Calculation of profiles of CIV,NV, OVI,and SiIV resonance lines formed in accretion shocks in T Tauri stars: A plane layer

We have calculated profiles of the CIV 1550, NV 1240, OVI 1035, and SiIV 1400 resonance doublets for a plane-parallel shock viewed at various angles. Calculations were performed for the range of preshock gas velocities V₀ and gas densities ρ₀ appropriate for classical T Tauri stars. The parameters of accretion shocks in young stars can be determined by comparing the calculated and observed profiles of the studied lines and their relative intensities. It is not possible to derive the parameters of the accreting gas from the line profiles without knowing the geometry of the accretion zone. The relation I _v (µ,V₀,ρ₀) for a plane shock, where I _v is the intensity μ=cosθ, can be used to determine the accretion parameters by either choosing a geometry for the radiating region or using a technique similar to Doppler tomography. The results obtained for DR Tau, T Tau, and RY Tau indicate that, in contrast to current concepts, the inner regions of the accretion disk are not disrupted by the magnetic field of the star, and the disk reaches the stellar surface. As a result, only a small fraction of the accreted matter passes through the shock and falls onto the star.     

Intercombinational line profiles in the UV spectra of T Tauri stars and analysis of the accretion zone

We have analyzed for the first time profiles of the SiIII 1892 Å and CIII 1909 Å intercombinational lines in HST spectra of the stars RY Tau and RU Lup. The widths of these optically thin lines exceeded 400 km/s, ruling out formation in the stellar chromosphere. Since the intensity of the Si line exceeds that of the C line, it is unlikely that a large fraction of the observed line flux is formed in a stellar wind. The observed profiles can be reproduced in the framework of an accretion shock model if the velocity field in the accretion zone is appreciably nonaxisymmetric. In this case, the line profiles should display periodic variations, which can be used to determine the accretion zone geometry and the topology of the magnetic field near the stellar surface; corresponding formulas are presented. In addition, periodic variations of the 0.3–0.7 keV X-ray flux should be observed.     

Mathematical modeling of the structure of an accretion disk in a stellar binary system

Numerical simulations of gas-dynamical processes taking place in the accretion disk of a stellar binary system are presented. The initial state of the disk is an equilibrium gaseous configuration. Mechanisms for the development of spiral waves and associated variations in the angular momentum of the gas are considered. The influence of the ratio of the binary-component masses and the initial disk configuration are investigated. It is concluded that the existence of a steady-state disk is impossible without a flow of gas from the donor star.     

Dependence of the absorption-line profiles and radial-velocity curve of the optical star in an X-ray binary on the orbital inclination and component-mass ratio

Theoretical absorption-line profiles and radial-velocity curves for tidally deformed optical stars in X-ray binary systems are calculated assuming LTE. The variations in the profile shapes and radial-velocity curve of the optical star are analyzed as a function of the orbital inclination of the X-ray binary system. The dependence of the shape of the radial-velocity curve on the orbital inclination i increases with decreasing component-mass ratio q = m _x /m _v . The integrated line profiles and radial-velocity curves of the optical star are calculated for the Cyg X-1 binary, which are then used to estimate the orbital inclination and mass of the relativistic object: i < 43° andm _x = 8.2–12.8 M_⊙. These estimates are in good agreement with earlier results of fitting the radial-velocity curve of Cyg X-1 using a simpler model (i < 45°, m _x = 9.0–13.2 M_⊙).     

The motion of a star in the vicinity of a globular cluster in an elliptical galaxy

The spatial motion of a star in the vicinity of a globular cluster located in an inhomogeneous, rotating elliptical galaxy (EG) is considered. Perturbations due to the gravitation of the galaxy are taken into account, taking it to be a two-layer system together with its halo: an inner ellipsoid, representing the lumious part of the galaxy, and a homeoid, representing space filled with dark matter between inner and outer ellipsoidal boundaries. The ellipsoids are taken to be homothetic and to have a common center, with the boundary of the outer ellipsoid coincident with the boundary of the galactic halo. The luminous part of the EG and the homeoid have different densities. The motion of the star near a globular cluster occurs outside the luminous part of the EG, but inside the homeoid. The concept of the “vicinity of the globular cluster” is concretized using the concept of a “sphere of influence” (and the gravitational sphere and Hill gravitational sphere). Stellar motions inside and outside the sphere of influence of the globular cluster are considered, and the region of possible motions is determined. A quasi-integral and surfaces of minimum energy are found, which under certain conditions can be transformed into an analog of the Jacobi integral and surfaces of zero velocity. The Lyapunov stability of the stationary solutions obtained is established. The results are applied to model EGs whose parameters coincide with those of NGC 4472 (M49), NGC 4636, and NGC 4374, which contain a large number of globular clusters, and are presented in the form of figures and tables. Using these galaxies as examples, it is shown that studying stellar motions, and also determining the libration points and establishing their stability, requires use of an exact, rather than an approximate, expression for the potential of the luminous part of the elliptical galaxy.     

Comparison between two-and three-dimensional modeling of the structure of an accretion disk in a binary system

We compare two-and three-dimensional modeling of gas-dynamical processes in the accretion disk of a binary system. The origin of spiral waves and the loss of the angular momentum related to them are considered. It is concluded that a steady state of the disk cannot exist without taking into account t he gas inflow from the donor star.     

Reverse rotation of the accretion disk in RW Aur A: Observations and a physical model

Speckle interferometry of the young binary system RW Aur was performed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences using filters with central wavelengths of 550 nm and 800 nm and pass-band halfwidths of 20 nm and 100 nm, respectively. The angular separation of the binary components was 1.448?? ± 0.005?? and the position angle of the system was 255.9° ±0.3° at the observation epoch (JD 2 454 255.9). We find using published data that these values have been changing with mean rates of +0.002??/yr and +0.02°/yr, respectively, over the past 70 years. This implies that the direction of the orbital motion of the binary system is opposite to the direction of the disk rotation in RW Aur A. We propose a physical model to explain the formation of circumstellar accretion disks rotating in the reverse direction relative to young binary stars surrounded by protoplanetary disks. Our model can explain the characteristic features of the matter flow in RWAur A: the high accretion rate, small size of the disk around the massive component, and reverse direction of rotation.     

Propagation of a fast magnetoacoustic shock wave in the magnetosphere of an active region

The propagation of a fast magnetoacoustic shock wave the magnetosphere of a solar active region is considered the nonlinear geometrical acoustics approximation. The magnetic field is modeled as a subphotospheric magnetic dipole embedded in the radial field of the quiet corona. The initial parameters of the wave are specified at a spherical surface in the depths of the active region. The wave propagates asymmetrically and is reflected from regions of the strong magnetic field, which results in the radiation of the wave energy predominantly upwards. Substantial gradients in the Alfvén speed facilitate appreciable growth in the wave intensity. Non-linear damping of the wave and divergence of the wave front lead to the opposite effect. Analysis of the joint action of these factors shows that a fast magnetoacoustic perturbation outgoing from an active region can correspond to a shock wave of moderate intensity. This supports the scenario in which the primary source of the coronal wave is an eruptive filament that impulsively expands in the magnetosphere of an active region.     

P–T paths in the Handöl area, central Scandinavia: record of Caledonian accretion of outboard rocks to the Baltoscandian margin

The Seve–Köli Nappe Complex is widespread in the Scandinavian Caledonides and is composed of units representing parts of the Baltoscandian margin (Seve Nappes) now overlain by magmatic–sedimentary rocks (Köli Nappes) derived from west of this margin. The metamorphic evolution of Köli and Seve units has been studied in the Handöl area, central Scandinavian Caledonides, where a fragmented ophiolite with cover sequence in the lower Köli units is thrust over the higher grade Seve units. Thermobarometry constrains metamorphic conditions to 490–570° C/950–600 MPa, with a slight downwards increase in grade, for the lower Köli (Bunnerviken lens), 520–620° C/1000–600 MPa for the upper Seve (Täljstensvalen Complex), 630–740° C/750–650 MPa for the middle Seve (Snasahögarna Nappe) and 480–600° C/1150–1000 MPa for the lower Seve (Blåhammarfjället Nappe).
P–T paths during garnet growth have been constructed for all units, except the highest grade middle Seve. These paths record heating at the base of the Köli and cooling in the underlying Seve units. Pressure increase during garnet growth is indicated for all units leading to anticlockwise P–T paths in the Seve. The results imply thermal convergence with time for all units and spatial convergence in metamorphic grade in the Köli. It is suggested that the contrasting metamorphic histories on either side of the Seve–Köli boundary resulted from the emplacement of relatively colder Köli rocks on top of relatively hotter Seve rocks and that emplacement of structurally higher units contributed to the increase in pressure.     

Gamma-ray bursts as a result of an interaction of a supernova shock with the star-companion in a binary system

Interaction of a fast shock wave generated during a supernova explosion with a magnetized star-companion of the supernova precursor produces a current sheet. We consider an evolution of this current sheet and show that a singularity (shock) is formed in finite time within the ideal magnetohydrodynamics framework. Charged particles (electrons) are accelerated in the vicinity of the singularity, and their distribution function has a plateau up to the energies of the order of 10⁴ mc ². These fast particles radiate in the γ-range in the strong magnetic field of the current sheet (B ≃ 10⁶ G). Radiation is concentrated within a narrow angle around the current sheet, Δθ ≃ 3 × 10⁻⁴, and its spectrum has the maximum at several hundreds of keV. Presented calculations confirm the model of cosmological GRBs proposed earlier by Istomin & Komberg.     

Widespread relics of high-pressure metamorphism confirm major terrane accretion in Ecuador: a new example from the Northern Andes

Metamorphic rocks from two sections through the North Andean Cordillera Real in Ecuador contain phengites as relics of high-pressure (HP) metamorphism. Their maximum Si contents per formula unit are (1) 3.25 in Al-rich metapelites, (2) 3.35 in ordinary metapelites and garnet-rich metabasites, and (3) 3.45 in garnet-bearing orthogneisses. Detailed pressure–temperature paths were derived by calculating P–T pseudosections. Garnet-bearing metapelite and metabasite yielded a P–T path starting at 1.4 GPa and 525°C. Peak temperature conditions were 560°C at <1.4 GPa. The retrograde path passed through 0.7–0.8 GPa at 500°C. An adjacent metagranitoid experienced a similar P–T evolution at slightly lower temperatures of about 30–50°C. This is the first reported occurrence of HP metamorphism in the Cordillera Real; it is widespread and evidently was the result of continental collision. We hypothesize that a single microcontinent collided with the South American continental margin in Early Cretaceous time. In contrast to our findings, previous models have simply suggested the amalgamation of several terranes in the late Mesozoic or earlier in the region of Ecuador. Therefore, we propose that a widespread search for HP relics should be undertaken. Such relics provide a general criterion for defining subducted terranes and their respective boundaries.     

The Araçuaí-West-Congo Orogen in Brazil: an overview of a confined orogen formed during Gondwanaland assembly

The Neoproterozoic Adamastor-Brazilide Ocean was generated during the breakup of the Rodinia supercontinent, and remnants of its oceanic lithosphere have been found in the Brasiliano-Pan African orogenic system that includes the Araçuaí, West-Congo, Brasília, Ribeira, Kaoko, Dom Feliciano, Damara and Gariep belts. The Araçuaí and the West-Congo belts are counterparts of the same Neoproterozoic orogen. The first belt comprises two thirds of the Araçuaí-West-Congo Orogen. This orogen is rather unique owing to its confined nature within the embayment outlined by the São Francisco and Congo cratons. In spite of this, the presence of ophiolitic remnants, and a calc-alkaline magmatic arc, indicate that the basin/orogen evolution comprise both oceanic spreading and consumption. It is assumed that coeval Paramirim and Sangha aulacogens played a key role by making room for the Araçuaí-West-Congo Basin. Sedimentary successions record all major stages of a basin that evolved from continental rift, when glaciation-related sedimentation was very significant, to passive margin. Rifting started around 1.0–0.9 Ga. The oceanic stage is constrained by an ophiolitic remnant dated at 0.8 Ga. If the cratonic bridge that once linked the São Francisco and Congo palaeocontinental regions did not hinder the opening of an ocean basin, it certainly limited its width. As a consequence, only a narrow oceanic lithosphere was generated, and it was subducted afterwards. This is also suggested by orogenic calc-alkaline granitoids occuping a small area of the orogen. Geochronological data for pre-, syn- and late-collisional granitoids indicate that the orogenic stage lasted from 625 Ma to 570 Ma. A period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535–500 Ma. The features of the Araçuaí-West-Congo Orogen suggest the development of a complete Wilson Cycle in a branch of the Adamastor Ocean, which can be interpreted as a gulf with limited generation of oceanic lithosphere.     

Light-curve synthesis for close binaries: An extended shock as an analog of a hot spot in cataclysmic variables

An algorithm is presented for the synthesis of the light curve of a close binary system consisting of a red dwarf that fills its Roche lobe and a spherical white dwarf. The spherical component is surrounded by an elliptical, geometrically thick accretion disk. The code models an extended shock located along the edge of the stream near the outer boundary of the disk. The observational manifestations of the shock show that it can be considered as an analog of a hot spot at the edge of the disk. Synthetic light curves for the SU UMa system OY Car at various phases of its activity indicate that the model can describe both typical and peculiar light curves for this cataclysmic variable reasonably well.     

The Marl Slate: a model for the precipitation of calcite, dolomite and sulphides in a newly formed anoxic sea

Detailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ¹³C, δ¹⁸O, δ³⁴S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite-rich and dolomite-rich carbonates. C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ¹⁸O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for ¹⁸O and 1·5% for ¹³C in calcite. The δ³⁴S values of pyrite are isotopically light (mean value = - 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins. The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in the Zechstein Sea.     

Structural kinematics,metamorphic P–T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south‐central Tibet: implication for a revised channel flow

A specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ～749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ～4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.     

Anti-clockwise P–T paths in the lower crust: an example from a kyanite-bearing regional aureole, George Sound, New Zealand

The George Sound Paragneiss (GSP) represents a rare Permo-Triassic unit in Fiordland that occurs as a km-scale pillar to gabbroic and dioritic gneiss of c . 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser-ablation U–Pb ages as young as c . 190 Ma and ¹⁷⁶Hf/¹⁷⁷Lu >0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S₁ foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S₂ foliation. S₁ staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T  <   650 °C and P <  8 kbar. Contact aureole and S₂ assemblages include Mg-rich, Ca-poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S₂ kyanite-bearing matrix assemblages and Ca-enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post-S₂ high- P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high- P anti-clockwise P–T history for the GSP involving: (i) mid- P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high- P and (iv) high- P cooling induced by tectonic juxtaposition of cooler country rock.     

The Gundahl Complex of the New England fold belt,Eastern Australia: a tectonic mélange formed in a palaeozoic subduction complex

The New England Fold Belt of eastern Australia preserves a Palaeozoic fore-arc terrain with a magmatic arc, fore-arc basin and a subduction complex. The Gundahl Complex is a tectonic mélange of regional extent in the subduction complex. The matrix and slabs of the Gundahl Complex have six mappable lithofacies: argillite, greywacke-argillite, greywacke, argillite-tuff, bedded chert and greenstone. The argillite matrix is pervasively sheared with many slickensided shear fractures. Locally the matrix is formed by highly sheared greenstone. Greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell. Folds, in places with axial-surface spaced cleavage, are common within those slabs comprised of well-bedded sequences. Bedding-plane shear and faulting at a high angle to bedding also occur in these slabs. On a map-scale much of the Gundahl Complex comprises slabs up to 10 km long in imbricate fault-bounded slices which repeat the disrupted pre-mélange stratigraphic sequence. Elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the Gundahl Complex. The unit is believed to have formed by accretion, imbrication and subsequent tectonic disruption of arc-derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex.