No abstract available

[en] We consider the size of region of the main energy release of black hol acretion discs. This problem is deeply connected with accretion process at the innermost region of accretion disc and with boundary condition at its inner edge. Ou main result is demonstration that the region of the main energy release is strongly localized. It is shown that for accretion onto non-rotating black hol the main portion of the liberated energy is released within a rather narrow radial range 5N≤13M. This property can have profound effect on spectrum and variability of the radiation of the acretion disc as a whole. In particular, the asymmetry of the main energy release region is favourable for the mechanism of the millisecond variability proposed earlier by the authors. (author). 15 refs.; 5 figs

[en] It is shown that in thin disks with an angular velocity distribution according to Kepler's law both equilibria cannot be fulfilled simultaneously. Depending on the amount of viscosity in the disk either thermally or dynamically driven circulation will occur. The velocity of this circulation is kept low because of a slightly baroclinic structure of the disk, which is produced by the circulation currents. (orig.)

[en] Small-scale hydrodynamic instabilities - the shear instability and thermal convections, which can provide turbulization of accretion disks of compact objects have been analysed. Actually domains of the existence of each of these instabilities are determined; it is shown that shear turbulence has no gyrotropic nature, while the thermal convection has and can generate a large-scale magnetic field; sequences of the difference in statistic characteristics of these turbulences are described depending on the extensions of the domains of their existence. (author). 16 refs.; 2 figs

[en] Accretion disks that become gravitationally unstable can fragment into stellar or substellar companions. The formation and survival of these fragments depends on the precarious balance between self-gravity, internal pressure, tidal shearing, and rotation. Disk fragmentation depends on two key factors: (1) whether the disk can get to the fragmentation boundary of Q = 1 and (2) whether fragments can survive for many orbital periods. Previous work suggests that to reach Q = 1, and have fragments survive, a disk must cool on an orbital timescale. Here we show that disks heated primarily by external irradiation always satisfy the standard cooling time criterion. Thus, even though irradiation heats disks and makes them more stable in general, once they reach the fragmentation boundary, they fragment more easily. We derive a new cooling criterion that determines fragment survival and calculate a pressure-modified Hill radius, which sets the maximum size of pressure-supported objects in a Keplerian disk. We conclude that fragmentation in protostellar disks might occur at slightly smaller radii than previously thought and recommend tests for future simulations that will better predict the outcome of fragmentation in real disks.

No abstract available

[en] The IRAS star β Pictoris is still the only one around which a disk made of both dust and gas has been detected. Spectroscopic observations of β Pic have revealed that some metallic absorption lines present redshifted additional components which strongly variate with time. This has been interpreted as the result of the infall of small evaporating cometary-like bodies toward the star. To improve this model, a theoretical description of such an event has been purchased. Numerical simulations of such infalling bodies have been able to reproduce quite faithfully the observed behaviour of some metallic ions, and we have explained the observed difference between these behaviours by means of radiation pressure. Some constraints on the orbits of the bodies have been deduced by through the simulations which should be related to a possible interpretation concerning β Pic's disk

[en] We identify the optical counterpart of the ultraluminous X-ray source (ULX) NGC 1313 X-1 and discuss constraints on its physical nature from multiband optical spectra. There is a single object on Hubble Space Telescope images within the aspect-corrected Chandra X-ray error circle; a fainter, possibly extended, feature lies near the edge of the error circle. The brighter object showed prominent variation in the F555W band, but was constant in the F814W band. The spectrum was consistent with a single power law on 2003 November 17, but deviated from this on 2004 July 17, suggestive of more than one emission component. Based on the location, magnitudes, spectral shape, and variability of the bright object, it is likely the ULX counterpart. The red wing of the spectrum around F814W may be due to emission from the companion star, and the blue wing is likely from disk emission. The stellar population around X-1 has an age older than 30 Myr, without very blue stars or young clusters. This places a constraint on the companion mass of the ULX as no more than 10 M_sun.

[en] We present deep high-angular resolution observations of the high-mass protostar NGC 7538 S, which is in the center of a cold dense cloud core with a radius of 0.5 pc and a mass of ∼2000 M _sun. These observations show that NGC 7538 S is embedded in a compact elliptical core with a mass of 85-115 M _sun. The star is surrounded by a rotating accretion disk, which powers a very young, hot molecular outflow approximately perpendicular to the rotating accretion disk. The accretion rate is very high, ∼(1.4-2.8) x 10^-3 M _sun yr^-1. Evidence for rotation of the disk surrounding the star is seen in all largely optically thin molecular tracers, H¹³CN J = 1 → 0, HN¹³C J = 1 → 0, H¹³CO⁺ J = 1 → 0, and DCN J = 3 → 2. Many molecules appear to be affected by the hot molecular outflow, including DCN and H¹³CO⁺. The emission from CH₃CN, which has often been used to trace disk rotation in young high-mass stars, is dominated by the outflow, especially at higher K levels. Our new high angular resolution observations show that the rotationally supported part of the disk is smaller than we previously estimated. The enclosed mass of the inner, rotationally supported part of the disk (D ∼ 5'', i.e., 14,000 AU) is ∼14-24 M _sun.

[en] The massive stars in the Galactic center inner arcsecond share analogous properties with the so-called Hot Jupiters. Most of these young stars have highly eccentric orbits and were probably not formed in situ. It has been proposed that these stars acquired their current orbits from the tidal disruption of compact massive binaries scattered toward the proximity of the central supermassive black hole. Assuming a binary star formed in a thin gaseous disk beyond 0.1 pc from the central object, we investigate the relevance of disk-satellite interactions to harden the binding energy of the binary, and to drive its inward migration. A massive, equal-mass binary star is found to become more tightly wound as it migrates inward toward the central black hole. The migration timescale is very similar to that of a single-star satellite of the same mass. The binary's hardening is caused by the formation of spiral tails lagging the stars inside the binary's Hill radius. We show that the hardening timescale is mostly determined by the mass of gas inside the binary's Hill radius and that it is much shorter than the migration timescale. We discuss some implications of the binary's hardening process. When the more massive (primary) components of close binaries eject most their mass through supernova explosion, their secondary stars may attain a range of eccentricities and inclinations. Such processes may provide an alternative unified scenario for the origin of the kinematic properties of the central cluster and S-stars in the Galactic center as well as the high-velocity stars in the Galactic halo.