The nonthermal radio emission of WR 140 in a model with colliding two-phase winds

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.     

Thermal stability of wind near a hot star

We present computations of the ionization and thermal balance in the wind from WR and OB stars. The star’s radiation field is modeled using a Planck function with a temperature from 2 to 6 eV. We assume that the principal contribution from gas heating and cooling is from ions and atoms of hydrogen, carbon, nitrogen, oxygen, and neon. We take into account photoionization from the ground state and excited states, ionization by electron collisions, radiative and two-electron recombination, and excitation and de-excitation of discrete levels by the star’s dilute radiation and electron collisions. It is demonstrated that the thermal regime of gas in the conditions typical of the winds near WR and OB stars is stable.     

Calculation of profiles of the CIV 1550 doublet formed in an accretion shock in a T Tauri star: Axially symmetric radial accretion

We have calculated profiles for the CIV 1550 doublet arising in an accretion shock in a T Tauri star assuming that (i) the accretion zone at the stellar surface is axially symmetric (a circular spot or spherical belt), (ii) the velocity and density of the gas in front of the shock do not vary within the accretion zone, and (iii) the gas falls radially inward toward the star. The calculated CIV 1550 profiles differ qualitatively from those observed in the spectra of T Tauri stars, probably because the velocity of the infalling gas in T Tauri stars has a tangential component of some tens of km/s due to the nonradial magnetic field near the stellar surface.     

Close binaries containing Supermassive Black Holes

We consider the evolution of binary systems formed by a Supermassive Black Hole (SMBH) residing in the center of a galaxy or a globular cluster and a star in its immediate vicinity. The star is assumed to fill its Roche lobe, and the SMBH accretes primarily the matter of this star. The evolution of such a system is mainly determined by the same processes as for an ordinary binary. The main differences are that the donor star is irradiated by hard radiation emitted during accretion onto the SMBH; in a detached system, nearly all the donor wind is captured by the black hole, which strongly affects the evolution of the semi-major axis; it is not possible for companions of the most massive SMBHs to fill their Roche lobes, since the corresponding orbital separations are smaller than the radius of the last stable orbit in the gravitational field of the SMBH. Moreover, there may not be efficient exchange between the orbital angular momentum and the angular momentum of the overflowing matter in such systems. Our computations assumed that, if the characteristic timescale for mass transfer is smaller than the thermal timescale of the star, no momentum exchange occurs. Absorption of incident external radiation in the stellar envelope was treated using the same formalism that was used when computing the radiative transfer in the stellar atmosphere. Numerical simulations show that Roche-lobe overflow is possible for a broad range of initial system parameters. The evolution of semi-detached systems containing a star and a SMBH nearly always ends with the dynamical disruption of the star. Stars with masses close to the solar mass are destroyed immediately after they fill their Roche lobes. During the accretion of matter of disrupted stars, the SMBH can achieve quasar luminosities. If the SMBH accretes ambient gas as well as gas stripped from stars, the star is subject to additional radiation in the detached phase of its evolution, strengthening its stellar wind. This leads to an increase of the semi-major axis and subsequent decrease of the probability of Roche-lobe overflow during the subsequent evolution of the system.     

Dust formation in binaries with OB and WR components in a two-phase stellar-wind model

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 3×10⁸ K along both sides of the interface boundary. During isobaric deexcitation to (0.5–0.7)×10⁴ 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 10⁶ 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 10⁶ s, and the dust is heated to (1.0–1.4)×10³ K, observed as an IR “lare.” 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.     

Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature

We have studied the crystallization sequences, mineral chemistries, and textures that develop when an average Type B Ca-Al-rich inclusion composition is cooled in air from 1275–1580° C to below 1000°C at rates between 0.5 and 1000°C/hr. Crystallization sequences, the textures of all the major phases, pyroxene chemistry, and melilite zoning patterns are functions of both the cooling rate and the temperature from which cooling begins. Determination of the order of pyroxene and plagioclase crystallization has been identified as an important goal for petrographic studies of CAIs because it can be used to set constraints on the cooling rate experienced by an individual inclusion. Overall textures plus melilite zoning patterns and pyroxene chemistry can give important clues as to whether pyroxene or plagioclase began to crystallize first. Melilite texture and chemistry appear to yield the most valuable information on the maximum temperature to which an inclusion was raised prior to cooling.Comparison of our experimental results with petrographic observations of Type B CAIs suggests that most inclusions were partially melted and then cooled at rates on the order of a few tenths to tens of degrees per hour. Maximum temperatures of about 1400°C appear most likely for intermediate Type B Allende inclusions. Our results do not support the suggestion that the textures observed in these inclusions formed by crystallization of supercooled, metastable melt droplets condensed from nebular gas. The slow cooling rates we infer for CAIs are difficult to reconcile with models for their origin that imply simple radiative cooling of individual molten or partially molten droplets in a cold, low density environment. On the other hand, cooling rates of the nebular cloud are believed to have been much slower than those we have inferred for Type B CAIs. Scenarios that could be reconciled with the thermal history that we have inferred include drag heating of particles falling through nebular gas, heating by intense radiation (e.g., via flares) from the early sun, heating in nebular shock fronts, or other thermal heterogeneities in the early nebula allowing time scales for cooling (and heating) of CAIs much shorter than those for the nebular cloud as a whole. Successful models for the origin of Type B CAIs must account for the fact that most Type B CAIs cooled relatively slowly from a partially molten state.     

Coronal Mass Ejection Effect on Envelopes of Hot Jupiters

Abstract—Currently, hot Jupiters have extended gaseous (ionospheric) envelopes extending far beyond the Roche lobe. The envelopes are loosely bound to the planet and are subject to a strong influence by stellar wind fluctuations. Since hot Jupiters are close to the parent star, the magnetic field of the stellar wind is an important factor which defines the structure of their magnetospheres. For a typical hot Jupiter, the velocity of stellar wind plasma flowing around the atmosphere is close to the Alfvén velocity. Thus, fluctuations of the stellar wind parameters (density, velocity, magnetic field) can affect conditions for the formation of the bow shock around a hot Jupiter, such as transforming the flow from sub-Alfvén to super-Alfvén regime and back. The study results of three-dimensional numerical MHD simulations confirm that, in a hot Jupiter’s envelope located near the Alfvén point of the stellar wind, both the disappearance and appearance of a detached shock can occur under the influence of a coronal mass ejection. The study also shows that this process can affect the observational manifestations of a hot Jupiter, including the radiation flux in the spectrum’s hard region.     

The dust envelope of the symbiotic nova HM Sge

We have studied the brightness and color variations of the symbiotic nova HM Sge based on longterm UBVRJHKLM photometry of the star and data on its energy distribution in the middle infrared (7.7–22.7 µm) obtained with the low-resolution spectrometers of the IRAS satellite and ISO orbital observatory. We have also calculated models for the steady-state, spherically symmetrical, extended dust envelope of the star for two extreme heating cases: heating only by radiation from the cool component of the system and by the combined radiation from both components. Model fitting to the IRAS and ISO data indicates that models with a single, central Mira-type source are more appropriate. This indicates that the radiation of the hot component is largely processed by the surrounding gas, and does not substantially affect the infrared spectrum of the symbiotic nova directly. The mean spectral energy distribution based on 1983 IRAS data differs appreciably from the ISO spectrum obtained on October 1, 1996. The observed evolution of the envelope spectrum probably reflects an increase of the density and decrease of the temperature of the dust grains near the inner boundary of the envelope, related to a decrease of the luminosity and increase of the temperature of the hot component. We estimate the total mass-loss rate, velocity of gas expansion at the outer envelope boundary, and upper limit for the mass of the central source of radiation.     

北极地区地面辐射量和云辐射强迫特征

In this paper the characteristics of surface radiative fluxes and cloud-radiative forcing are reviewed with a focus on the Arctic. Three aspects are addressed, including (i) changes in radiation flux over the global surface; (ii) characteristics of surface fluxes in the Arctic; and (iii) characteristics of cloud-radiative forcing in the Arctic. The clouds not only significantly reduce the peak summer radiative heating of the surface but also reduce the wintertime radiative cooling at the surface in　higher latitudes. The downward longwave fluxes dominates the incident radiative fluxes in the Arctic during most of the year. Incoming shortwave fluxes are negligible during late fall, winter and early spring, and even during the midsummer the incoming shortwave fluxes are only slightly greater than the downward longwave fluxes. The total net surface radiative flux is negative for most of the year and only positive during midsummer in the Arctic. The global net cloud-radiative forcing is negative, but the cloud-radiative forcing is positive in the Arctic, showing a warming effect, except for a short period in mid-summer. Positive cloud-radiative forcing in the Arctic is attributed　to the presence of snow and ice with high albedo and the absence of solar radiation during the polar night.     

Retardation of angular velocity of a rotating star due to mass loss

Variations of angular velocity of a rotating star on the upper main sequence due to mass loss driven by various mechanisms, like radiation, corpuscule ejection, and stellar wind, are examined. Expressions for the variations of angular velocity are derived by considering a model of a rotating star. The theoretical results show that the angular velocity decreaseswith time due tomass loss. The obtained results are applied to a hot fast-rotating star V1182 Aql (O9 V) and to Y Cyg (B0 V).     

The role of metals in cooling gas behind shock fronts in the atmospheres of cool stars

The influence of various chemical elements on radiative cooling of the gas flowing from a viscous jump is investigated in a model with a stationary shock in the atmosphere of a cool star. A closed system of equations is written for the thermal energy per heavy particle, the electron temperature, and the relative concentrations of elements in all ionization states. In addition to hydrogen and helium, atomic, singly ionized, and doubly ionized carbon, nitrogen, oxygen, sodium, magnesium, aluminum, silicon, sulfur, potassium, calcium, and iron are included, assuming they have their normal cosmic abundances. The high optical depth in Lyman-series lines leads to a return of the thermal energy to electrons via secondary collisions. As a result, the contribution of hydrogen to the cooling rate falls to the level of the contribution of metals, mainly carbon, magnesium, and iron. Thus, such shock models are able to explain the presence of bright metal lines in the spectra of cool and solar-type stars.     

The influence of variations of the elemental composition on the thermal properties of gas behind shock fronts

The homogenization of inhomogeneities in the elemental composition of the interstellar medium due to stellar evolution and weak mixing are inevitably related to the action of shocks. This paper considers the influence of variations in the elemental composition on the thermal and ionizational evolution of a collisional gas with the solar metallicity that is cooled behind a shock front with a velocity of 50–120 km/s. The intensities of lines of heavy elements in plasma cooling behind a shock front depend not only on variations in the elemental composition, but also on the shock velocity, due to the different values of the critical density for the transition to the equilibrium level populations in atoms and ions of heavy elements. This circumstance can be used to determine the elemental composition of cool and warm gas of the interstellar medium, as well as the thermal history of the gas.     

Accretion of neutral hydrogen and helium onto a neutron star. The X-ray and gamma-ray radiation of cool neutron stars

The accretion of neutral gas (hydrogen and helium) onto a neutron star is studied. The gas is gravitationally captured into the magnetosphere of the star, where it is ionized by thermal radiation from the stellar surface and accelerated by the electric field at the light cylinder and in a tube of open magnetic lines. Particles accelerated at light cylinder generate gamma-ray, some particles move to the star and heat its polar regions, resulting in the emission of X-rays. Our calculations of the model parameters of the X-ray and gamma-ray radiation indicate that the radiation intensities should be sufficient to be observed.     

The origin of giant molecular clouds

The parameters of molecular clouds formed via the Parker instability with dominant radiative losses are estimated. In this scenario, the cloud parameters (such as their mean densities and masses) should depend on galactocentric radius. This dependence is determined mainly by radial variations of the gas metallicity and the flux of heating radiation in the Galaxy. Due to the development of the interchange mode of the Parker instability, the angular momentum of the clouds will not necessarily be parallel to the galactic rotation axis.     

Infrared emission and the destruction of dust in HII regions

The generation of infrared (IR) radiation and the observed IR-intensity distribution at wavelengths of 8, 24, and 100 µm in the ionized hydrogen region around a young, massive star is investigated. The evolution of the HII region is treated using a self-consistent chemical-dynamical model in which three dust populations are included—large silicate grains, small graphite grains, and polycyclic, aromatic hydrocarbons (PAHs). A radiative transfer model taking into account stochastic heating of small grains and macromolecules is used to model the IR spectral energy distribution. The computational results are compared with Spitzer and Herschel observations of the RCW 120 nebula. The contributions of collisions with gas particles and the radiation field of the star to stochastic heating of small grains are investigated. It is shown that a model with a homogeneous PAH content cannot reproduce the ring-like IR-intensity distribution at 8 µm. A model in which PAHs are destroyed by ultraviolet radiation of the star, generating region HII, provides a means to explain this intensity distribution. This model is in agreement with observations for realistic characteristic destruction times for the PAHs.     

The nature of peculiar stellar complexes

The nature of stellar complexes with peculiar populations and morphologies is investigated. The existence in the LMC of complexes made up of isolated stars, on the one hand, and consisting exclusively of clusters, on the other hand, could be due to different turbulence patterns in the initial gaseous medium. Arc-shaped stellar complexes are unlikely to be the result of star formation in a gaseous shell swept up by a central source of pressure, and instead probably reflect the shape of a bow shock that develops when a sufficiently dense cloud is subject to dynamical pressure. A peculiar arc-shaped complex in NGC 6946, which contains a young, massive cluster, may be the result of an oblique infall of a high-velocity cloud onto a region of the gaseous disk of the Galaxy with a strong, regular magnetic field; the properties of this complex can be explained as the result of a collision of the resulting shocks. The arc-shaped complexes in the LMC were also probably produced by high-velocity clouds moving obliquely through the more tenuous gas of the LMC disk. A similar complex in NGC 300 may owe its origin to the effect produced on a dense cloud by the shock from an extremely powerful external explosion, whose stellar remnant may have survived as an X-ray source now located along the line of symmetry of the arc of the complex. The rareness of such structures can be explained by the narrow range of conditions under which they can develop.     

On Possible Types of Magnetospheres of Hot Jupiters

As a rule, the orbits of “hot Jupiter” exoplanets are located close to the Alfven point of the stellar wind of the host star. Many hot Jupiters could be in the sub-Alfven zone, where the magnetic pressure of the stellar wind exceeds the dynamical pressure. Therefore, the magnetic field in the wind should play an extremely important role in the process of stellar wind flowing around the atmosphere of a hot Jupiter. This must be taken into account when constructing theoretical models and interpreting observational data. Analyses show that many typical hot Jupiters should have shockless induced magnetospheres, which have no analogs in the solar system. Such magnetospheres are characterized first and foremost by the fact that there is no bow shock, and the magnetic barrier (ionopause) is formed by induced currents in upper layers of the ionosphere. This conclusion is confirmed here using three-dimensional numerical simulations of the flow of the stellar wind from the host star around the hot Jupiter HD 209458b, taking into account both the intrinsic magnetic field of the planet and the magnetic field in the wind.

     

VLBI of supernovae and gamma-ray bursts

Supernovae and gamma-ray bursts (GRBs) are among the brightest events in the universe. Excluding Type Ia supernovae and short GRBs, they are the result of the core collapse of a massive star with material being ejectedwith speeds of several 1000 km/s to nearly the speed of light, and with a neutron star or a black hole left over as the compact remnant of the explosion. Synchrotron radiation in the radio is generated in a shell when the ejecta interact with the surrounding medium and possibly also in the central region near the compact remnant itself. VLBI has allowed resolving some of these sources and monitoring their expansion in detail, thereby revealing characteristics of the dying star, the explosion, the expanding shock front, and the expected compact remnant. We report on updates of some of the most interesting results that have been obtained with VLBI so far. Movies of supernovae are available from our website. They show the evolution from shortly after the explosion to decades thereafter, in one case revealing an emerging compact central source, which may be associated with shock interaction near the explosion center or with the stellar corpse itself, a neutron star or a black hole.     

Circumstellar envelopes of hot stars: Formation of extended regions of weakly ionized gas

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.     

Hydrodynamical modeling of mass transfer in the close binary system <Emphasis Type="Italic">β</Emphasis> Lyr

We present results of two-dimensional hydrodynamical simulations of mass transfer in the close binary system β Lyr for various radii of the accreting star and coefficients describing the interaction of the gaseous flow and the main component (primary). We take the stellar wind of the donor star into account and consider various assumptions about the radiative cooling of the gaseous flow. Our calculations show that the initial radius of the flow corresponding to our adopted mass-transfer rate through the inner Lagrange point (L₁) of (1–4) × 10^?5M_⊙/yr is large: 0.22–0.29 (in units of the orbital separation). In all the models, the secondary loses mass through both the inner and outer (L₁ and L₂) Lagrange points, which makes the mass transfer in the system nonconservative. Calculations for various values of the primary radius show a strong dependence on the coefficient fv that models the flow-primary interaction. When the radius of the primary is 0.5, there is a strong interaction between the gas flow from L1 and the flow reflected from the primary surface. For other values of the primary radius (0.1 and 0.2), the flow does not interact directly with the primary. The flow passes close to the primary and forms an accretion disk whose size is comparable to that of the Roche lobe and a dense circum-binary envelope surrounding both the disk and the binary components. The density in the disk varies from 10¹² to 10¹⁴ cm^?3, and is 10¹⁰–10¹² cm^?3 in the circum-binary envelope. The temperature in the accretion disk ranges from 30000 to 120000 K, while that in the circum-binary envelope is 4000–18000 K. When radiative cooling is taken into account explicitly, the calculations reveal the presence of a spiral shock in the accretion disk. The stellar wind blowing from the secondary strongly interacts with the accretion disk, circum-binary envelope, and flow from L₂. When radiative cooling is taken into account explicitly, this wind disrupts the accretion disk.