The role of the energy equation in the fragmentation of protostellar discs during stellar encounters

In this paper, we use high-resolution smoothed particle hydrodynamics (SPH) simulations to investigate the response of a marginally stable self-gravitating protostellar disc to a close parabolic encounter with a companion discless star. Our main aim is to test whether close brown dwarfs or massive planets can form out of the fragmentation of such discs. We follow the thermal evolution of the disc by including the effects of heating due to compression and shocks and a simple prescription for cooling and find results that contrast with previous isothermal simulations. In the present case we find that fragmentation is inhibited by the interaction, due to the strong effect of tidal heating, which results in a strong stabilization of the disc. A similar behaviour was also previously observed in other simulations involving discs in binary systems. As in the case of isolated discs, it appears that the condition for fragmentation ultimately depends on the cooling rate.Comment: 9 pages, 10 figures, accepted in MNRA

Studies of auroral X-ray imaging from high altitude spacecraft

Results of a study of techniques for imaging the aurora from a high altitude satellite at X-ray wavelengths are summarized. The X-ray observations allow the straightforward derivation of the primary auroral X-ray spectrum and can be made at all local times, day and night. Five candidate imaging systems are identified: X-ray telescope, multiple pinhole camera, coded aperture, rastered collimator, and imaging collimator. Examples of each are specified, subject to common weight and size limits which allow them to be intercompared. The imaging ability of each system is tested using a wide variety of sample spectra which are based on previous satellite observations. The study shows that the pinhole camera and coded aperture are both good auroral imaging systems. The two collimated detectors are significantly less sensitive. The X-ray telescope provides better image quality than the other systems in almost all cases, but a limitation to energies below about 4 keV prevents this system from providing the spectra data essential to deriving electron spectra, energy input to the atmosphere, and atmospheric densities and conductivities. The orbit selection requires a tradeoff between spatial resolution and duty cycle

APPLICATION OF THE MODULARIZATION CONCEPT TO SATELLITE TAPE RECORDERS

Application of the modularization concept to satellite tape recorder

Global properties of the nearby spiral M101

M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission

Quantum Oscillations in Magnetic Field Induced Antiferromagnetic Phase of Underdoped Cuprates : Application to Ortho-II YBa2Cu3O6.5

Magnetic field induced antiferromagnetic phase of the underdoped cuprates is studied within the t-t'-J model. A magnetic field suppresses the pairing amplitude, which in turn may induce antiferromagnetism. We apply our theory to interpret the recently reported quantum oscillations in high magnetic field in ortho-II YBa2Cu3O6.5 and propose that the total hole density abstracted from the oscillation period is reduced by 50% due to the antiferromagnetism.Comment: 5 pages, 3 figure

Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3

The electronic structure of the corundum-type transition-metal oxides V2O3 and Ti2O3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3d a_{1g} density of states, which is present in both compounds. The a_{1g} states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a_{1g} states, which is caused by hybridization with the e_g^{pi} bands. In contrast, the antibonding a_{1g} states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a_{1g} orbitals for the metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi} hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Rotational Symmetry Breaking in Sodium Doped Cuprates

For reasonable parameters a hole bound to a Na^{+} acceptor in Ca_{2-x}Na_{x}CuO_{2}Cl_{2} has a doubly degenerate ground state whose components can be represented as states with even (odd) reflection symmetry around the x(y) -axes. The conductance pattern for one state is anisotropic as the tip of a tunneling microscope scans above the Cu-O-Cu bonds along the x(y)-axes. This anisotropy is pronounced at lower voltages but is reduced at higher voltages. Qualitative agreement with recent experiments leads us to propose this effect as an explanation of the broken local rotational symmetry.Comment: 10 pages, 4 figure

Interlayer couplings and the coexistence of antiferromagnetic and d-wave pairing order in multilayer cuprates

A more extended low density region of coexisting uniform antiferromagnetism and d-wave superconductivity has been reported in multilayer cuprates, when compared to single or bilayer cuprates. This coexistence could be due to the enhanced screening of random potential modulations in inner layers or to the interlayer Heisenberg and Josephson couplings. A theoretical analysis using a renormalized mean field theory, favors the former explanation. The potential for an improved determination of the antiferromagnetic and superconducting order parameters in an ideal single layer from zero field NMR and infrared Josephson plasma resonances in multilayer cuprates is discussed.Comment: 6 pages, 2 figure