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The formation of planets around binary stars maybe more difficult than around single stars (1-3). In a close binary star (with a separation of less than a hundred astronomical units), [...]

The formation of very wide binary systems (1-3), such as the α Centauri system with Proxima (also known as α Centauri C) separated from a Centauri (which itself is a [...]

We present numerical simulations of near-Earth asteroid (NEA) tidal disruption resulting in bound, mutually orbiting systems. Using a rubble pile model we have constrained the relative likelihoods for possible physical and dynamical properties of the binaries created. Overall 110,500 simulations were run, with each body consisting of ~1000 particles. The encounter parameters of close approach distance and velocity were varied, as were the bodies' spin, elongation and spin axis direction. The binary production rate increases for closer encounters, at lower speeds, for more elongated bodies, and for bodies with greater spin. The semimajor axes for resultant binaries are peaked between 5 to 20 primary radii, and there is an overall trend for high eccentricity, with 97% of binaries having e > 0.1. The secondary-to-primary size ratios of the simulated binaries are peaked between 0.1 and 0.2, similar to trends among observed asteroid binaries. The spin rates of the primary bodies are narrowly distributed between 3.5- and 6-h periods, whereas the secondaries' periods are more evenly distributed and can exceed 15-h periods. The spin axes of the primary bodies are very closely aligned with the angular momenta of the binary orbits, whereas the secondary spin axes are nearly random. The shapes of the primaries show a large distribution of axis ratios, where those with low elongation (ratio of long and short axis) are both oblate and prolate, and nearly all with large elongation are prolate. This work presents results that suggest tidal disruption of gravitational aggregates can make binaries physically similar to those currently observed in the NEA population. As well, tidal disruption may create an equal number of binaries with qualities different from those observed, mostly binaries with large separation and with elongated primaries. Keywords: Asteroids; Dynamics; Rotational dynamics; Tides, solid body

The common envelope phase of binary star evolution is necessary in formation of short-period systems having a compact object.Significant mass and angular momentum are lost. The result is that a wide-progenitor system becomes a close-remnant binary. Conditions required for survival of the binary according to the results to three-dimensional hydrodynamics calculations are emphasized with discussion of evolution of high-mass systems having neutron stars. Thorne-Zytkow objects, binary pulsars, and high- and low-mass X-ray binaries are covered.

Assuming current models of terrestrial planet formation in the Solar System, we numerically investigate the conditions under which the secondary star in a binary system will inhibit planet growth in the circumstellar habitable zone. Runaway accretion is assumed to be precluded if the secondary (1) causes the planetesimal orbits to cross within the runaway accretion time scale and (2) if, during crossing, the relative velocities of the planetesimals have been accelerated beyond a certain critical value which results in disruption collisions rather than accretion. For a two solar mass binary with planetesimals in circular orbits about one star at 1 AU, and a typical wide binary eccentricity of 0.5, the minimum binary semimajor axis which would not inhibit planet formation, [a.sub.c], is 32 AU. If the planetesimals orbit the center of mass of the binary system, [a.sub.c] = 0.10 AU, which is inside the tidal circularization radius. We obtain an empirical formula giving the dependencies of [a.sub.c] on the binary eccentricity, secondary mass, planetesimal location, and critical disruption velocity. based on the distributions of orbital elements of a bias-corrected sample of nearby G-dwarfs, we find that [approximately equal to]60% of solar-type binaries cannot be excluded from having a habitable planet solely on the basis of the perturbative effect of the secondary star. This conclusion is independent of when the secondary star formed, nebula dissipative mechanisms, and the time scale for runaway planetesimal accretion, and is relatively insensitive to the mass of the secondary star, the critical disruption velocity, and the location of planetesimals within the circumstellar habitable zone. An earlier study of planet formation in binary star systems came to a different conclusion, namely that planet formation, even at Mercury's distance, is unlikely except in widely separated systems ([greater than or equal to]50 AU), or when the secondary has a very low mass and near circular orbit as in the Sun-Jupiter system. The discrepancy with the present numerical study is due in part to the different runaway accretion time scales assumed and the neglect in the earlier study of an exact criterion for crossing orbits. Key Words: planet formation; binary stars; life.

The role of mergers in producing galaxies, together with the finding that most large galaxies harbour black holes in their nuclei (1), implies that binary supermassive black hole systems should be common. Here we report that the quasar SDSS J153636.22+ 044127.0 is a plausible example of such a system. This quasar shows two broad-line emission systems, separated in velocity by 3,500 km [s.sup.-1]. A third system of unresolved absorption lines has an intermediate velocity. These characteristics are unique among known quasars. We interpret this object as a binary system of two black holes, having masses of [10.sup.7.3] and [10.sup.8.9] solar masses separated by ~0.1 parsec with an orbital period of ~100 years., The present object is the first known with two broad-line systems, and builds on the recent interest in the quasar SDSS J092712.65+294344.0, which shows two sets of narrow emission lines [...]

Neutron stars and black holes can find themselves with close stellar companions through birth or later accident (section 1). The systems give rise to intense variable X-ray sources and born again pulsars (section 2) and perhaps to still odder things (section 6). They provide the best measurements of neutron star masses and the most persuasive evidence for the existence of black holes in the real world (section 3). Most of the basic evolutionary (section 1) and radiation mechanisms (section 4) required to model the observations have been in place for a decade or more and have survived discovery of new phenomena every couple of years. Many of these involve disks of gas accreting onto the stars (section 5). These disks are perhaps prototypes for those in the cores of quasars. Some problems remain with (a) relative numbers of objects in various evolutionary phases and (b) a few extreme individual systems such as SS 433 and 1957 + 20 (section 6).