Wolf-Rayet stars with relativistic companions

The evolution of close binary systems containing Wolf-Rayet (WR) stars and black holes (BHs) is analyzed numerically. Both the stellar wind from the donor star itself and the induced stellar wind due to irradiation of the donor with hard radiation arising during accretion onto the relativistic component are considered. The mass and angular momentum losses due to the stellar wind are also taken into account at phases when the WR star fills its Roche lobe. It is shown that, if a WR star with a mass higher than ～10M _⊙ fills its Roche lobe in an initial evolutionary phase, the donor star will eventually lose contact with the Roche lobe as the binary loses mass and angular momentum via the stellar wind, suggesting that the semi-detached binary will become detached. The star will remain a bright X-ray source, since the stellar wind that is captured by the black hole ensures a near-Eddington accretion rate. If the initial mass of the helium donor is below ～5M _⊙, the donor may only temporarily detach from its Roche lobe. Induced stellar wind plays a significant role in the evolution of binaries containing helium donors with initial masses of ～2M _⊙. We compute the evolution of three observed WR-BH binaries: Cyg X-3, IC 10 X-1, and NGC 300 X-1, as well as the evolution of the SS 433 binary system, which is a progenitor of such systems, under the assumption that this binary will avoid a common-envelope stage in its further evolution, as it does in its current evolutionary phase.     

Partial mixing in early-type main-sequence B stars

Partial mixing of material in the radiative envelopes and convective cores of rotating main sequence stars with masses of 8 and 16 M _⊙ is considered as a function of the inital angular momentum of the stars. Losses of rotational kinetic energy to the generation of shear turbulence in the radiative envelope and the subsequent mixing of material in the envelope are taken into account. With an initial equatorial rotational velocity of 100 km/s, partial mixing develops in the upper part of the layer with variable chemical composition and the lower part of the chemically homogeneous radiative envelope. When the initial equatorial rotational velocity is 150–250 km/s, the joint action of shear turbulence and semi-convection leads to partial mixing in the radiative envelope and central parts of the star. The surface abundance of helium is enhanced, with this effect increasing with the angular momentum of the star. With an initial equatorial rotational velocity of 250 km/s, the ratio of the surface abundances of helium and hydrogen grows by ~30% and ~70% toward the end of the main-sequence evolution of an 8 M _⊙ and 16 M _⊙ star, respectively. The transformation of rotational kinetic energy into the energy of partial mixing increases with the angular momentum of the star, but does not exceed ~2%?3% in the cases considered.     

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.     

Models for highly flattened, rapidly rotating cool stars in a Newtonian approximation

We investigate the physical characteristics of single, rapidly rotating white dwarfs, which could form as a result of a merger of two white dwarfs with different masses and filled Roche lobes, due to the radiation of gravitational waves. When the merging of the binary components occurs without loss of mass and angular momentum, the merger products are subject to secular instability, and the density in their cores does not exceed ～10⁸ g/cm³. Models are constructed for rapidly rotating neutron stars, which could form after the collapse of rotating iron cores of evolved massive stars. Dynamically unstable neutron-star models are characterized by a shift of the maximum density from the rotational axis. The total momentum of such neutron stars is about half the maximum possible momentum for the evolved cores of massive stars.     

Analysis of magnetic activity of the rapidly rotating stars He 373 and AP 225

Spectroscopic and photometric data for the two rapidly rotating members of the α Persei cluster He 373 and AP 225 are analyzed. Improved estimates have been obtained for the projected equatorial rotation velocities: v sin i = 164 km/s for He 323 and v sin i = 129 km/s for AP 225. Multi-band photometric mapping is used to map the spot distributions on the surfaces of the two stars. The fractional spotted areas S and mean temperature difference ΔT between the unspotted photosphere and the spots are estimated (S = 7% and ΔT = 1000 K for He 373; S = 9% and ΔT = 800 K for AP 225). The H _α line profiles of both stars have variable emission components whose widths are used to deduce the presence of extended regions of emission reaching the corotation radius.     

Turbulent entrainment at the boundaries of the convective cores of main-sequence stars

Extra mixing of matter in stellar interiors at the boundaries of the convective cores of mainsequence stars is considered for the first time using the physical model of turbulent entrainment developed by Arnett and collaborators based on three-dimensional hydrodynamical simulations. The model takes into account the energy that goes into mixing the matter of the convective core and layers stable against convection located above the core. It is shown that the extent of the region of extra mixing expressed in units of the pressure scale height is not constant, and decreases as the star evolves along the main sequence. Adequate allowance for extra mixing at the boundaries of convective cores is necessary to clarify the relative importance of different mixing mechanisms in stellar interiors, as well as to determine stellar parameters using asteroseismlogy.     

Partial mixing of material in the radiative envelope and convective core of a rotating main-sequence star with mass 16M ⊙

The evolution of a rotating main-sequence star with a mass of 16M _⊙ is studied. The flow of material in the radiative envelope is turbulent. When the exchange of energy and chemical elements is taken into account, this turbulence transfers helium and momentum from the convective core to the radiative envelope. The helium content at the stellar surface is enhanced toward the end of the evolution on the main sequence by 0.006–0.306, depending on the character of the star’s initial rotation and the horizontal coefficient of the turbulent viscosity.     

Multiplicity of main-sequence turn-offs in globular clusters

We present color-magnitude diagrams of globular clusters for models with self-enrichment and primordial enrichment. The models with self-enrichment turn out to have two or more main-sequence turn-off points in the color-magnitude diagram if the fraction of mass lost by the globular cluster as a result of supernova explosions does not exceed 95–97%. The models with primordial enrichment can have only one main-sequence turn-off point. We argue that the cluster ω Cen evolved according to a self-enrichment scenario.     

The influence of Population III stars on the early evolution of galaxies

Numerical simulations of the chemical evolution of disk galaxies taking into account the influence of Population III stars are considered. The probability that stars with peculiar chemical compositions are present in the solar neigborhood is analyzed, and possibilities for their detection considered. For various assumptions about the slope of the initial mass function for Population III stars and the critical metallicity, the radius surrounding the Sun containing at least one such star is 10–12 pc. Such objects could be studied using modern large telescopes. The influence of Population III stars on the chemical evolution of disk galaxies is investigated. Taking into account the first stars in early stages leads to an earlier onset of chemical enrichment of the ISM and a characteristic chemical composition of the gas, but all traces of this enrichment have disappeared by the current epoch.     

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.     

Evolution of stars with high metallicities

We have calculated evolutionary tracks for stars with high abundances of heavy elements. An abundance increase from the solar level (～0.02) to 0.1 (for ΔY/ΔZ=0–2.4), which corresponds to the central regions of the disk components of giant galaxies, shifts the main sequence towards lower effective temperatures; however, it does not appreciably affect either the luminosity of the stars or their lifetime on the main sequence. Increasing the heavy-element abundance to 0.2 for ΔY/ΔZ=2.4 shifts the main sequence towards higher effective temperatures, appreciably increases the luminosity of the stars, and substantially accelerates their evolution.     

Observational constraints on potassium synthesis during the formation of stars of the Galactic disk

The non-LTE potassium abundances in the atmospheres of 33 Galactic-disk stars are derived and the parameters of the atmospheres of 23 of the stars are determined. Neglecting departures from LTE results in a systematic overestimation of the potassium abundances and an increase in their dispersion, even for differential analyses relative to the Sun. The non-LTE corrections are significant ((?0.2)–(?0.6) dex) and depend on the surface gravities and effective temperatures of the stars. The mean potassium abundance for a sample of ten stars with [Fe/H]～0.0 is in agreement with the solar and meteoritic abundances (log ? _⊙(K)=5.12). As the stellar metallicity increases from [Fe/H]=(?1.0) to (0.2) dex, the [K/Fe] ratio decreases systematically from 0.3 dex to ?0.1 dex. The derived dependence [K/Fe]-[Fe/H] is in agreement with the results of published model calculations of the chemical evolution of the Galaxy. This indicates the dominance of explosive oxygen burning in massive type II supernovae during the synthesis of potassium in the Galactic disk.     

Identification of gamma-ray sources with Wolf-Rayet stars

An analysis of unidentified discrete sources of gamma-rays with energies E>100 MeV demonstrates that the spatial characteristics of this group of gamma-ray sources coincides with those of Wolf-Rayet stars. It is concluded that Wolf-Rayet stars are potential steady sources of high-energy gamma rays with mean luminosities L(>100 MeV)≈10³⁵ erg/s.     

Galactic orbits of selected companions of the Milky Way

High-accuracy absolute proper motions, radial velocities, and distances have now been measured for a number of dwarf-galaxy companions of the Milky Way, making it possible to study their 3D dynamics. Galactic orbits for 11 such galaxies (Fornax, Sagittarius, Ursa Minor, LMC, SMC, Sculptor, Sextans, Carina, Draco, Leo I, Leo II) have been derived using two previously refined models for the Galactic potential with the Navarro–Frenk–White and Allen–Santillán expressions for the potential of the dark-matter halo, and two different masses for the Galaxy within 200 kpc—0.75 × 10¹² M _⊙ and 1.45 × 10¹² M _⊙. The character of the orbits of most of these galaxies indicates that they are tightly gravitationally bound to the Milky Way, even with the lower-mass model for the gravitational potential. One exception is the most distant galaxy in the list, Leo I, whose orbit demonstrates that it is only weakly gravitationally bound, even using the higher-mass model of the gravitational potential.     

Population synthesis for massive close WR20a-type binaries

We have used the “Scenario Machine” to carry out a population synthesis for close binary systems with the masses of both components and their orbital periods similar to those for the WR20a system. The possible qualitative composition of WR20a, the most massive known binary with non-degenerate components (commonly classified as Wolf-Rayet stars according to their observational parameters), has been studied. Meridional circulation may enrich the envelope of a rapidly rotating main-sequence star in CNO elements. In the most likely model, WR20a consists of a Wolf-Rayet star and a main-sequence star.     

The evolution of stars paired with supermassive black holes

A star located in the close vicinity of a supermassive black hole (SMBH) in a galactic nucleus or a globular-cluster core could form a close binary with the SMBH, with the star possibly filling its Roche lobe. The evolution of such binary systems is studied assuming that the SMBH mainly accretes matter from the companion star and that the presence of gas in the vicinity of the SMBH does not appreciably influence variations in the star’s orbit. The evolution of the star–SMBH system is mainly determined by the same processes as those determining the evolution of ordinary binaries. The main differences are that the star is subject to an incident flux of hard radiation arising during the accretion of matter by the SMBH, and, in detached systems, the SMBH captures virtually all the wind emitted by its stellar companion, which appreciably influences the evolution of the major axis of the orbit. Moreover, the exchange between the orbital angular momentum and the angular momentum of the overflowing matter may not be entirely standard in such systems. The computations assume that there will be no such exchange of angular momentum if the characteristic timescale for mass transfer is shorter than the thermal time scale of the star. The absorption of external radiation in the stellar envelope was computed using the same formalism applied when computing the opacity of the stellar matter. The numerical simulations show that, with the adopted assumptions, three types of evolution are possible for such a binary system, depending on the masses and the initial separation of the SMBH and star. Type I evolution leads to the complete destruction of the star. Only this type of evolution is realized for low-mass main-sequence (MS) stars, even those with large initial separations from their SMBHs. Massive MS stars will also be destroyed if the initial separation is sufficiently small. However, two other types of evolution are possible for massive stars, with a determining role in the time variations of the parameters of the star–SMBH system being played by the possible growth of the massive star into a red giant during the time it is located in the close vicinity of the SMBH. Type II evolution can be realized for massive MS stars that are initially farther from the SMBH than in the case of disruption. In this case, the massive star fills its Roche lobe during its expansion, but is not fully destroyed; the star retreats inside its Roche lobe after a period of intense mass loss. This type of evolution is characterized by an increase in the orbital period of the system with time. As a result, the remnant of the star (its former core) is preserved as a white dwarf, and can end up at a fairly large distance from the SMBH. Type III evolution can be realized formassiveMSstars that are initially located still farther from their SMBHs, and also for massive stars that are already evolved at the initial time. In these cases, the star moves away from the SMBH without filling its Roche lobe, due to its intense stellar wind. The remnants of such stars can also end up at a fairly large distances from their SMBHs.     

Radio pulsars in close binaries with neutron stars

The initial distribution of young radio pulsars, reconstructed from the observed distribution of their spatial velocities distorted by observational selection effects, taking into account the age and spatial distribution of radio pulsars with measured spatial velocities, appears to be bimodal. Most young pulsars are formed with velocities of ～100 km/s. Some fraction of young radio pulsars display an almost flat velocity distribution (dv/dv ≈ const) from 150 to 1000 km/s. Scenario modeling in the absence of an additional (kick) velocity acquired by the young neutron star during its formation in a supernova explosion can reproduce the initial velocity distribution of radio pulsars, but results in a higher fraction of radio pulsars in binaries than is observed. Assuming a random initial Maxwellian kick velocity of ～100 km/s makes it possible to reduce the fraction of radio pulsars in binaries to the observed value (<1%), while leaving the velocity distribution for radio pulsars close to the observed bimodal initial distribution. Such kick velocities are also able to explain the observed distribution of radio pulsars in close binaries in the eccentricity—orbital period plane.     

Patterns of activity in stars with cycles becoming established

Late-type stars with chromospheric and coronal activities exceeding those of the Sun and other stars with well-defined cycles are considered. These rotate more rapidly than stars with well established cycles; for single stars, this appears to be due to their younger ages. The spots on such stars cover several per cent of the total area, which is an order of magnitude higher than for the Sun at its activity maximum. Our wavelet analysis of the chromospheric-emission variability, which has been observed since 1965 in the framework of the HK project, indicates that the period of the axial rotation of some of these starts varies from year to year. This is most pronounced in two “Good” stars according to the classification of Baliunas et al., HD 149661 and HD 115404, and also in a star with a more complex variability, HD 101501. No similar effect is exhibited by the “Excellent” cyclic-activity stars. Such variations in the period can be observed during epochs of appreciable rotational modulations of the chromospheric-emission fluxes, most likely, immediately after the maximum of a long-period wave (cycle?). This seems to provide evidence for the existence of huge activity complexes in the chromospheres of these stars, whose longitudes remain virtually constant over several years; they drift from fairly high latitudes to the equator at speeds close to the value typical of sunspots. The observed period variations are most likely due to differential rotation of the same sign that is known for the Sun. Our results provide independent confirmation of similar conclusions obtained by us previously using zonal models for highly spotted stars. Other activity features of a selected star group and the implications of the results for the theory of stellar and solar dynamos are discussed.     

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.     

Neutron stars in close binaries with elliptical component orbits

We consider the evolution of close binaries in which the initial secondary component is a nondegenerate helium star with mass M_He = 0.4–60 M_⊙, while the initially more massive primary has evolved into a black hole, neutron star, or degenerate dwarf. The neutron star is assumed to originate as a result of the evolution of a helium star with a mass of 2.5 M_⊙ ≤ M_He ≤ 10 M_⊙ after the explosion of a type Ib,c supernova. If the axial rotation of the helium star before the explosion is rigid-body and synchronized with the orbital rotation, for P_orb ≤ 0.16 day, the rotational energy of the young neutron star will exceed the energy of an ordinary supernova. If the magnetic field of the neutron star is sufficiently strong, the necessary conditions for a magnetic-rotational supernova are provided. The initial rotational period of a young neutron star originating in a system with an orbital period shorter than ～50 days is shorter than ～4 s, which, according to observations, is required for the appearance of a radio pulsar. A helium star whose mass exceeds ～10 M_⊙ in a close binary with an orbital period shorter than one day and with the axial rotation of the helium presupernova synchronous with the orbital rotation evolves into a Kerr black hole, whose formation is likely to be accompanied by a gamma-ray burst with a duration longer than two seconds. In particular, we consider close binaries in which the second supernova results in the formation of a neutron star that remains in the binary. The theoretical distribution of orbital periods and eccentricities for such systems is consistent with that observed for radio pulsars in the Galactic disk in binaries with compact components and orbital eccentricities exceeding ～0.09, providing an explanation for the observed correlation between the orbital eccentricities and orbital periods for these systems.