The evolutionary status of the most massive WNh stars in close binary systems. NGC 3603-A1

The evolution of the components of the unique, massive, close binary system NGC 3603-A1, which consists of stars of spectral types WN6ha and WN6h, is analyzed. The component masses are estimated to be 116 and 89M _⊙, close to the highest measured stellar masses. Numerical modeling of the evolution of the components has been carried out, taking into account mass loss via the stellar winds of the two massive stars. It is shown that the maximum possible initial component masses are close to 140 and 125M _⊙. The components are currently slightly evolved main-sequence stars, with a comparative low degree of helium enrichment at their surfaces. Further evolution of the system will lead to filling of the Roche lobe of the primary and subsequent evolution in a common envelope. This may lead to the merger of the components, with the evolution of the system ending in the formation of a singlemassive black hole after the second supernova explosion. Otherwise, depending on the masses of the resulting black holes, either a binary system of two black holes or two unbound black holes may form, accompanied by gamma-ray bursts.     

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.     

Evolution of the components of the close binary WR 20a

We have analyzed the evolution of the components of the unique massive binary system WR 20a, which consists of a Wolf-Rayet nitrogen star and an Of star with an extremely small separation. The estimated masses of the components are 83 and 82 M _⊙, which are among the highest stellar mass inferred. We have carried out numerical modeling of the evolution of the components, taking into account the mass loss due to the stellar wind inherent to massive stars. In a scenario in which the systemis detached from the time the components reach the main sequence until its present state, the initial component masses are inferred to be close to 110 M _⊙, if the initial masses of the stars were equal, or 120 and 100 M _⊙, if they were different. Currently, the components are evolved main-sequence stars, whose surfaces are relatively little enriched by helium. The further evolution of the system will result in one of the components filling its Roche lobe and evolution within a common envelope. As a result, the components may coalesce, leading to the formation of a single massive black hole the supernova explosion. Otherwise, depending on the masses of the resulting black holes, either a binary system with two black holes or two free black holes will be formed. In the latter case, gamma-ray bursts will be observed.     

Formation of low-mass X-ray novae with black holes from triple systems

We apply a population synthesis technique to study the formation and evolution of low-mass X-ray binaries with black holes, observed as X-ray novae, from hierarchical triple systems. A scenario is suggested in which an inner close binary system evolves into an X-ray system with a large mass ratio. The high rate of accretion onto the neutron star leads to a common envelope stage, which may result in the formation of a Thorne-Zytkow (TZ) object. During its evolution, the envelope of the TZ object expands, encompassing the third star. The recurrent common-envelope stage decreases the size of the orbit of the third star, leading to the formation of a lowmass X-ray nova with a black hole. The dynamical stability of triple systems automatically ensures that only lowmass X-ray novae form. We also consider the possible formation of an X-ray nova from a binary in the case of asymmetrical core collapse during a supernova explosion.     

The nature hypervelocity stars

We list and analyze the main currently known mechanisms for accelerating the space motions of stars. A high space velocity of a star can be a consequence of its formation in the early stages of the evolution of a massive galaxy, when it was spheroidal and non-stationary, so that stars were born with velocities close to the escape velocity for the galaxy. Another possibility is that the star arrived from another galaxy with a velocity that is high for our Galaxy. The decay of unstable close multiple stars or supernova explosions in close binaries can also provide velocities of up to several hundreds of km/s to main-sequence stars and velocities of up to ∼1000 km/s to degenerate stars, neutron stars, and stellar-mass black holes. The merger of components of a binary system containing two neutron stars or a neutron star and a black hole due to gravitational-wave radiation can accelerate the nascent black hole to a velocity∼1000 km/s. Hypervelocity relativistic stars can be born due to asymmetric neutrino ejection during a supernova explosion. Stars can be efficiently accelerated by single and binary supermassive black holes (with masses from several millions to several billions of solar masses) in the nuclei of galaxies. Thanks to their gravitational field and fast orbital motion (in the case of binary objects), supermassive black holes are able to accelerate even main-sequence stars to relativistic velocities.     

Magnetars,gamma-ray bursts,and very close binaries

We consider the possible existence of a common channel of evolution of binary systems, which results in a gamma-ray burst during the formation of a black hole or the birth of a magnetar during the formation of a neutron star. We assume that the rapid rotation of the core of a collapsing star can be explained by tidal synchronization in a very close binary. The calculated rate of formation of rapidly rotating neutron stars is qualitatively consistent with estimates of the formation rate of magnetars. However, our analysis of the binarity of newly-born compact objects with short rotational periods indicates that the fraction of binaries among them substantially exceeds the observational estimates. To bring this fraction into agreement with the statistics for magnetars, the additional velocity acquired by a magnetar during its formationmust be primarily perpendicular to the orbital plane before the supernova explosion, and be large.     

The evolution of close binary systems with intermediate-mass black holes and ultra-luminous X-ray sources

The results of numerical studies of the evolution of a close binary system containing a black hole with a mass of ～3000M_⊙ are presented. Such a black hole could form in the center of a sufficiently rich and massive globular cluster. The secondary could be a main-sequence star, giant, or degenerate dwarf that fills or nearly fills its Roche lobe. The numerical simulations of the evolution of such a system take into account the magnetic wind of the donor together with the wind induced by X-ray irradiation from the primary, the radiation of gravitational waves by the system, and the nuclear evolution of the donor. Mass transfer between the components is possible when the donor fills its Roche lobe, and also via the black hole’s capture of some material from the induced stellar wind. The computations show that the evolution of systems with solar-mass donors depends only weakly on the mass of the accretor. We conclude that the observed ultra-luminous X-ray sources (L _X ? 10³⁸ erg/s) in nearby galaxies could include accreting black holes with masses of 10²?10⁴M_⊙. Three scenarios for the formation of black holes with such masses in the cores of globular clusters are considered: the collapse of superstars with the corresponding masses, the accretion of gas by a black hole with a stellar initial mass (<100M_⊙), and the tidal accumulation of stellar black holes. We conclude that the tidal accumulation of stellar-mass black holes is the main scenario for the formation of intermediate-mass black holes (10²?10⁴M_⊙) in the cores of globular clusters.     

The evolution of GRS 1915+105—an X-ray binary with a black hole

A brief review of the observed parameters of binary systems with black holes is presented. We discuss in detail the evolutionary status of the X-ray binary GRS 1915+105, which contains a massive black hole. Numerical simulations of the evolution of GRS 1915+105 at the X-ray stage indicate that the most probable initial mass of the optical component (donor star) is (1.5–)M_⊙. Two possible scenarios are suggested for the evolution of the system prior to the formation of the black hole. If the initial mass of the optical component was (2.5–)M_⊙, the system underwent a common-envelope phase; in this case, the initial mass of the black hole progenitor did not exceed ～50M_⊙. If the initial mass of the donor was (1.5–2.5)M_⊙, a scenario without a common envelope is possible, with the initial mass of the black hole progenitor being smaller than ～50M_⊙. The lack of information about the initial mass-ratio distribution for binary components for small q and the uncertainty of the system parameters make it impossible to give preference to a particular scenario for the system's prior evolution.     

Evolution of close binaries and gamma-ray bursts

We analyze the late stages of evolution of massive (M ₀ ? 8 M _⊙) close binaries, from the point of view of possible mechanisms for the generation of gamma-ray bursts. It is assumed that a gamma-ray burst requires the formation of a massive (～1 M _⊙), compact (R ? 10 km) accretion disk around a Kerr black hole or neutron star. Such Kerr black holes are produced by core collapses of Wolf-Rayet stars in very close binaries, as well as by mergers of neutron stars and black holes or two neutron stars in binaries. The required accretion disks can also form around neutron stars that were formed via the collapse of ONeMg white dwarfs. We estimate the Galactic rate of events resulting in the formation of rapidly rotating relativistic objects. The computations were carried out using the “Scenario Machine.”     

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.     

The disruption of close binaries in the gravitational field of a supermassive black hole and the formation of hypervelocity stars

The formation of hypervelocity stars due to the dynamical capture of one component of a closebinary system by the gravitational field of a supermassive black hole (SMBH) is modeled. The mass of the black hole was varied between 10⁶ and 10⁹ M _⊙. In the model, the problem was considered first as a three-body problem (stage I) and then as an N-body problem (stage II). In the first stage, the effect of the inclination of the internal close-binary orbit (the motion of the components about the center of mass of the binary system) relative to the plane of the external orbit (the motion of the close binary around the SMBH) on the velocity with which one of the binary components is ejected was assessed. The initial binary orbits were generated randomly, with 10 000 orbits considered for each external orbit with a fixed pericenter distance r _p . Analysis of the results obtained in the first stage of the modeling enables determination of the binary-orbit orientations that are the most favorable for high-velocity ejection, and estimation of the largest possible ejection velocities V _max. The boundaries of the region of stellar disruption derived from the balance of tidal forces and self-gravitation are discussed using V _max-r _p plots, which generalize the results of the first stage of the modeling. Since a point-mass representation does not enable predictions about the survival of stars during close passages by a SMBH, there is the need for a second stage of the modeling, in which the tidal influence of the SMBH is considered. An approach treating a star like a structured finite object containing N bodies (N = 4000) enables the derivation of more accurate limits for the zone of efficient acceleration of hypervelocity stars and the formulation of conditions for the tidal disruption of stars.     

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.     

Different magneto-rotational supernovae

We present the results of two-dimensional calculations of a magneto-rotational (MR) supernova explosion with a collapsing core for various core masses, rotational angular momenta, and magnetic-field configurations. It is shown that the MR mechanism produces an explosion energy that corresponds to observed values. The form of the explosion depends substantially on the initial configuration of the magnetic field. MR instability develops during the evolution of the magnetic field in an MR supernova explosion, resulting in an exponential increase of all components of the magnetic field, thereby substantially decreasing the time scale of the MR explosion. The energy of the supernova increases with the core’s mass and initial rotational energy.     

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.     

Did the SN 1987A outburst leave a compact remnant?

We have computed the ejection of a massive envelope by a star during a type II supernova explosion in the presence of a compact remnant (a neutron star or black hole). This problem is of interest because of the possible presence of a compact remnant following the SN 1987A explosion. The computations demonstrate that a fairly large amount of matter is left in the neighborhood of the compact gravitating body. We present computations of the accretion rate onto the surface of the compact remnant. The estimated luminosity exceeds that observed for SN 1987A in various frequency ranges by several orders of magnitude.     

Wolf-Rayet stars,black holes,and gamma-ray bursters in close binaries

We consider the evolutionary status of observed close binary systems containing black holes and Wolf-Rayet (WR) stars. When the component masses and the orbital period of a system are known, the reason for the formation of a WR star in an initial massive system of two main-sequence stars can be established. Such WR stars can form due to the action of the stellar wind from a massive OB star (M_OB≥50M_⊙), conservative mass transfer between components with close initial masses, or the loss of the common envelope in a system with a large (up to ～25) initial component mass ratio. The strong impact of observational selection effects on the creation of samples of close binaries with black holes and WR stars is demonstrated. We estimate theoretical mass-loss rates for WR stars, which are essential for our understanding the observed ratio of the numbers of carbon and nitrogen WR stars in the Galaxy \(\dot M_{WR} (M_ \odot yr^{ - 1} ) = 5 \times 10^{ - 7} (M_{WR} /M_ \odot )^{1.3} \). We also estimate the minimum initial masses of the components in close binaries producing black holes and WR stars to be ～25M_⊙. The spatial velocities of systems with black holes indicate that, during the formation of a black hole from a WR star, the mass loss reaches at least several solar masses. The rate of formation of rapidly rotating Kerr black holes in close binaries in the Galaxy is ～3×10^?6 yr^?1. Their formation may be accompanied by a burst of gamma radiation, possibly providing clues to the nature of gamma-ray bursts. The initial distribution of the component mass ratios for close binaries is dN～dq=dM₂/M₁ in the interval 0.04?q₀≤1, suggesting a single mechanism for their formation.     

Acceleration of spatial motions of stars by close-binary supermassive black holes in galactic nuclei

The conditions for the acceleration of the spatial motions of stars by close-binary supermassive black holes (SMBHs) in galactic nuclei are analyzed in order to derive the velocity distribution for stars ejected from galaxies by such black holes. A close binary system consisting of two SMBHs in circular orbits was subject to a spherically symmetrical “barrage” of solar-mass stars with various initial velocities. The SMBHs were treated as point objects with Newtonian gravitational fields. Models with binary component-mass ratios of 1, 0.1, 0.01, and 0.001 were studied. The results demonstrate the possibility of accelerating neutron stars, stellar-mass black holes, and degenerate dwarfs to velocities comparable to the relative orbital velocities of the binary-SMBH components. In the stage when the binary components are merging due to the action of gravitational-wave radiation, this velocity can approach the speed of light. The most massive binary black-holes (M ? 10⁹M_⊙) can also accelerate main-sequence stars with solar or subsolar masses to such velocities.     

The probability of forming hyperveloicty stars in the Galaxy

The probability of forming a Galactic hypervelocity star is estimated for the scenario of Hills, which describes the dynamical capture of one component of a binary star by the gravitational field of the supermassive black hole in the Galactic center, leading to the ejection of the other component. Ten thousand initial orientations of the binary orbits were considered, and the semi-major axes of the binary orbits were varied in a wide range from 11.3 R _⊙ to 425 R _⊙. Two series of computations were carried out, in which the mass of the supermassive black hole was taken to be 10⁶ M _⊙ and 3.4 × 10⁶ M _⊙. Numerical simulations of encounters of the binary and black hole in the framework of the three-body and N-body problems are used to localize regions favorable for the formation of hypervelocity stars. The motion of the ejected star in the regular field of the Galaxy is calculated, and the conditions under which the star escapes the Galaxy defined. The probability of escaping the Galaxy is caluclated as a function of various parameters the initial separation of the binary components and the distance of the binary from the black hole. On average, the probability of forming a hypervelocity star is higher for closer encounters and more tightly bound binary pairs.

     

Merging of components in close binaries: Type Ia supernovae,massive white dwarfs,and Ap stars

The “Scenario Machine” (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes. We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance must be taken into account at estimating the parameters of the Universe expansion acceleration. According to theoretical estimates, the most massive—as a rule, magnetic—white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main-sequence stars (M ? 1.5 M _⊙) with convective envelopes.     

The dynamical evolution of galaxy clusters taking into account the deceleration of disk galaxies by the gaseous component of the cluster

Numerical simulations of the dynamical evolution of a galaxy cluster in the framework of the N-body problem taking into account dark matter are presented. These simulations are aimed at studying the role of intergalactic gas in the cluster (the ICM) in the formation of a central, supermassive cD galaxy. The numerical models indicate that deceleration of the galaxies by intergalactic gas supports the observed high temperature of this gas, and accelerates the formation of a supermassive cD galaxy in the cluster core. The accretion of interstellar gas by the cluster core can support a high accretion rate by the central, supermassive black hole associated with the nucleus of the cD galaxy. As a result, this nucleus harbors a bright quasar. The mass of the black hole can grow with time to values 10¹⁰ M _⊙, as are observed for the brightest quasars.