14820089
On Kinetic Theory Viscosity in a Rotating Gas
Clarke and Pringle (2004) derived a proper viscosity formula in a rotating gas by applying mean free path theory. We study their argument in detail and show that their result can be derived with a much simpler calculational procedure and a physically clearer picture.
119460856
Is Angular Momentum in an Accretion Disk Transported Inwards
The derivation of the viscosity formula of accretion disks appearing in a number of textbooks is based on a mean free path theory of gas particles. However, this procedure, when followed precisely, leads to the incorrect conclusion that the angular momentum of an accretion disk flows from the outer part to the inner part.
17908059
0 Numerical simulation of accretion discs in close binary systems and discovery of spiral shocks
The history of hydrodynamic numerical simulations for accretion disks in close binary systems is reviewed, in which emphasis is placed, in particular, on the facts that spiral shock waves were numerically found in 1986 by researchers including one of the present authors and that spiral structure was discovered in IP Pegasi in 1997 by Steeghs et al. The results of our two and three-dimensional numerical simulations in recent years are then summarized, with comparison being made with observations.
16747752
Comments on “Angular Momentum Transport in Quasi-Keplerian Accretion Disks”
Subramanian, Pujari and Becker (2004) claim that the correct expression for the angular momentum transport in an accretion disc, which is proportional to dΩ/dR, can be derived on the basis of the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disc. We study their argument and show that their derivation contains several fundamental errors: 1) the biased choice of the desired formula from an infinite number of formulae; 2) the biased choice of parcel trajectories; and 3) confusion regarding the reference frames. Following 1) we could derive, for example, a (invalid) formula in which the angular momentum transport is proportional to dv Φ /dR, and from 2) we could even prove that the angular momentum transport is either inward or null. We present the correct approach to the problem of angular momentum transport in an accretion disc in terms of mean free path theory.
119334547
THE STRUCTURAL EVOLUTION OF SUBSTRUCTURE
We investigate the evolution of substructure in cold dark matter (CDM) halos using N-body simulations of tidal stripping of substructure halos within a static host potential. We find that halos modeled following the Navarro, Frenk, & White (NFW) mass profile lose mass continuously because of tides from the massive host, leading to the total disruption of satellite halos with small tidal radii. Although mass is predominantly stripped from the outer regions, tidal heating also causes the halo to expand and the central density to decrease after each pericentric passage, when mass loss preferentially occurs. As a result, simple models based on the tidal-limit approximation underestimate significantly the tidal mass loss over several orbits. The equilibrium structure of stripped NFW halos depends mainly on the fraction of mass lost and can be expressed in terms of a simple correction to the original NFW profile. We apply these results to substructure in the Milky Way and conclude that the dark matter halos surrounding its dwarf spheroidal (dSph) satellites have circular velocity curves that peak well beyond the luminous radius at velocities significantly higher than expected from the stellar line-of-sight velocity dispersion. Our modeling suggests that the true tidal radii of dSphs lie well beyond the putative tidal cutoff observed in the surface brightness profile, suggesting that the latter are not really tidal in origin but rather features in the light profile of limited dynamical relevance. For Draco in particular, our modeling implies that its tidal radius is much larger than derived by Irwin & Hatzidimitriou, lending support to the interpretation of recent Sloan Digital Sky Survey data by Odenkirchen et al. Similarly, our model suggests that Carina’s halo has a peak circular velocity of order � 55 km s � 1 , which may help explain how this small galaxy has managed to retain enough gas to undergo several bursts of star formation. Our results imply a close correspondence between the most massive substructure halos expected in a CDM universe and the known satellites of the Milky Way and suggest that only substructure halos with peak circular velocities below 35 km s � 1 lack readily detectable luminous counterparts. Subject headings: cosmology: theory — dark matter — galaxies: evolution — galaxies: halos — galaxies: structure — methods: N-body simulations
59385487
Beam position monitor for the LNLS UVX synchrotron light source
The beam orbit measuring system for the LNLS UVX ring is presented. The system consists of 24 strip-line beam position monitors and associated electronics. The beam position monitors (BPM's) are capable of detecting down to 10 mA of stored beam current or 100 ns-long LINAC injection pulses (single pulse detection mode) with 0.05 mm resolution. Acquisition electronics is divided into two parts: four fast-acquisition channels for the pulsed mode operation (used during injection) and a single PIN-switch multiplexed channel for CW operation (closed orbit measurement). Experimental results obtained on a characterization bench are presented and compared with theoretical estimates.
119946613
Mass production of tile/fiber units for the CDF plug upgrade EM calorimeter
Abstract We have fabricated scintillating tile/fiber units for the CDF plug upgrade electromagnetic calorimeter. The total number of tile/fiber systems amounts to ∼23 000. We describe the quality control tests of scintillator plates, optical fibers, and tile/fiber units which we placed at various stages in the fabrication procedures. We have completed the production retaining the tile/fiber units which satisfy our requirements.
14663098
Disk Galaxy Rotation Curves in Triaxial CDM Halos
We use N-body hydrodynamical simulations to study the structure of disks in triaxial potentials resembling CDM halos. Our analysis focuses on the accuracy of the dark mass distribution inferred from rotation curves derived from simulated long-slit spectra. We consider a massless disk embedded in a halo with axis ratios of 0.5:0.6:1.0 and with its rotation axis aligned with the minor axis of the halo. Closed orbits for the gaseous particles deviate from coplanar circular symmetry, resulting in a variety of long-slit rotation curve shapes, depending on the orientation of the disk relative to the line of sight. Rotation curves may thus differ significantly from the spherically-averaged circular velocity profile of the dark matter halo. "Solid-body" rotation curves--typically interpreted as a signature of a constant density core in the dark matter distribution--are obtained about 25% of the time for random orientations although the dark matter follows the cuspy density profile proposed by Navarro, Frenk & White (NFW). We conclude that the discrepancies reported between the shape of the rotation curve of low surface brightness galaxies and the structure of CDM halos may be resolved once the complex effects of halo triaxiality on the dynamics of the gas component is properly taken into account.
124098674
Search for new neutral gauge bosons in 1.8-TeV proton-antiproton collisions