-down and spin-up driven crustal motion and magnetic field changes. Applications and unsolved problems are discussed for millisecond pulsar evolution and properties, glitches and post-glitch responses, and transcient gamma-ray emission.     

Nonstationarity of the <Emphasis Type="Italic">α</Emphasis> CYG atmosphere: IV. Some peculiarities of time variation in line profiles

We use 240 CCD spectra taken in 1998–2000 with the coude echelle spectrograph of the 2-m telescope of the National Academy of Sciences of Azerbaijan to study temporal radial velocity and line profile variations of the ion, HeI, and Hβ lines in the spectrumof the α Cyg supergiant. We demonstrate that these variations are caused by pulsation-type motions in the star’s atmosphere. Ion and HeI lines oscillate in the main fundamental mode with a period of about 12.0 ± 0.5 ^d and an amplitude of 5.0 ± 0.5 km/s. These ion-line oscillations continue for about 35 days. Then the difference between the radial velocities of strong and weak ion lines results in a gradual decay of oscillations over a time interval of about 5.0 ± 1.0 ^d . Thereafter the process repeats itself. For the Hβ line we found two significant periods, two amplitudes, and three characteristic radial velocity variability behaviors for the blue and red halves of the absorption profile: with equal variability parameters (period P and amplitude A); with equal P and A, but with a phase shift between the radial velocity variations of the blue and red halves of the absorption profile; with different P and A for the two halves of the absorption profile. The star’s center of mass radial velocity as inferred from the γ-velocity is −4.5 ± 0.5 km/s. The average expansion velocity of the atmospheric layers, where the Hβ line forms, amounts to about −16.5 ± 0.5 km/s and varies temporally with an amplitude of about 3.0 km/s.     

Starspot and flare activity of the dwarf system CM Dra

The spotting activity of the dwarf system CM Dra (dM4.5+dM4.5) is analyzed using new photometric observations taken in the spring of 2005 using the multichannel photometer and 70-cm telescope at the Astronomical Observatory of the Urals State University. A light curve constructed for this system revealed a rotational brightness modulation of low amplitude, 0^m.016, owing to cold spots analogous to those on the sun. The longitude of maximum spotting is found to be 263°±4° perpendicular to the line between the centers of the components. A comparison with our earlier observations in 1996-1997, as well as with published data, shows that during different epochs a spotting maximum is observed in the hemisphere of the principal component which faces the secondary component and is shifted by 30° relative to the line between the centers along the direction of rotation of the star. This may indicate a role for tidal effects in creating the magnetic activity of CM Dra. Grouping of flares in time is observed, even for long-term observations. This appears to be related to the passage of large active regions across the star’s disk and may indicate a possible cyclical activity of CM Dra.     

On the progenitors of core-collapse supernovae

Theory holds that a star born with an initial mass between about 8 and 140 times the mass of the Sun will end its life through the catastrophic gravitational collapse of its iron core to a neutron star or black hole. This core collapse process is thought to usually be accompanied by the ejection of the star’s envelope as a supernova. This established theory is now being tested observationally, with over three dozen core-collapse supernovae having had the properties of their progenitor stars directly measured through the examination of high-resolution images taken prior to the explosion. Here I review what has been learned from these studies and briefly examine the potential impact on stellar evolution theory, the existence of “failed supernovae”, and our understanding of the core-collapse explosion mechanism.