Nonlinear radial pulsations of Wolf-Rayet stars

The evolution of Population I stars with initial masses 60 M _⊙ ≤ M _ZAMS ≤ 120 M _⊙ is computed up to the Wolf-Rayet stage, when the central helium abundance decreases to Y _c ≈ 0.05. Several models from evolutionary sequences in the core helium-burning stage were used as initial conditions when solving the equations of radiative hydrodynamics for self-exciting stellar radial pulsations. The low-density envelope surrounding the compact core during the core helium burning is unstable against radial oscillations in a wide range of effective temperatures extending to T _eff ～ 10⁵ K. The e-folding time of the amplitude growth is comparable to the dynamical time scale of the star, and, when the instability ceases growing, the radial displacement of the outer layers is comparable to the stellar radius. Evolutionary changes of the stellar radius and luminosity are accompanied by a decrease in the amplitude of radial pulsations, but, at the effective temperature T _eff ≈ 10⁵ K, the stellar oscillations are still nonlinear, with a maximum expansion velocity of the outer layers of about one-third the local escape velocity. The period of the radial oscillations decreases from 9 hr to 4 min as stellar mass decreases from M = 28 M _⊙ to M = 6 M _⊙ in the course of evolution. The nonlinear oscillations lead to a substantial increase of the radii of the Lagrangian mass zones compared to their equilibrium radii throughout the instability region. The instability of Wolf-Rayet stars against radial oscillations is due to the action of the κ mechanism in the iron-group ionization zone, which has a temperature of T ～ 2 × 10⁵ K.     

Distribution of inhomogeneities in the interstellar plasma in the directions of three distant pulsars from observations with the RadioAstron ground–space interferometer

The RadioAstron ground–space interferometer has been used to measure the angular sizes of the scattering disks of the three distant pulsars B1641–45, B1749–28, and B1933+16. The observations were carried out with the participation of the Westerbork Synthesis Radio Telescope; two 32-m telescopes at Torun, Poland and Svetloe, Russia (the latter being one antenna of the KVAZAR network); the Saint Croix VLBA antenna; the Arecibo radio telescope; the Parkes, Narrabri (ATCA), Mopra, Hobart, and Ceduna Australian radio telescopes; and the Hartebeesthoek radio telescope in South Africa. The full widths at half maximum of the scattering disks were 27 mas at 1668 MHz for B1641–45, 0.5 mas at 1668 MHz for B1749–28, and 12.3 at 316 MHz and 0.84 mas at 1668 MHz for B1933+16. The characteristic time scales for scatter-broadening of the pulses on inhomogeneities in the interstellar plasma τ_sc were also measured for these pulsars using various methods. Joint knowledge of the size of the scattering disk and the scatter-broadening time scale enables estimation of the distance to the effective scattering screen d. For B1641–45, d = 3.0 kpc for a distance to the pulsar D = 4.9 kpc, and for B1749–28, d = 0.95 kpc for D = 1.3 kpc. Observations of B1933+16 were carried out simultaneously at 316 and 1668 MHz. The positions of the screen derived using the measurements at the two frequencies agree: d ₁ = 2.6 and d ₂ = 2.7 kpc, for a distance to the pulsar of 3.7 kpc. Two screens were detected for this pulsar from an analysis of parabolic arcs in the secondary dynamic spectrum at 1668 MHz, at 1.3 and 3.1 kpc. The scattering screens for two of the pulsars are identified with real physical objects located along the lines of sight toward the pulsars: G339.1–04 (B1641–45) and G0.55–0.85 (B1749–28).     

On the effect of the coastal zone wind regime on the optical characteristics of haze

Based on an experimental study of directed dissipation coefficients and aerosol microstructure in the coastal area the problem of wind regime manifestation is considered. It is shown that the main changes of parameters of dissipation indicatrix form near angles =15°–165°, which are conditioned by submicron fractions of aerosol particles, take place without regard for wind regime. Usually they are defined by effect of relative air humidity. It is found that specific manifestations of wind regime are observed close to small dissipation angles and are conditioned by the effect of coarse-dispersed aerosol fractions.UDK 551.593     

Simulation of orographically generated waves in a nonhydrostatic model of an adiabatic atmosphere

Results of numerical experiments on the simulation of a flow moving around an isolated mountain are presented. The influence of the sizes of a barrier and of the flow velocity on characteristics of wave oscillations is discussed. All calculations are carried out with the authors’ two-dimensional (in the vertical plane) version of a nonhydrostatic dynamic scheme, in which equations of the dry quasi-incompressible atmosphere are solved with a semi-implicit semi-Lagrangian method. This method uses large time steps as compared to explicit-implicit Eurlerian methods. The results of calculations agree with results obtained by other authors, which gives hope for finding physically correct solutions in the simulation of nonhydrostatic processes in the atmosphere.     

Multiscale Global Atmosphere Model SL-AV: the Results of Medium-range Weather Forecasts

Development of the multiscale version of the global atmosphere model SL-AV required many improvements in the dynamical core, replacement or refinement of parameterization algorithms and complex tuning of the model. These modifications were initially tested with the experiments on modern climate simulation and then incorporated into the model configuration for medium-range numerical weather prediction. The impact of these model improvements on forecast quality is studied in this paper. The increase in accuracy of model climate characteristics has led to the reduction of forecast errors. The comparison of quality for numerical forecasts starting from the initial data of Hydrometcenter of Russia and ECMWF is carried out. The effect of replacing the initial data turned out to be comparable to the effect of multi-year works on model development. This shows the importance and necessity of development and improvement of the Hydrometcenter of Russia data assimilation system.     

On the potentiality of using the fluorescence of humic substances for the determination of hydrological structures in coastal sea waters and in inland water basins

This paper describes the potentialities of the characteristics of the fluorescence of humic substances (HS), namely, their intensity normalized to the Raman band intensity of water and the shape of their fluorescence band, as indications of the hydrological structures of the surface water layer in marine coastal areas (using the example of the North Sea region near the East Frisian Islands and the estuary zone of the Weser River) and in inland water basins (using the example of the Rybinsk Reservoir). We demonstrate that the HS fluorescence band??s shape parameters allow stricter discrimination of the dominant water masses as compared to the admixtures?? concentration-related indicators (the salinity, electric conductivity, and HS fluorescence intensity).     

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.     

Raman scattering and fluorescence spectra of water from the sea surface microlayer

The Raman scattering and fluorescence spectra were first obtained for the water of sea surface microlayers (SML) of 1 and 0.2 mm thickness (sampled with a Garrett net and a Lapshin capillary sampler, respectively). For reference, samples of water below the SML (from the layer of 0.5 m) were also taken. Substantial differences were found for the values of the normalized intensity of the fluorescence (the number of photons from the volume unit in response to a unit of excitation) between the aqueous humic substances and, presumably, oil hydrocarbons and proteins. Some slight but analyzable differences in the shapes and location of spectral bands were also found. These latter allow one to determine the content of salts and the characteristics of complicated organic compounds in the SML and to compare them to those within the water volume.     

The results of gravity surveys at geophysical grounds in the Kaluga region

The scientific educational practice on gravimetry for the second-year students of the geophysical specialty in 2012–2013 was executed in Central Russia (Kaluga region, the Village of Aleksandrovka). It allowed us to provide an educational process with modern high-precision gravimeters and to fully preserve the curricula and to add elements of topography and geodesic practice. Over these 2 years reliable facts and data were received, which illustrated the possibilities of high-precision gravity measurements in studying the sedimentary cover features of a geological structure.