Disk wind in young binaries and its impact on the infrared excesses of young stars

We consider the effect of binarity of young stars on the spectral energy distribution of the IR radiation from circumstellar dust. The formation of a common dust envelope in a binary system with a low-mass secondary component is strongly affected by the disk wind from the secondary. The small velocities in peripheral areas of the wind are such that it can be partially or entirely captured by the primary, even when the distance between the components is up to several astronomical units. As a result, an envelope with a rather complex spatial and kinematic structure is formed. Its mass is many orders of magnitude smaller than that of the accretion disk around the binary. However, the thermal radiation emitted by dust particles of the envelope can be comparable to the total radiation of the accretion disk. This result is discussed in the context of the deficit at near-IR wavelengths (2–10µm) in current models for accretion disks around young stars.     

Enhancement of surface helium abundance in intermediate-mass main-sequence stars

The evolution of rapidly rotating 8, 4, and 2 M _⊙ main-sequence stars is considered together with hydrodynamical transfer in their interiors. The conditions under which turbulent erosion, semiconvection, and shear turbulence lead to partial mixing of the matter in the radiative envelope and central regions of the stars are determined. The enhancement of the surface helium abundance with time depends on both the intensity of partial mixing in their interiors and mass loss by the stellar wind. The ratio of the number densities of helium and hydrogen at the surface can rise by the end of main-sequence stage by ～30% for a 8 M _⊙ star and ～10?20% for a 4 M _⊙ star, depending on the mass-loss rate. Partial mixing of the matter in the radiative envelope and in the central region of the star can provide an explanation for the observed enhancement of the atmospheric helium abundances of early B stars toward the end of their main-sequence evolution. The enhancement of the surface helium abundance in a 2 M _⊙ star is so small that it cannot be detected, and is appreciably lower than the enhancement beneath the surface.     

Collision-induced absorption of radiation in the atmospheres of late-type stars

Problems associated with taking into account absorption induced by collisions between hydrogen and helium atoms, helium atoms and hydrogen molecules, and hydrogen molecules, resulting in the formation of short-lived, quasi-molecular complexes are discussed, together with opacity in the atmospheres of late-type stars due to such absorption. There is good agreement between such opacities computed using codes developed by the author and by R. Kurucz. To demonstrate the importance of including collision-induced opacity, theoretical fluxes are compared to the observed spectral energy distribution of the metal-poor L subdwarf SDSS J125637.13-022452.4. The spectral energy distribution of this object can be reproduced with an effective temperature of T_eff = 2600 K only if collision-induced absorption is taken into account.     

Gravitational radiation during coalescence of neutron stars

The coalescence of components of a binary star with equal masses (M ₁ = M ₂ = M _⊙) and moving in circular orbits is considered. The equation of state for degenerate neutrons is used, leading to the equation of state for an ideal gas. The initial model has zero temperature, corresponding to a polytrope with n = 1.5. To reduce the required computational time, the initial close binary is constructed using the self-consistent field method. The computations use Newtonian gas dynamics, but the back reaction of the gravitational radiation is taken into account in a PN2.5 post-Newton approximation, obtained using ADM formalism. This makes it possible to apply previous experienceof constructing high-order Godunov-type difference schemes, which are suitable for end-to-end calculations of discontinuous solutions of the gas-dynamics equations on a fixed Eulerian grid. The Poisson equations were solved using an original spherical-function expansion method. The 3D computations yielded the parameters of the gravitational signal. Near the radiation maximum, the strain amplitude is rh ～ 4 × 10⁴ cm, the power maximum is 4 × 10⁵⁴ erg/s, and the typical radiation frequency is ?1 kHz. The energy carried away by gravitational waves is ?10⁵² erg. These parameters are of interest, since they form an inherent part of a rotational mechanism for the supernova explosion. They are also of interest for the planning of gravitational-wave detection experiments.     

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.     

Shocks at the bases of non-relativistic jets from young stars

A cylindrical magneto-hydrodynamical model for the transverse structures of the nonrelativisitic jets observed from young stars is proposed. The importance of the temperature terms in the equations describing one-dimensional cylindrical flows is discussed. It is shown that taking into account heating at an oblique shock at the base of the jet makes it possible to obtain physical parameters of the jet that are in good agreement with observations. In particular, the jet can be confined by an external magnetic field of the order of 10⁻⁶ G at a distance of 100 AU from the rotational axis.     

Meridional circulation in young massive stars with laminar rotation

We study the rotation of a chemically homogeneous star with a mass of 16M_⊙, assuming that the angular-momentum distribution in its radiative envelope is determined by hydrodynamical processes—flows and turbulent diffusion. Meridional circulation and horizontal shear turbulence are the main hydrodynamical processes forming the radial distribution of the angular momentum in young massive stars in the absence of magnetic fields. The rotation of such stars is close to steady-state. The angular velocity of rotation of the convective core can be ～5–20% higher than the surface value. Under these conditions, the characteristic time for the radial transport of angular momentum by meridional flows and shear turbulence is comparable to the nuclear time scale.     

Structure of the circumbinary envelopes of young binary stars with elliptical orbits

The results of two-dimensional gas-dynamical numerical simulations of the structure of matter flows in the envelopes of a number of T Tauri binary systems with elliptical orbits are considered. The main flow elements in inner regions of protoplanetary disks of these stars are described. The influence of shocks on the size of the gap—a rarified region in the inner parts of the protoplanetary disk—is analyzed. A method is proposed for estimating the size of this gap from the numerical simulations, and the gap sizes for the studied stars are determined and compared with observational results. The flow dynamics in the gap is considered, and the periodic variations of the gap size on time scales of several orbital periods are analyzed. Possible observational manifestations of the studied flows are discussed.     

Circumstellar disks around young low-mass stars: Observed properties and lifetime

Unveiling the physical processes leading to planet formation is one of the major challenges of modern astrophysics. Until the last fifteen years, the origin of planetary systems was mostly investigated from observations and modelling of our own Solar System. However, the new generation of astronomical facilities is slowly but surely changing this. Observations of circumstellar disks around young stars similar to the Sun in its infancy are now routinely provided by large astronomical facilities, from the optical wavelengths to the millimeter range. These observations begin to have the sensitivity and angular resolution needed to provide invaluable information inside the regions of disks where planets should form. In this review, I show some new results which illustrate how fast this new domain is evolving and which kind of physical/chemical parameters can be explored. In particular, I present what recent observations tell us about disk lifetime and evolution.I conclude by some open questions which represent important astrophysical challenges for tomorrow in order to understand how planetary systems form.     

Studies of intermediate-mass stellar models using eclipsing binaries

Evolutionary computations for intermediate-mass stars are analyzed using observed parameters for eclipsing SB2 binaries and theoretical parameters based on evolutionary tracks. Modern observations cannot be used to distinguish between models with and without convective overshooting for stars in the vicinity of the main sequence. Statistically significant discrepancies between the observed and computed stellar parameters are associated with systematic errors in photometric effective temperatures. After taking into account systematic effects, the theoretical computations fit the observational data uniformly well throughout the entire mass interval studied. Empirical and semiempirical (i.e., reduced to the ZAMS and with solar elemental abundances) formulas for the mass-luminosity, mass-effective temperature, and mass-radius relations are proposed.     

Influence of mass-loss due to stellar wind on the orbits of binary stars to a second order theory

We examine the influence of mass-loss due to stellar wind using the celestial mechanics of variable mass, and derive first-and second-order solutions, taking into account the decreasing mass due to the stellar wind. The theoretical results show that the semi-major axis exhibits secular and periodic variations in the first-and second-order theory. The orbital eccentricity exhibits periodic, but no secular variation or changes. The longitude of periastron exhibits only periodic variations in the first-order solution, but also secular variations in the second-order solution. The theoretical results are applied to the binary star HD 698, and variability of the orbital elements of this star due to stellar-wind mass loss calculated. Published in Russian in Astronomicheskiĭ Zhurnal, 2008, Vol. 85, No. 10, pp. 896–900. The article was translated by the authors.     

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.     

Cobalt in stars and the galaxy

An analysis of the abundance of cobalt in atmospheres of red giants, indicates they can be divided into two groups: stars with the normal [Co/Fe] abundance and those with a small [Co/Fe] excess. A comparative analysis of the spectrograms taking into account the effect of superfine splitting provides evidence for a [Co/Fe] excess in some stars. We have also detected physical and kinematical differences between these groups. Stars with a [Co/Fe] excess are related to the thick-disk population of the Galaxy. These stars are older and less massive, display lower metallicities, and have Galactic velocities corresponding to those of thick-disk objects. It is suggested that the observed pattern of a [Co/Fe] excess in the halo and thick disk reflects the chemical composition of the Galaxy at a very early stage of its evolution, when Population III objects existed. The lower abundance excess in the thick disk compared to the halo and the absence of an excess in the thin disk are due to the contributiuon from Type I supernovae at later stages of the Galaxy’s evolution. We have found that the thick disk of the Galaxy displays gradients of its cobalt and iron abundances, possibly providing evidence that the thick disk formed as a result of the collapse of a protogalactic cloud.     

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.     

Molecular bands in the spectra of M stars

The profiles of the main molecular bands in the spectral-energy distributions (SEDs) of M stars have been calculated. The calculations of the individual band profiles were performed using the just-overlapping-lines approximation. Information about the oscillator strengths and the sources of the spectroscopic data for specific transitions between electronic levels of molecules is provided. The calculations of theoretical SEDs for M stars were performed using available lists of molecular lines for sources of bound-bound opacity in the atmospheres of oxygen-sequence stars. The observed SEDs of the oxygen-sequence red giant HD 148783 (30 Her) and the M dwarf 2MASS J22424129?2659272 are reproduced. The dependence of the calculated SEDs of the M giant on the adopted metallicity and carbon abundance is studied. The observed SEDs of HD 148783 and 2MASS J22424129?2659272 are described well by theoretical spectra calculated for model atmospheres with T _eff/log g/[Fe/H] = 3250/ ? 0.4/0 and 3000/5.0/0, respectively.     

Kinematics of stars in the Pleiades open cluster

A master catalog of stellar proper motions in the field of the Pleiades young open cluster has been created based on data adopted from 12 catalogs. A total of 340 likely cluster members are identified, 80% with linear-velocity components in the plane of the sky estimated to better than 0.7 km/s. The average radial and tangential motions are analyzed. No rotation of the cluster or expansion or compression of the cluster as a whole or of its parts was found. The dispersion of the residual velocities does not depend on the masses of the stars, as is demonstrated by an absence of mass segregation in the spatial distribution of the stars in the cluster. Most of the stellar orbits in the cluster are strongly elongated.     

Thermal instability in the envelopes of Wolf-Rayet stars

Astronomy Reports - We briefly consider thermal instabilities developing in the expanding envelopes of Wolf-Rayet stars and their possible implications. Due to the specific physical conditions in...     

Variability of stars in the Pulkovo spectrophotometric catalog

We present the results of a statistical study of brightness variability for 693 stars of the Pulkovo spectrophotometric database in five spectral bands in the range λλ 320–1080 nm. Significant brightness variations were detected in at least one spectral band against the background of the random noise for one-third of the stars not earlier believed to be variable. A comparison of the distributions of these variations in amplitude and spectral band for the normal and variable stars shows that variability is inherent to most stars to some extent and is often wavelength dependent.     

Chemical composition of stars in the galactic halo

The chemical compositions of the atmospheres of six metal-poor stars are analyzed. Spectra with signal-to-noise ratios of no less than 100 and a resolution of R≈17 000 were obtained using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na, K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-process elements (Y, Ba) are derived. The star G251-54 ([Fe/H]=?1.55, T _eff=5541 K, logg=3.58) is deficient in some elements compared to both stars with similar metallicities and the Sun. The atmosphere of G251-54 has the following elemental abundances relative to iron: [O/Fe]=+0.47, [α/Fe]≈?0.3, [Na/Fe]=?0.60, [Sc/Fe]=?0.57, [Cr, Ni, Fe]≈0, [Zn/Fe]=+0.16, [Cu/Fe]=?0.66, [Y/Fe]=?0.70, and [Ba/Fe]=?1.35. The remaining five stars have metallicities in the range ?1.6<[Fe/H]相似文献   

Role of climate changes for wind gap formation in a young,actively growing mountain range

Wind gaps in actively growing mountain ranges are unique geomorphological features testifying to the competition between tectonics and fluvial incision. Although it is clear that these landforms reflect the defeat of rivers during sustained rock uplift, the role of climate changes in their formation has never been explored. Here, we use a coupled tectonics–landscape evolution model to show that temporal changes in precipitation rate exert an important control on wind gap formation. In models with a constant precipitation rate, rivers flowing across a growing range are either defeated at an early stage or they abandon their valleys very late, if at all. If precipitation varies, wind gaps form mostly c. 100–200 ka after a transition to drier conditions because of sediment aggradation upstream of the range. Our results suggest that the Pliocene–Quaternary aridification of Central Asia contributed to wind gap formation in active mountain ranges in the foreland of northeastern Tibet.