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The paper presents the results of an analysis of observations of an eruptive prominence on the MFS and HSFA2 spectrographs of the Ondřejov Observatory (Astronomical Institute, Czech Republic) in the hydrogen, helium, and calcium lines. After spectral processing, the integral radiation fluxes in the lines were determined and the physical parameters of the plasma were calculated theoretically using a model in the absence of local thermodynamic equilibrium. Comparison of the observed and calculated values showed that the observed radiation fluxes in the lines can be explained in a model of stationary gas radiation taking into account the opacity in the spectral lines. To calculate the theoretical fluxes, in some cases, it was necessary to introduce radiation from several layers with different temperatures and heights. The calculated radiation fluxes agree with the observed ones to within 10%. As a result of the simulation, the main parameters of the plasma of the prominence were obtained: temperature, concentration, etc. The values of the radiation fluxes in the spectral lines are evidence of inhomogeneity of the emitting gas, and there may be regions next to each other with temperatures differing by an order of magnitude. [ABSTRACT FROM AUTHOR]

The emission from the gas behind the front of a stationary shock wave is calculated for the conditions in the atmospheres of red dwarf stars for velocities u0 from 30 to 100 km/s. Energy exchange between the electron and atom-ion components is taken into account through elastic collisions, free-free, bound-bound, and bound-free collisional and radiative transitions of hydrogen in the radiation field of a star's photosphere. Cooling by the following chemical elements is included: C, N, O, Mg, Si, S, Ca, and Fe. The following results are obtained: the post-shock gas remains transparent in the optical range of the continuum throughout the emission time; hence, it cannot be a source of the black-body radiation that appears at times during flares. The recombination and bremsstrahlung radiation of the transparent gas, as well as the flux in the Balmer series lines represent a few percent of the energy flux of matter through the viscous jump. The ratio of the fluxes in the spectrum lines and the continuum depends on u0 and on the parameters of the atmosphere. These results are consistent with the idea of multicomponent emission in flares, specifically, line emission is determined by the shock wave in layers above the photosphere, while black-body radiation comes from the photosphere heated by a flux of suprathermal particles. [ABSTRACT FROM AUTHOR]

The effect of the number K of atomic hydrogen levels taken into account on the cooling of the gas behind a shock front is studied. The calculations are done for the conditions in the atmospheres of long-period Mira Ceti type variables. K ranges from 2 to 25. The electron temperature Te(t; K) and ionization state x(r,K) asymptotically approach limiting functions Te(t) and x(t) that are independent of K. After the maximum electron temperature is reached, a partial equilibrium phase sets in, during which the populations of the highly excited discrete levels with principal quantum numbers ≥ 8 obey the Saha equation for the instantaneous electron temperature and density. [ABSTRACT FROM AUTHOR]

Using a two-component model for the structure of the circumstellar nebulae of WR and O stars, we predict that their emission may be variable on time scales from several months to several years. This variability should be manifest as an enhancement of the emission luminosities of separate compact regions in the hydrogen Balmer lines, HeII ion line, and nebular lines of the ions [OII], [OIII], [NeIII], and [NeIV]. In the model considered, this line variability reflects non-stationary ionization, cooling, and recombination behind a shock propagating inward in the cloud. The set of lines that are variable depends on the shock velocity, while the scale of the variability decreases with increasing cloud density. [ABSTRACT FROM AUTHOR]

We use a two-phase model for the structure of the circumstellar nebulae of hot stars to analyze the radiative cooling of a dense, compact cloud behind the shock produced by the compression of the cloud by hot gas from the stellar wind, taking into account ionization and heating by radiation from the central star. We can distinguish three stages of the evolution of the cloud during its compression. In the first stage, relevant for the entire cloud before compression and the gas ahead of the shock front, the state of the gas is determined purely by ionization by the stellar radiation. The next stage is characterized by the simultaneous action of two gas excitation mechanisms—photoionization by the stellar radiation and shock heating. In this stage, the gas intensively radiates thermal energy received at the shock front. After radiative cooling, in the final stage, ionization and heating of the gas are again determined mainly by the star. To compute the spectrum of the cloud radiation, we solved for the propagation of a plane-parallel, homogeneous flux through the shock front in the radiation field of the hot star. The computations show that a combination of two excitation mechanisms considerably enriches the theoretical spectrum. The relative intensities of emission lines of a single cloud may resemble either those for an HII region or of a supernova remnant. [ABSTRACT FROM AUTHOR]

We suggest a model that explains the stratification peculiarities of the [O III] and H α line emission from some of the ring nebulae around Wolf-Rayet stars. These peculiarities lie in the fact that the [O III] line emission regions are farther from the central star than the H α regions, with the distance between them reaching several tenths of a parsec. We show that the radiative shock produced by a Wolf-Rayet stellar wind and propagating with a velocity of ∼100 km s−1 cannot explain such large distances between these regions due to the low velocity of the gas outflow from the shock front. The suggested model takes into account the fact that the shock produced by a Wolf-Rayet stellar wind propagates in a two-phase medium: a rarefied medium and dense compact clouds. The gas downstream of a fast shock traveling in a rarefied gas compresses the clouds. Slow radiative shocks are generated in the clouds; these shocks heat the latter to temperatures at which ions of doubly ionized oxygen are formed. The clouds cool down, radiating in the lines of this ion, to temperatures at which Balmer line emission begins. The distance between the [O III] and H α line emission regions is determined by the cooling time of the clouds downstream of the slow shock and by the velocity of the fast shock. Using the ring nebula NGC 6888 as an example, we show that the gas downstream of the fast shock must be at the phase of adiabatic expansion rather than deceleration with radiative cooling, as assumed previously. [ABSTRACT FROM AUTHOR]

We propose a model explaining the presence of vast regions of partially ionized gas in the interstellar medium. The circumstellar envelope of a hot star absorbs soft ionizing radiation, but transmits an appreciable fraction of the hard photons, which are absorbed much more weakly than photons with energies close to the ionization limit. For this reason, the radiation attenuated by the envelope becomes harder, and can penetrate to larger distances. For stars hotter than 50 000 K, the transition zone between the ionized and neutral gas can extend to tens or hundreds of parsecs. Thus, a region of partially ionized hydrogen, with a small gradient of the degree of ionization without a well-defined inner HII zone, can form in the interstellar medium. [ABSTRACT FROM AUTHOR]

A number of features are detected outside the nebula NGC 6888, within 1.2° (30 pc) of the star WR 136, which can be explained in a two-phase stellar-wind model. These include regions with fine filamentary gas structure that do not contain sources of stellar wind, extended radial “streams,” ultra-compact HII regions with high-velocity gas motions, and high-velocity gas motions outside the envelope of NGC 6888. The two-phase wind consists of a rarefied component and dense compact condensations, or “bullets.” The bullets generate cylindrical shocks in the interstellar gas, resulting in the presence of high-velocity gas up 20–30 pc from the star, outside the cavity formed by the rarified component of the wind.© 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We describe the formation of carbon dust in binary systems with hot components as a result of the collisions of clouds in a two-phase stellar-wind model. Calculations are made for the well studied system WR 140. The collisions lead to the formation of composite clouds and shock waves, with the temperature at the shock front equal to about 3 × 10[sup 8] K along both sides of the interface boundary. During isobaric deexcitation to (0.5-0.7) × 10[sup 4] K, the cloud density increases by a factor of several thousand; its thickness in the direction of the shock decreases by the same factor. After deexcitation, the hydrogen inside the composite cloud is in its atomic state, while the carbon remains ionized. The deexcitation is followed by expansion of the cloud, which moves away from both stars. During the first 10[sup 6] s, its thickness remains relatively small, so that the expansion is one-dimensional. The radiation field inside the cloud decays, resulting in the recombination of the carbon. Further expansion of the cloud leads to adiabatic cooling, and the formation of dust particles becomes possible. After the dimensions of the cloud have become roughly the same in all directions, its expansion is isotropic, so that it becomes transparent within approximately 10[sup 6] s, and the dust is heated to (1.0-1.4) × 10[sup 3] K, observed as an IR "flare." The time required for the cloud to move from the exciting star and heat the dust is comparable to the observed delay in the increased IR emission relative to the time of periastron. .. [ABSTRACT FROM AUTHOR]

A model with interacting two-phase winds, each consisting of rarified gas and dense, compact clouds, is proposed to explain the weakness of the variability of the X-ray emission of the binary system HD 193793. The flowing together of the rarified components, collisions of clouds, and the motion of the clouds in the rarified component of the other wind are considered. Computations of the interactions of the rarified components were carried out using the method of Chernyı for standard, two-dimensional, gas–dynamical problems, taking into account differences in the ion and electron temperatures. Due to the adiabatic expansion of the rarified gas behind the shock front, the X-ray luminosity decreases at 1/r, where r is the distance between the stars in the system. The gas in the dense clouds radiates, leading, on the contrary, to a gradual growth in their luminosity in the transition from periastron to apoastron, approximately proportional to r[sup 1/3]. The joint action of these two factors could give rise to the observed nearly constant luminosity of the system at 2–5 and 2–6 keV. The contribution of the clouds exceeds that of the rarified component by a substantial factor. © 2000 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We present a model in which the nonthermal radio emission of binary systems containing Wolf-Rayet and O components is due to collisions between clouds belonging to dense phases of the wind of each star. The relativistic electrons are generated during the propagation of fast shock waves through the clouds and their subsequent de-excitation. The initial injection of superthermal particles is due to photoionization of the de-excited cold gas by hard radiation from the shock front. Therefore, the injection takes place in cloud regions fairly far from the front. Further, the superthermal electrons are accelerated by the betatron mechanism to relativistic energies during the isobaric compression of the cloud material, when most of the gas radiates its energy. Collisions between the clouds can occur far beyond the contact boundary between the rarefied wind components. Thus, the model avoids the problem of strong low-frequency absorption of the radiation. [ABSTRACT FROM AUTHOR]