9,808 research outputs found
Hydrodynamic Simulations of Counterrotating Accretion Disks
Hydrodynamic simulations have been used to study accretion disks consisting
of counterrotating components with an intervening shear layer(s).
Configurations of this type can arise from the accretion of newly supplied
counterrotating matter onto an existing corotating disk. The grid-dependent
numerical viscosity of our hydro code is used to simulate the influence of a
turbulent viscosity of the disk. Firstly, we consider the case where the gas
well above the disk midplane rotates with angular rate +\Omega(r) and that well
below has the same properties but rotates with rate -\Omega(r). We find that
there is angular momentum annihilation in a narrow equatorial boundary layer in
which matter accretes supersonically with a velocity which approaches the
free-fall velocity and the average accretion speed of the disk can be
enormously larger than that for a conventional \alpha-disk rotating in one
direction. Secondly, we consider the case of a corotating accretion disk for
rr_t. In this case we observed, that
matter from the annihilation layer lost its stability and propagated inward
pushing matter of inner regions of the disk to accrete. Thirdly, we
investigated the case where counterrotating matter inflowing from large radial
distances encounters an existing corotating disk. Friction between the
inflowing matter and the existing disk is found to lead to fast boundary layer
accretion along the disk surfaces and to enhanced accretion in the main disk.
These models are pertinent to the formation of counterrotating disks in
galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary
systems.Comment: LaTeX, 18 pages, to appear in Ap
Interspecific differences in the larval performance of Pieris butterflies (Lepidoptera: Pieridae) are associated with differences in the glucosinolate profiles of host plants
The tremendous diversity of plants and herbivores has arisen from a coevolutionary relationship characterized by plant defense and herbivore counter adaptation. Pierid butterfly species feed on Brassicales plants that produce glucosinolates as a chemical deterrent against herbivory. In turn, the larvae of pierids have nitrile specifier proteins (NSPs) that are expressed in their gut and disarm glucosinolates. Pierid butterflies are known to have diversified in response to glucosinolate diversification in Brassicales. Therefore, each pierid species is expected to have a spectrum of host plants characterized by specific glucosinolate profiles. In this study, we tested whether the larval performance of different Pieris species, a genus in Pieridae (Lepidoptera: Pieridae), was associated with plant defense traits of putative host plants. We conducted feeding assays using larvae of three Pieris species and 10 species of the Brassicaceae family possessing different leaf physical traits and glucosinolate profile measurements. The larvae of Pieris rapae responded differently in the feeding assays compared with the other two Pieris species. This difference was associated with differences in glucosinolate profiles but not with variations in physical traits of the host plants. This result suggests that individual Pieris species are adapted to a subset of glucosinolate profiles within the Brassicaceae. Our results support the idea that the host ranges of Pieris species depend on larval responses to glucosinolate diversification in the host species, supporting the hypothesis of coevolution between butterflies and host plants mediated by the chemical arms race
The Dense Plasma Torus Around the Nucleus of an Active Galaxy NGC 1052
A subparsec-scale dense plasma torus around an active galactic nucleus (AGN)
is unveiled. We report on very-long-baseline interferometry (VLBI) observations
at 2.3, 8.4, and 15.4 GHz towards the active galaxy NGC 1052. The convex
spectra of the double-sided jets and the nucleus imply that synchrotron
emission is obscured through free--free absorption (FFA) by the foreground cold
dense plasma. A trichromatic image was produced to illustrate the distribution
of the FFA opacity. We found a central condensation of the plasma which covers
about 0.1 pc and 0.7 pc of the approaching and receding jets, respectively. A
simple explanation for the asymmetric distribution is the existence of a thick
plasma torus perpendicular to the jets. We also found an ambient FFA absorber,
whose density profile can be ascribed to a spherical distribution of the
isothermal King model. The coexistence of torus-like and spherical
distributions of the plasma suggests a transition from radial accretion to
rotational accretion around the nucleus.Comment: 10 pages, to appear in Publ. Astron. Soc. Japan, vol.53, No.2 (2001
A new method of evaluating the side wall interference effect on airfoil angle of attack by suction from the side walls
A quantitative evaluation method of the suction effect from a suction plate on side walls is explained. It is found from wind tunnel tests that the wall interference is basically described by the summation form of wall interferences in the case of two dimensional flow and the interference of side walls
Hydrometeorology, Suspended Sediment and Conductivity in a Large Glacierized Basin, Slims River, Yukon Territory, Canada (1993-94)
The Slims River was monitored for global solar radiation, air temperature, discharge, suspended sediment, and dissolved load in 1993 and 1994. Peak seasonal discharge occurred late in the summer and reflects a typical glacierized basin hydrograph, with increased bare ice surfaces contributing strongly to discharge in July and August. Air temperature, rather than global solar radiation, was most strongly correlated with discharge in both years, but during sustained ablation, air temperature becomes a poor index of meltwater production. Precipitation was infrequent and of low magnitude. The variance in suspended sediment concentration could be explained only in part by discharge; frequent clockwise hysteresis and seasonal sediment concentration peaks unrelated to discharge variations also contributed to this variance. High concentrations of Ca2+ and Mg2+ in meltwaters reflect the lithological influence of carboniferous sedimentary rocks in the basin. Conductivity and individual cation concentrations decreased during both seasons and were inversely related to discharge. Diurnal conductivity amplitude was greatest during glacier melt, and frequent clockwise hysteresis was observed in both years.En 1993 et 1994, on a installé des appareils sur la rivière Slims pour mesurer le rayonnement solaire global, la température de l'air, le débit, les sédiments en suspension et la charge dissoute. Le débit saisonnier de pointe se produisait tard durant l'été et reflète l'hydrographe typique d'un bassin englacé, où les surfaces de glace vive plus étendues en juillet et en août contribuent fortement au débit. Au cours de ces deux années, la température de l'air plutôt que le rayonnement solaire global était très fortement corrélée au débit, mais la température de l'air devient un index médiocre de la production d'eau de fonte durant une période d'ablation intense. Les précipitations étaient rares et de faible intensité. La variance dans la concentration des sédiments en suspension pourrait s'expliquer en partie seulement par le débit; une hystérésis dextrorse fréquente et des pointes non reliées à la variation du débit dans la concentration saisonnière de sédiments contribuaient également à cette variance. De fortes concentrations de Ca2+ et de Mg2+ dans l'eau de fonte reflètent l'influence lithologique des roches sédimentaires carbonifères dans le bassin. La conductivité et la concentration en cations individuels diminuaient durant les deux saisons et étaient inversement reliées au débit. L'amplitude de conductivité diurne était la plus grande durant la fonte glaciaire, et on a observé une fréquente hystérésis dextrorse au cours des deux années
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
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