778 research outputs found
3D General Relativistic Simulations of Coalescing Binary Neutron Stars
We develop a 3 dimensional computer code to study a coalescing neutron star
binary. The code can currently follow the evolution up to two stars begin to
merge from two spherical stars of mass 1 solar mass and radius 8.9km with
separation 35.4km. As for coordinate conditions, we use conformal slicing and
pseudo-minimal distortion conditions. The evolution equations for the metric is
integrated using the CIP method while the van Leer's scheme is used to
integrate the equations for the matter. We present a few results of our
simulations including gravitational radiation.Comment: invited talk at Yukawa Internatinal Seminar (YKIS99) 17 pages, 11
figures, for associated movie files, see
http://astro.sc.niigata-u.ac.jp/~oohara/ykis99
Hoop Conjecture and Black Holes on a Brane
The initial data of gravity for a cylindrical matter distribution confined to
a brane are studied in the framework of the single-brane Randall-Sundrum
scenario. In this scenario, the 5-dimensional nature of gravity appears in the
short-range gravitational interaction. We find that a sufficiently thin
configuration of matter leads to the formation of a marginal surface, even if
the configuration is infinitely long. This implies that the hoop conjecture
proposed by Thorne does not hold on the brane: Even if a mass does not
become compacted into a region whose circumference in every
direction satisfies , black holes with horizons can form in
the Randall-Sundrum scenario.Comment: 7 pages, 1 figure, To be published in Progress of Theoretical Physics
Supplement No. 148 "Brane World: New Perspective in Cosmology
Numerical study on the hydrodynamic instability of binary stars in the first post Newtonian approximation of general relativity
We present numerical results on the hydrodynamic stability of coalescing
binary stars in the first post Newtonian(1PN) approximation of general
relativity. We pay particular attention to the hydrodynamical instability of
corotating binary stars in equilibrium states assuming the stiff polytropic
equation of state with the adiabatic constant . In previous 1PN
numerical studies on corotating binary stars in equilibrium states, it was
found that along the sequence of binary stars as a function of the orbital
separation, they have the energy and/or angular momentum minima where the
secular instability sets in, and that with increase of the 1PN correction, the
orbital separation at these minima decreases while the angular velocity there
increases. In this paper, to know the location of the innermost stable circular
orbit(ISCO), we perform numerical simulations and find where the hydrodynamical
instability along the corotating sequences of binary sets in. From the
numerical results, we found that the dynamical stability limit seems to exist
near the energy and/or angular momentum minima not only in the Newtonian, but
also in the 1PN cases. This means that the 1PN effect of general relativity
increases the angular frequency of gravitational waves at the ISCO.Comment: 16 pages(11 figures). To appear in Prog. Theor. Phys. vol.98(1997
Soft-breaking correction to hard supersymmetric relations: QCD correction to squark decay
Supersymmetric relations between dimensionless couplings receive finite
correction at one-loop when supersymmetry is broken softly. We calculate the
correction to the squark decay width to a quark and an
electroweak gaugino, which is found to be nonvanishing. Logarithmic correction
appears when the gluino is heavy.Comment: 12 pages plus 5 figures, macros include
Axionic Mirage Mediation
Although the mirage mediation is one of the most plausible mediation
mechanisms of supersymmetry breaking, it suffers from two crucial problems. One
is the \mu-/B \mu-problem and the second is the cosmological one. The former
stems from the fact that the B parameter tends to be comparable with the
gravitino mass, which is two order of magnitude larger than the other soft
masses. The latter problem is caused by the decay of the modulus whose
branching ratio into the gravitino pair is sizable. In this paper, we propose a
model of mirage mediation, in which Peccei-Quinn symmetry is incorporated. In
this axionic mirage mediation, it is shown that the PQ symmetry breaking scale
is dynamically determined around 10^{10-12} GeV due to the supersymmetry
breaking effects, and the \mu-problem can be solved naturally. Furthermore, in
our model, the lightest supersymmetric particle (LSP) is the axino, that is the
superpartner of the axion. The overabundance of the LSPs due to decays of
modulus/gravitino, which is the most serious cosmological difficulty in the
mirage mediation, can be avoided if the axino is sufficiently light. The
next-LSPs (NLSPs) produced by the gravitino decay eventually decay into the
axino LSPs, yielding the dominant component of the axinos remaining today. It
is shown that the axino with the mass of O(100) MeV is naturally realized,
which can constitute the dark matter of the Universe, with the free-streaming
length of the order of 0.1 Mpc. The saxion, the real scalar component of the
axion supermultiplet, can also be cosmologically harmless due to the dilution
of the modulus decay. The lifetime of NLSP is relatively long, but much shorter
than 1 sec., when the big-bang nucleosynthesis commences. The decay of NLSP
would provide intriguing collider signatures.Comment: reference added, typo correcte
Contact magnetoresistive head for perpendicular magnetic recording
科研費報告書収録論文(課題番号:07405052・基盤研究(A)(2)・H7~H9/研究代表者:中村, 慶久/テラビット・スピニック・データストレージの基礎検討
A Neural Correlate of Predicted and Actual Reward-Value Information in Monkey Pedunculopontine Tegmental and Dorsal Raphe Nucleus during Saccade Tasks
Dopamine, acetylcholine, and serotonin, the main modulators of the central nervous system, have been proposed to play important roles in the execution of movement, control of several forms of attentional behavior, and reinforcement learning. While the response pattern of midbrain dopaminergic neurons and its specific role in reinforcement learning have been revealed, the role of the other neuromodulators remains rather elusive. Here, we review our recent studies using extracellular recording from neurons in the pedunculopontine tegmental nucleus, where many cholinergic neurons exist, and the dorsal raphe nucleus, where many serotonergic neurons exist, while monkeys performed eye movement tasks to obtain different reward values. The firing patterns of these neurons are often tonic throughout the task period, while dopaminergic neurons exhibited a phasic activity pattern to the task event. The different modulation patterns, together with the activity of dopaminergic neurons, reveal dynamic information processing between these different neuromodulator systems
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