Spots and activity of solar-type stars from Kepler observations

The spot coverages S for 2846 solar-type stars with effective temperatures from 5700 K to 5800 K and gravities from 4.4 to 4.5 have been measured. An analysis based on the MAST catalog, which presents photometric measurements obtained with the Kepler Space Telescope during Q9 is presented. The existence of two groups of solar-type stars, with S values between 0.001 and 0.007 and with S > 0.007, is inferred. The second group (active stars) contains 279 stars (about 10% of the total number of stars analyzed). The mean S parameter for the entire sample is 0.004, comparable to the mean spot coverage of the Sun. In general, the dependence of S on the rotation period for solar-type stars has characteristics similar to those found earlier for stars with exoplanets. For the vast majority of the stars in the sample, the activity is constant, and independent of age. The activity of the small number of active stars with S > 0.007 decreases with age. The age variations of the chromospheric activity index R′_HK are compared to variations of the spot coverage S. The relations analyzed have common characteristic features. It is likely that both the spot activity level and the chromospheric activity level abruptly decrease for stars older than 4 billion yrs.     

Generation of large-scale magnetic fields in young solar-type stars

Differential-rotation and dynamo models are computed for a young, solar-mass star at the initial stage of the formation of its radiative core. It is argued that the global magnetic fields in the radiative zones of the contemporary Sun and similar stars are due to the action of a hydromagnetic dynamo at early evolutionary stages. Our computations suggest that this field should be nonaxisymmetric. Physical reasons for departures from axial symmetry are discussed in detail. It is suggested that nonaxisymmetric relic fields are responsible for the phenomenon of active longitudes.     

Non-LTE analysis of the formation of EuII lines in the atmospheres of solar-type stars

A method to analyze the statistical equilibrium of the EuII ion based on a 36-level model atom has been developed. The formation of EuII lines without assuming local thermodynamic equilibrium (LTE) is considered for T _eff=5500–7000 K, logg=4.0, and metallicities [A] from 0 to ?1.5. Non-LTE effects in the level populations are primarily due to radiative pumping of excited states from the ground and low-lying levels, which leads to over-population of upper relative to lower levels. As a result, the studied λ4129 and λ6645 Å lines are weaker than in the LTE case. However, due to the small energy differences between even low-lying EuII levels, collisional coupling is strong, and deviations from LTE in EuII lines are modest: for the Sun, non-LTE corrections to the abundance are only 0.04 dex. The non-LTE effects grow with an increase in the effective temperature and with a decrease in the metallicity, so that non-LTE abundance corrections can reach 0.12 dex for T _eff=5500K, logg=4.0, [A]=?1.5 and 0.1 dex for T _eff=7000K, logg=4.0, [A]=0. The effect of inaccuracy in the atomic parameters for EuII on the non-LTE calculations is examined. Analysis of the profiles of the solar EuII λ4129 and λ6645 Å lines is used to empirically refine estimates of the efficiency of collisional processes in forbidden transitions in establishing the distribution of EuII ions over excited states.     

Three-dimensional gas-dynamical modeling of changes in the flow structure during the transition from the quiescent to the active state in symbiotic stars

The results of three-dimensional modeling of the flow structure in the classical symbiotic system Z Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady-burning range. Therefore, in order to realize the transition from a quiescent to an active state, it is necessary to find a mechanism capable of sufficiently increasing the accretion rate on the time scales typical for outburst development. Our calculations provide support for a mechanism for the transition from quiescence to outburst in classical symbiotic systems suggested earlier based on two-dimensional calculations (Bisikalo et al., 2002). Our results show that an accretion disk forms in the system for a wind velocity of 20 km s^?1. The accretion rate for the solution with the disk is ～22.5–25% of the mass-loss rate of the donor, which is ～4.5?5 × 10^?8M_⊙ yr^?1 for Z And. This value is in agreement with the steady-burning range for the white-dwarf masses usually accepted for this system. When the wind velocity increases from 20 to 30 km s^?1, the accretion disk is destroyed and the disk material falls onto the accretor surface. This process is followed by an approximately twofold jump in the accretion rate. The resulting growth in the accretion rate is sufficient so as to exceed the upper limit of the steady-burning range, thus bringing the system into an active state. The time during which the accretion rate is above the steady-burning value is in very good agreement with observations. Our analysis leads us to conclude that small variations in the donor wind velocity can lead to the transition from disk accretion to wind accretion and, as a consequence, to the transition from a quiescent to an active state in classical symbiotic stars.     

On the reconstruction of the surface structure of the spotted stars

We have developed and tested a light-curve inversion technique for photometric mapping of spotted stars. The surface of a spotted star is partitioned into small area elements, over which a search is carried out for the intensity distribution providing the best agreement between the observed and model light curves within a specified uncertainty. We have tested mapping techniques based on the use of both a single light curve and several light curves obtained in different photometric bands. Surface reconstruction artifacts due to the ill-posed nature of the problem have been identified.     

The structure of shocks in the atmospheres of pulsating stars

The structure of shocks propagating through partially ionized hydrogen gas with characteristics typical of the atmospheres of RR Lyr, W Vir, and RV Tau type variables is analyzed in terms of a self-consistent solution of the equations of gas dynamics, atomic kinetics, and radiation transfer. The solutions were obtained for shock waves with velocities 20 km/s≤U ₁≤90 km/s and unperturbed hydrogen gas with temperatures 3000 K≤T ₁≤9000 K and density ρ₁=10^?10 g/cm³. The fraction of the energy of the gas-dynamic flux converted into radiation increases with the shock amplitude, and the ratio of the radiation flux emitted by the shock to the gas kinetic energy flux is 0.4???0.8 for the velocities U ₁ considered. This ratio also increases slightly with the ambient gas temperature T ₁ due to an increase in hydrogen ionization in the radiative precursor. The flux emitted by the leading edge of the shock opposite to the gas flow is several percent higher than the flux emitted in the opposite direction by the trailing edge of the shock. Radiation is mostly concentrated in the Balmer continuum, and the region of efficient Lyman radiation transfer includes gas layers located near the viscous jump (δX=±10⁴ cm). The final gas-compression ratio in units of the limiting compression corresponding to an isothermal approximation is virtually independent of the shock amplitude, and increases with the unperturbed gas temperature from r≈0.5 at T ₁=3000 K to r≈0.9 at T ₁=9000 K.     

The character of solar-type activity and the depth of the convective zone

The origin of solar-type activity for low-mass stars of late spectral types is considered. Spectroscopic data were used to study the dependence of the activity level logR′ _HK on the lithium abundance logA(Li) and axial rotation rate. A close correlation between logA(Li) and logR′ _HK is found for two groups of G stars, hotter and cooler than the Sun. This relation is most clearly expressed in the case of high activity, and is somewhat more strongly expressed for G6-K3 dwarfs, which includes many BY Dra variables, than for F8-G5 stars. It is confirmed that, for stars with high activity, both the lithium abundance and the activity level are determined by the rotation rate, which depends on the age. The lithium abundance exhibits different dependences on the chromospheric activity, depending on the level of this activity. Cooler stars, with detectable lithium and solar-like chromospheres, possess much stronger coronas. This change in the relationship between the relative luminosities of the chromosphere and corona can be reliably traced using larger datasets. The different ratios between the activity of the choromosphere and corona for cooler and hotter G stars may reflect the fact that their convective zones become deeper or shallower than some critical value. This is consistent with observations of parameters describing rotational modulation and the correlation and anti-correlation of chromospheric and photospheric activity indices for stars hotter and cooler than the Sun. Physically, this means that the character of the activity could be related to a changing contribution of the large-scale and local magnetic fields to the generation of the activity. The results of this study confirm the earlier idea that there may be different evolution paths associated with solar-type activity. The results can be used to refine methods for estimating ages of stars from their activity levels (gyrochronology).     

A model for the population of helium stars in the Galaxy: Low-mass stars

We model the Galactic ensemble of helium stars using population synthesis techniques, assuming that all helium stars are formed in binaries. In this picture, single helium stars are produced by mergers of helium remnants of the components of close binaries (mainly, the merging of helium white dwarfs) or in the disruption of binaries with helium components during supernova explosions. The estimated total birthrate of helium stars in the Galaxy is 0.043 yr^?1; the total number is 4 × 10⁶; and the binarity rate is 76%. We construct a subsample of low-mass (M_He ? 2M_⊙) helium stars defined by observational selection effects: the limiting magnitude (V_He ≤ 16), ratio of the magnitudes of the components in binaries (V_He ≤ V_comp), and lower limit for the semiamplitude of the radial velocity required for detecting binarity (K_min = 30 km s^?1). The parameters of this subsample are in satisfactory agreement with observations of helium subdwarfs. In particular, the binarity rate in the selection-limited sample is 58%. We analyze the relations between the orbital periods and masses of helium subdwarfs and their companions in systems with various combinations of components. We predict that the overwhelming majority (～97%) of unobserved companions to helium stars will be white dwarfs, predominantly, carbon-oxygen white dwarfs.     

Equation of state of Fe3+-bearing phase-X

We investigated the high-pressure behaviour of Fe³⁺-bearing hydrous phase-X, (K_1.307Na_0.015)(Mg_1.504Fe _0.373 ³⁺ Al_0.053Ti _0.004 ⁴⁺ )Si₂O₇H_0.36, up to 34?GPa at room temperature by synchrotron X-ray powder diffraction. The lattice parameters behave anisotropically, with the [001] direction stiffer than [100]. In the 10^?4 to 22?GPa pressure range, the axial bulk moduli are K _0a ?=?112(3) GPa and K′?=?4, and K _0c ?=?158(2) GPa and K′?=?4, and the anisotropy of the lattice parameters is β_0c :β_0a ?=?0.71:1. The cell volumes are fitted by a second-order Birch–Murnaghan equation of state giving a bulk modulus of K ₀?=?127(1) GPa and K′?=?4 in the same pressure range. After 22?GPa, a discontinuity in volume and lattice parameters can be recognized. Sample did not become amorphous up to 34?GPa. The coupled substitution K?+?Mg?=?[]?+?Fe³⁺ has only a limited influence on the bulk modulus and structural stability of phase-X.     

Equation of state and melting point studies of forsterite

We report in this paper results of lattice dynamical calculations of the thermodynamic properties, namely, the equation of state and the melting point of forsterite. The root mean square displacements of atoms and thermal parameters are also evaluated. It is observed from our results, that, a modified Lindemann criterion for melting may be proposed for an ionic ‘molecular’ solid like forsterite. Agreement among various theoretical and experimental results is satisfactory indicating that lattice dynamical studies based on microscopic quasiharmonic formulations help in understanding the macroscopic properties of forsterite, over a wide range of temperature and pressure.     

Equation of state for NaCl-H2O: comparison with mineral dehydration equilibria

 A previously published equation of state is examined with respect to validity of predicted fugacity values for comparison with mineral reaction equilibria. It is found that values of f _H2O should be valid for the range from 0.0 to 0.3 and probably to 0.4 in mole fraction NaCl for temperatures from 300° to 600° C and less accurately to 900° C. Comparison is made with existing experimental results near 630° C for the dehydration of brucite; the agreement is excellent. Comparison with recent results at 850° C from a different experiment is also satisfactory. However, fugacity of NaCl is not given on an absolute basis, even in this range below x _NaCl=0.4. Received: 8 October 1994 / Accepted: 11 May 1995     

Physico-chemical models of the internal structure of partially differentiated Titan

We analyze models of the internal structure of Titan, a large icy satellite of the Saturn system. Calculations are carried out using information on the mass, mean density, moment of inertia, orbital parameters, and elastic properties of the satellite obtained by the Cassini–Huygens mission, as well as geochemical data on the composition of chondrite materials, equations of state of water and ices I, III, V, VI, and VII, and thermodynamic models for conductive heat transfer in the outer icy crust and of global convection in the interior zones of the satellite. The analysis of the models shows that models of partially differentiated Titan are most consistent; they include an outer water–ice shell, an intermediate ice–rock mantle, and an inner rock–iron core. It is shown that for the models of this type the maximum thickness of the water–ice shell is 460–470 km; it can be composed of an outer conductive crust of Ih ice 80–110 km thick and a subsurface water ocean 200–300 km deep. The maximum radius of the central rock–iron core of Titan can reach ~1300 km. The thickness of Titan’s ice–rock mantle does not exceed 2100 km at a density of 1.22–2.64 g/cm³. The model of partially differentiated Titan is feasible in the moment of inertia range of 0.312 < I/MR ² < ~0.350.     

Changes in the internal spatial structure of post-communist Prague*

The article overviews the most important changes in the internal urban structure of Prague since 1989. Post-communist urban development has been influenced by government-directed reforms of political and economic system, internationalisation and globalisation, public policies favouring unregulated market development, economic restructuring in terms of deindustrialisation and growth of producer services, and increasing social differentiation. The three most transparent processes of urban change in Prague have been (1) commercialisation of the historical core; (2) revitalisation of some inner city neighbourhoods; and (3) residential and commercial suburbanisation in the outer city. This revised version was published online in July 2006 with corrections to the Cover Date.     

Morphology and internal structure of sandwaves in the Bay of Fundy

Intertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274-1.275 mm; velocity strength index (V¹)0.46-3.34; and velocity symmetry index (V²) 0.011-0.294), and they vary from symmetrical to strongly asymmetrical in cross-section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi-equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non-deposition or erosion and slowed or reversed migration (trough at brink). Dominant-current cross-bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross-Bedding—decimetre-scale cross-beds separated by gently inclined (9° average) erosional surfaces; and Large-Scale Foresets—cross-beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross-Bedded Cosets, that consists of nearly equal volumes of dominant- and subordinate-current cross-beds stacked without a preferred set-boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature-related mass-transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation.     

Equation of state of water based on speeds of sound measured in the diamond-anvil cell

Measurements of the speed of sound in water have been extended to 5.5 GPa and 400°C. The data were taken in the diamond-anvil cell using Impulsive Stimulated Scattering (Forced Rayleigh Scattering) at acoustic frequencies of ∼1 GHz. Measured sound speeds differ significantly from those given by previously published equations of state; at 5 GPa, measured speeds are 3% lower than those predicted by the 1995 formulation of the International Association for the Properties of Water and Steam (IAPWS). New thermal diffusivity measurements to 1 GPa are combined with previously reported thermal conductivities in a determination of specific heat. A complete equation of state (giving all thermodynamic quantities) was generated and subsequently extended to 40 GPa and 3000 K. This equation of state matches both our speed of sound data and previously published shock wave data; in a range extending to 3 GPa and 700-1600°C predicted densities are up to 5% higher than those reported from studies of inclusions, leading to a reinterpretation of those previous studies. Within the range of the new data, an experimentally determined estimate of uncertainty is given for the densities.     

The internal structure of the Arosa Zone (Swiss-Austrian Alps)

The Arosa Zone, part of the main Alpine suture zone between the Austroalpine and the Penninic realms, forms a heterogeneous unit composed of rocks of oceanic and continental origin. It exhibits mélange character due to minor sedimentary mixing and local penetrative tectonic deformation during Cretaceous and Early Tertiary imbrication. Competent blocks of both Austroalpine and Penninic origin, covering up to 2.5 km², are embedded in incompetent serpentinitic or shaly-calcareous matrix. On a mesoscale, disrupted strata occur in and adjacent to thrust and shear zones. Contrasting competence between blocks and matrix partitioned deformation into brittle and ductile processes. Extension veins and shear fractures affected the competent strata whereas the matrix developed a penetrative foliation during ductile flow and accommodated high strain. Flow was mainly non-coaxial in the matrix, and coaxial extension prevailed in the blocks.In a regional tectonic setting, we define the Arosa Zone as the tectonostratigraphic unit sandwiched between the Austroalpine and Penninic units. It forms a narrow and highly imbricated zone containing both South Penninic ophiolitic and sedimentary rocks as well as blocks and slices of Austroalpine origin.

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Zusammenfassung Die Arosa Zone ist Teil der Alpinen Suturzone zwischen Ostalpin und Pennimkum und bildet eine lithologisch heterogene Einheit aus kontinentalen und ozeanischen Gesteinen. Sie zeigt den Charakter einer Melange, die aus, untergeordnet, sedimentärer Durchmischung und lokaler, aber penetrativer tektonischer Deformation während kretazisch und alttertiärer Tektonik resultiert. Kompetente Blöcke ostalpiner und penninischer Herkunft, die bis zu 3 km im Streichen verfolgbar sind, schwimmen in einer inkompetenten serpentinitischen oder tonig-karbonatischen Matrix. Im Mesobereich treten zerbrochene Gesteinsabfolgen in oder direkt an Überschiebungs- und Scherzonen auf. Unterschiedliche Kompetenz zwischen Blöcken und Matrix teilt die Deformation in spröde und duktile Prozesse auf. Die kompetenten Gesteine zeigen Extensionsspalten und Scherbrüche, in der Matrix entwickelte sich durch duktiles Fließen eine penetrative Schieferung bei hoher Strainintensität. Die Deformation in der Matrix war hauptsächlich nicht-koaxial, koaxiale Extension herrschte in den Blöcken.Im regionalen tektonischen Rahmen definieren wir die Arosa Zone als tektonostratigraphische Einheit zwischen Ostalpin und Penninikum. Sie bildet eine schmale, stark imbrikierte Zone die, aus südpenninischen ophiolithischen und sedimentären Gesteinen, sowie aus Blöcken und Spänen ostalpiner Herkunft aufgebaut wird.

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Résumé La Zone d'Arosa, partie de la zone de suture alpine entre l'Austro-alpin et le Pennique, forme une unité hétérogène composée de roches d'origines océanique et continentale. Elle présente le caractère d'un mélange qui résulte d'un brassage sédimentaire mineur, et de déformations tectoniques, locales mais pénétratives, au cours du Crétacé et du Tertiaire ancien. Des blocs compétents de l'Austro-alpin et du Pennique qui couvrent jusqu'à 2.5 km², sont enrobés dans une matrice incompétente serpentineuse ou argilo-calcaire. A moyenne échelle, les couches disloquées se rencontrent dans les zones charriées et cisaillées, ou y sont adjacentes. La différence de compétence entre les blocs et la matrice répartit la déformation en processus cassants et ductiles. Les roches compétentes montrent des fentes d'extension et des fractures de cisaillement, alors que dans la matrice, sous l'action d'un flux ductile et d'une intensité de contrainte élevée, se développe une foliation pénétrative. La déformation fut principalement non coaxiale dans la matrice et coaxiale dans les blocs d'extension.Dans un cadre tectonique régional, nous définissons la Zone d'Arosa comme une unité tectonostratigraphique prise entre l'Austro-alpin et le Pennique. Elle constitue une zone étroite et fortement imbriquée composée aussi bien de roches ophiolitiques du Sud-Pennique et de roches sédimentaires, que de blocs et d'écaillés de l'Austro-alpin.

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The activity of late-type stars: the Sun among stars with cyclic activity

The coronal and chromospheric emission of several hundred late-type stars whose activity was recently detected are analyzed. This confirms the previous conclusion for stars of HK project that there exist three groups of objects: active red M dwarfs, G-K stars with cyclic activity, and stars exhibiting high but irregular activity. The X-ray fluxes, EUV-spectra, and X-ray cycles can be used to study the main property of stellar coronas—the gradual increase in the number of high-temperature (T ≥ 10 MK) regions in the transition from the Sun to cyclically active K dwarfs and more rapidly rotating F and G stars with irregular activity. The level of X-ray emission is closely related to the spottedness of the stellar surface. The correlation between the chromospheric and coronal emission is weak when the cycles are well-defined, but becomes strong when the activity is less regular. Unexpectedly, stars whose chromospheric activity is even lower than that of the Sun are fairly numerous. Common and particular features of solar activity among the activity of other cyclically active stars are discussed. Our analysis suggests a new view of the problem of heating stellar coronas: the coronas of stars with pronounced cycles are probably heated by quasistationary processes in loops, while prolonged nonstationary coronal events are responsible for heating the coronas of F and G stars with high but irregular activity.     

Possible scenarios for the formation of Ap/Bp stars

Possible paths for the formation of Ap/Bp stars—massive main-sequence stars with strong magnetic fields—are analyzed based on modern theories for the evolution of single and binary stars. Assuming that the strong magnetic fields of these stars are the main reason for their comparatively slow axial rotation and the observed anomalies in the chemical compositions of their atmospheres, possible origins for these high magnetic fields are considered. Analysis of several possible scenarios for the formation of these stars leads to the conclusion that their surface magnetic fields are probably generated in the convective envelopes of the precursor stars. These precursors may be young, single stars with masses 1.5–3 M _⊙ that formed at the peripheries of forming star clusters and ended their accretion at the Hayashi boundary, or alternatively close binaries whose components have convective envelopes, whose merger leads to the formation of an Ap/Bp star. Arguments are presented supporting the view that the merger of close binaries is the main channel for the formation of Ap/Bp stars, and a detailed analysis of this scenario is presented. The initial major axes of the merging binary systems must be in the range 6–12 R _⊙, and the masses of their components in the range 0.7–1.5 M _⊙. When the merging components possess developed convective envelopes and fairly strong initial magnetic fields, these can generate powerful magnetic fields “inherited” by the products of the merger—Ap/Bp stars. The reason the components of the close binaries merge is a loss of angular momentum via the magnetic stellar winds of the components.