The search for planets around eclipsing binary stars

A list of eclipsing binary stars that may have planetary systems is presented. Eclipsing binaries facilitate the search for planets in binary systems. The presence of eclipses strongly increases the probability that the observer is in the orbital plane of the system, since it is natural to expect that protoplanetary disks and planets are located in or close to this plane. The planets in the listed systems could be detected with the transit method using facilities that are available for a wide range of professional observers, as well as amateur astronomers.     

The evolution of close binary stars

A review of our current understanding of the physics and evolution of close binary stars with various masses under the influence of the nuclear evolution of their components and their magnetic stellar winds is presented. The role of gravitational-wave radiation by close binaries on their evolution and the loss of their orbital angular momentum is also considered. The final stages in the evolution of close binary systems are described. The review also notes the main remaining tasks related to studies of the physics and evolution of various classes of close binaries, including analyses of collisions of close binaries and supermassive black holes in galactic nuclei. Such a collision could lead to the capture of one of the components by the black hole and the acceleration of the remaining component to relativistic speeds.     

Formation and evolution of Ae and Be stars

Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30M _⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars.     

Formation of planetary systems and brown dwarfs around single stars

The conditions for the formation of planets and brown dwarfs around single main-sequence stars are considered in two scenarios. The formation of planets and brown dwarfs requires that the initial specific angular momentum of a solar-mass protostar be (0.32)×10¹⁸ cm²/s. The accreted matter of the protostar envelope forms a compact gas ring (disk) around the young star. If the viscosity of the matter in this ring (disk) is small, increasing its mass above a certain limit results in gravitational instability and the formation of a brown dwarf. If the viscosity of the gas is sufficiently large, the bulk of the protostar envelope material will be accreted by the young star, and the gas disk will grow considerably to the size of a protoplanetary dust disk due to the conservation of angular momentum. The formation of dust in the cool part of the extended disk and its subsequent collisional coalescence ultimately results in the formation of solar-type planetary systems.     

Influence of binary stars on the chemical evolution of damped Lyα systems

The impact of variations in the fraction of binary stars producing type Ia supernovae, β, on the chemical evolution of spiral galaxies is analyzed numerically. Even modest variations in β appreciably affect the evolution of the relative abundances of iron-group and alpha-process elements. If a substantial number of the damped Lα systems manifest in the spectra of quasars are due to spiral galaxies, the large scatter of the abundances of various elements displayed by these systems can be accounted for by variations in β.     

Massive close binary stars and gamma-ray bursts

We analyze the observed parameters of massive extremely close binaries containing Wolf-Rayet stars and black holes, and identify those systems whose supernova outbursts lead to the formation of rapidly rotating Kerr black holes. It is proposed that the formation of such a black hole is accompanied by a strong gamma-ray burst. Several types of observed systems satisfy the conditions necessary for the formation of a Kerr black hole: BH+WR, BH+OB, WR+O, and BH+K,M.     

Dynamical evolution of multiple stars

We have modeled the dynamical evolution of small groups of N=3–18 stars in the framework of the gravitational N-body problem, taking into account possible coalescences of stars and the ejection of single and binary stars from the system. The distribution of states is analyzed for a time equal to 300 initial crossing times of the system. The parameters of the binaries and stable triple systems formed, as well as those of ejected single stars, are studied. In most cases, the evolution of the group results in the formation of a binary or stable triple system. The orbital eccentricities of the binaries formed are distributed according to the law f(e)=2e. As a rule, stable triple systems display pronounced hierarchy (the mean ratio of the semimajor axes of the outer and inner binaries is about 20:1). Stars are ejected with velocities from several km/s to several tens of km/s. The results of the modeling are compared with the parameters of observed wide binaries and triple systems.     

The formation and evolution of the most massive stars

We consider the formation of massive stars under the assumption that a young star accretes material from the protostellar cloud through its accretion disk while losing gas in the polar directions via its stellar wind. The mass of the star reaches its maximum when the intensity of the gradually strengthening stellar wind of the young star becomes equal to the accretion rate. We show that the maximum mass of the forming stars increases with the temperature of gas in the protostellar cloud T ₀, since the rate at which the protostellar matter is accreted increases with T ₀. Numerical modeling indicates that the maximum mass of the forming stars increases to ～900 M _⊙ for T ₀ ～ 300 K. Such high temperatures of the protostellar gas can be reached either in dense star-formation regions or in the vicinity of bright active galactic nuclei. It is also shown that, the lower the abundance of heavy elements in the initial stellar material Z, the larger the maximum mass of the star, since the mass-loss rate due to the stellar wind decreases with decreasing Z. This suggests that supermassive stars with masses up to 10⁶ M _⊙ could be formed at early stages in the evolution of the Universe, in young galaxies that are almost devoid of heavy elements. Under the current conditions, for T ₀ = (30–100) K, the maximum mass of a star can reach ～100M _⊙, as is confirmed by observations. Another opportunity for the most massive stars to increase their masses emerges in connection with the formation and early stages of evolution of the most massive close binary systems: the most massive stars can be produced either by coalescence of the binary components or via mass transfer in such systems.     

Evolution of the masses of neutron stars in binary systems

We study the growth of the masses of neutron stars in binary systems due to the accumulation of mass from the optical donors accreted onto the neutron-star surface. Possible scenarios for this accretion are considered. The masses and magnetic-field strengths of radio pulsars derived using population-synthesis methods are compared to the observational data. The population-synthesis analysis indicates that a neutron star can increase its mass from the standard value of m _x ? 1.35M_⊙ to the Oppenheimer-Volkoff limit, m _x ? 2.5M_⊙, via accretion from a companion.     

Elliptical motions of stars in close binary systems

The motion of stars in close binary systems with conservative mass transfer is considered. It is shown that the Paczynski-Huang model that is currently used to determine the variations of the semimajor axis of the relative orbit of the stars is not correct, and leads to large errors in the derived semi-major axis. A new model is proposed, suitable for elliptical stellar orbits. The reaction forces and gravitational forces between the stars and the stream of overflowing matter are taken into account. The possibility of mass transfer in the presence of an accretion disk is considered.     

Photoelectric lunar-occultation observations of several close binary stars

The results of photoelectric lunar-occultation observations for several stars are presented. Some of these are obvious close binary systems, while others are suspected close binaries or multiple systems, or show evidence for the presence of complex structure of some other kind. It is expected that new, efficient interferometric systems will enable detailed studies of the structure of many “ordinary” stars with an angular resolutions of 10^?4–10^?5 arcsec.     

Hf-W chronology of the accretion and early evolution of asteroids and terrestrial planets

The ¹⁸²Hf-¹⁸²W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their ²⁶Al-²⁶Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of ²⁶Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf-W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf-W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO₂ in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical ¹⁸²W/¹⁸⁴W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of ¹⁸²Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical ¹⁸²W/¹⁸⁴W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.Hafnium-tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the ¹⁸²W/¹⁸⁴W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of ¹⁸²Hf. In contrast, no ¹⁸²W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a ¹⁴⁶Sm-¹⁴²Nd age of 50-200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf-W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of ¹⁴⁶Sm-¹⁴²Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined ¹⁸²W-¹⁴²Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf-W age for the formation of the Moon.     

A model for the population of binary stars in the Galaxy

A comparative investigation of the population of Galactic binary stars is performed for two modes of star formation: star formation at a constant rate over 10¹⁰ yrs, and a burst of star formation that reprocesses the same mass of gas into stars over 10⁹ yrs. Estimates are presented for the star-formation rates and populations of about 100 types of binaries and the products of their evolution. For most close binary systems, the models depend only weakly on the common-envelope parameter α_ce.     

Accurate relative positions and motions of poorly studied binary stars

We have studied the relative motions of the components of 25 visual binary and multiple stars for which only a modest number of observations of various types had accumulated due to their small relative component motions. A uniform series of photographic observations obtained over many years on the 26-inch refractor of the Pulkovo Observatory are used to calculate the parameters of the apparent relative positions and motions, which can be used for further kinematic and dynamical studies of these stars. A comparison of the relative and proper motions of the components indicates that they are physically bound. Optical components are identified in six systems??for the first time in the three systemsWDS00082+6217, ADS 830, and ADS 7361. The relative motions in ADS 861 and ADS 12925 suggest the presence of perturbing invisible companions, but this requires confirmation based on more accurate observations. The apparent motion parameters method yields a family of orbits with a minimum period of 26 000 yrs for the nearby wide visual binary Gliese 745, for which the parallax and relative radial velocity of the components are known.     

Gravitational darkening coefficients of the stars in the semidetached binary V Puppis

We have analyzed high-precision vby light curvesfor the semi-detached binary V Pup in a Roche model. They are consistent with the standard gravitational darkening coefficient for hot stars, β = 0.25, rather than the value β = 1.36 ± 0.04 derived by Kitamura and Nakamura [1] using a simpler model. We rigorously estimate the confidence intervals for the allowed gravitational darkening coefficients for a star filling its Roche lobe to be β = (?0.24, +1.29) for the 99% confidence level and β = (?0.21, +1.26) for the 67% confidence level.     

Magnetorotational supernova explosions and the formation of neutron stars in close binary systems

The formation of neutron stars in the closest binary systems (P _orb<12 h) gives the young neutron star/pulsar a high rotational velocity and energy. The presence of a magnetic field of 3×10¹¹–3×10¹³ G, as is observed for radio pulsars, enables the neutron star to transfer ～10⁵¹ erg of its rotational energy to the envelope over a time scale of less than an hour, leading to a magnetorotational supernova explosion. Estimates indicate that about 30% of all type-Ib,c supernovae may be the products of magnetorotational explosions. Young pulsars produced by such supernovae should exhibit comparatively slow rotation (P _rot>0.01 s), since a large fraction of their rotational angular momentum is lost during the explosion. The magnetorotational mechanism for the ejection of the envelope is also reflected by the shape of the envelope. It is possible that the Crab radio pulsar is an example of a product of a magnetorotational supernova. A possible scenario for the formation of the close binary radio pulsar discovered recently by Lyne et al. is considered.     

High-velocity stars as a result of encounters between stars and massive binary black holes in galactic nuclei

Numerical simulations of the motions of stars in the gravitational fields of binary black holes with various component mass ratios have been carried out. Two models are considered: (1) the two-body problem with two fixed centers; (2) the general three-body problem. The first model is applicable only over short times Δt ? T, where T is the period of the binary system. The second model is applicable at all times except for during close encounters of stars with one of the binary components, r ≤ 0.00002 pc, where r is the distance from the star to the nearer black hole. In very close passages, relativistic corrections must be taken into account. Estimates of the probability of formation of high-velocity stars as a result of such interactions are obtained. It is shown that this mechanism is not suitable for the nucleus of our Galaxy due to the probable absence of a second massive black hole in the central region of the Galaxy.     

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.     

The evolution of solar-like activity of low-mass stars

An analysis of data on chromospheric activity obtained in the framework of exoplanet-search programs is presented. Observations of 1334 stars showing that the chromospheric activity of the Sun is clearly higher than for the vast majority of stars in the solar vicinity are used. A comparison of chromospheric and coronal activity led to the identification of a significant group of stars with a low level of chromospheric activity, whose coronal radiation spans wide ranges. There are reasons to believe that the chromospheric and coronal activities of one group of stars decrease simultaneously as the rotation decelerates, while, in stars of the other group, the chromospheric activity diminishes, but their coronas remain stronger than that of the Sun. Features of cyclic activity of the Sun are discussed. This enables us to associate differences in the behavior of the activity with different depths of the convective zones of stars of spectral classes earlier and later than G6. Arguments in favor of a two-layer dynamo and different roles of the large-scale and small-scale magnetic fields in the formation and evolution of activity are formulated. Age estimations based on activity levels (gyrochronology) must be carried out differently for these different groups of stars.     

Dynamical evolution of gaseous clouds in the atmospheres of Wolf-Rayet stars

We have computed the dynamical evolution of homogeneous, spherical gaseous condensations in the atmosphere of a Wolf-Rayet star. The physical conditions in the condensations vary substantially in the course of their motion in the stellar wind, which should result in variations in the observed spectrum of the star. The condensations also move at velocities of up to 1000 km/s relative to the surrounding stellar wind. Variations of the physical conditions in these condensations should be taken into account in models of the stellar winds of Wolf-Rayet stars.