2,136 research outputs found
A redundant low power PCM telemeter for the orbiting solar observatory
Redundant low power PCM telemeter for OS
Report of the Explorations for a Route for the Pacific Railroad 1854
A description of Kansas land by Lt. E.G. Beckwith after his exploration of the area to discover a desierable route for the Pacific Railroad
Analyzing Extreme Programming and B-Trees with Sail
The emulation of Boolean logic is an unproven issue. Given the current status of metamorphic methodologies, hackers worldwide famously desire the evaluation of SMPs, which embodies the technical principles of complexity theory. In order to overcome this grand challenge, we propose a novel method for the development of expert systems (Sail), which we use to verify that randomized algorithms can be made symbiotic, âsmartâ, and concurrent
Colliding Particles in Highly Turbulent Flows
We discuss relative velocities and the collision rate of small particles
suspended in a highly turbulent fluid. In the limit where the viscous damping
is very weak, we estimate the relative velocities using the Kolmogorov cascade
principle.Comment: 5 pages, no figures, v2 contains additional result
A layered edge-on circumstellar disk around HK Tau B
We present the first high angular resolution 1.4mm and 2.7mm continuum maps
of the T Tauri binary system HK Tau obtained with the Plateau de Bure
Interferometer. The contributions of both components are well disentangled at
1.4mm and the star previously known to host an edge-on circumstellar disk, HK
Tau B, is elongated along the disk's major axis. The optically bright primary
dominates the thermal emission from the system at both wavelengths, confirming
that it also has its own circumstellar disk. Its non-detection in scattered
light images indicates that the two disks in this binary system are not
parallel. Our data further indicate that the circumprimary disk is probably
significantly smaller than the circumsecondary disk.
We model the millimeter thermal emission from the circumstellar disk
surrounding HK Tau B. We show that the disk mass derived from scattered light
images cannot reproduce the 1.4mm emission using opacities of the same
population of submicron dust grains. However, grain growth alone cannot match
all the observed properties of this disk. We propose that this disk contains
three separate layers: two thin outer surfaces which contain dust grains that
are very similar to those of the ISM, and a disk interior which is relatively
massive and/or has experienced limited grain growth with the largest grains
significantly smaller than 1mm. Such a structure could naturally result from
dust settling in a protoplanetary disk.Comment: Accepted fopr publication in A&A, 8 pages, 1 embedded figur
Vertical Structure of Stationary Accretion Disks with a Large-Scale Magnetic Field
In earlier works we pointed out that the disk's surface layers are
non-turbulent and thus highly conducting (or non-diffusive) because the
hydrodynamic and/or magnetorotational (MRI) instabilities are suppressed high
in the disk where the magnetic and radiation pressures are larger than the
plasma thermal pressure. Here, we calculate the vertical profiles of the {\it
stationary} accretion flows (with radial and azimuthal components), and the
profiles of the large-scale, magnetic field taking into account the turbulent
viscosity and diffusivity and the fact that the turbulence vanishes at the
surface of the disk.
Also, here we require that the radial accretion speed be zero at the disk's
surface and we assume that the ratio of the turbulent viscosity to the
turbulent magnetic diffusivity is of order unity. Thus at the disk's surface
there are three boundary conditions. As a result, for a fixed dimensionless
viscosity -value, we find that there is a definite relation between the
ratio of the accretion power going into magnetic disk winds to the
viscous power dissipation and the midplane plasma-, which is the ratio
of the plasma to magnetic pressure in the disk. For a specific disk model with
of order unity we find that the critical value required for a
stationary solution is , where the disk's
half thickness. For weaker magnetic fields, , we argue that
the poloidal field will advect outward while for it will
advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with .Comment: 5 pages, 3 figure
Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays
Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds
Molecular hydrogen emission in NGC 7027
The spatial distribution of the emission in the v = 1-0 S(1) line of H2 in the planetary nebula NGC 7027 is presented. The excited H2 molecules do not fill the same volume as the ionized gas and most likely reside near the outer edge of the nebula. The spatial resolution of the data, 5 arcsec, is insufficient to define the location of the molecules precisely. An upper limit to the strength of the v = 2-1 S(1) line is given; it is low enough to preclude simple ultraviolet fluorescence as the source of excitation. A simple shock model can fit the line ratio data, however, and there is enough energy in the expanding nebula to sustain such a shock. A rough estimate of 1 M_â to 4 M_â for the mass in the molecular cloud surrounding NGC 7027 is derived
Large dust particles in disks around T Tauri stars
We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars
in the Taurus-Auriga star-forming region obtained with the Very Large Array
with ~1.5" resolution and ~0.3 mJy rms sensitivity. For 10 objects, the
circumstellar emission has been spatially resolved. The large outer disk radii
derived suggest that the emission at this wavelength is mostly optically thin.
The millimetre spectral energy distributions are characterised by spectral
indices alpha = 2.3 to 3.2. After accounting for contribution from free-free
emission and corrections for optical depth, we determine dust opacity indices
beta in the range 0.5 to 1.6, which suggest that millimetre-sized dust
aggregates are present in the circumstellar disks. Four of the sources with
beta > 1 may be consistent with submicron-sized dust as found in the
interstellar medium. Our findings indicate that dust grain growth to
millimetre-sized particles is completed within less than 1 Myr for the majority
of circumstellar disks.Comment: 11 pages, 4 figure
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