1,480 research outputs found
General Aviation Turbine Engine (GATE) study
The feasibility of turbine engines for the smaller general aviation aircraft was investigated and a technology program for developing the necessary technology was identified. Major results included the definition of the 1988 general aviation market, the identification of turboprop and turboshaft engines that meet the requirements of the aircraft studies, a benefit analysis showing the superiority of gas turbine engines for portions of the market studied, and detailed plans for the development of the necessary technology
Colloidal diffusion and hydrodynamic screening near boundaries
The hydrodynamic interactions between colloidal particles in small ensembles are measured at varying distances from a no-slip surface over a range of inter-particle separations. The diffusion tensor for motion parallel to the wall of each ensemble is calculated by analyzing thousands of particle trajectories generated by blinking holographic optical tweezers and by dynamic simulation. The Stokesian
Dynamics simulations predict similar particle dynamics. By separating the dynamics into three classes of modes: self, relative and collective diffusion, we observe qualitatively different behavior depending on the relative magnitudes of the distance of the ensemble from the wall and the inter-particle separation. A simple picture of the pair-hydrodynamic interactions is developed, while many-body-hydrodynamic interactions give rise to more complicated behavior. The results demonstrate that the
effect of many-body hydrodynamic interactions in the presence of a wall is much richer than the single
particle behavior and that the multiple-particle behavior cannot be simply predicted by a superposition of pair interactions
G328.4+0.2 : A large and luminous Crab-like supernova remnant
We report on radio continuum and HI observations of the radio source
G328.4+0.2 using the Australia Telescope Compact Array. Our results confirm
G328.4+0.2 to be a filled-center nebula with no surrounding shell, showing
significant linear polarization and an almost flat spectral index. These
results lead us to conclude that G328.4+0.2 is a Crab-like, or ``plerionic'',
supernova remnant (SNR), presumably powered by an unseen central pulsar. HI
absorption towards G328.4+0.2 puts a lower limit on its distance of 17.4 +/-
0.9 kpc, making it the largest (D=25 pc) and most luminous (L_R = 3e35 erg/s)
Crab-like SNR in the Galaxy. We infer G328.4+0.2 to be significantly older than
the Crab Nebula, but powered by a pulsar which is fast spinning (P<20 ms) and
which has a comparatively low magnetic field (B<1e12 G). We propose G328.4+0.2,
G74.9+1.2 and N157B as a distinct group of large-diameter, high-luminosity
Crab-like SNRs, all powered by fast-spinning low-field pulsars.Comment: 7 pages, 3 embedded EPS figures, uses emulateapj.sty. Accepted to
ApJ. Abstract corrected so that distance is now in kpc, not pc
Structure formation in binary colloids
A theoretical study of the structure formation observed very recently [Phys.
Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model
solely the dipole-dipole interaction of the particles is considered,
electrohidrodynamic effects are excluded. Based on molecular dynamics
simulations and analytic calculations we show that the total concentration of
the particles, the relative concentration and the relative dipole moment of the
components determine the structure of the colloid. At low concentrations the
kinetic aggregation of particles results in fractal structures which show a
crossover behavior when increasing the concentration. At high concentration
various lattice structures are obtained in a good agreement with experiments.Comment: revtex, 4 pages, figures available from authors due to size problem
Infrared Excess and Molecular Gas in the Galactic Worm GW46.4+5.5
We have carried out high-resolution (~3') HI and CO line observations along
one-dimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI
data with IRAS data, we have derived the distributions of I_100 excess and
tau_100 excess, which are respectively the 100 mum intensity and 100 mum
optical depth in excess of what would be expected from HI emission. In two
observed regions, we were able to make a detailed comparison of the infrared
excess and the CO emission. We have found that tau_100 excess has a very good
correlation with the integrated intensity of CO emission, W_CO, but I_100
excess does not. There are two reasons for the poor correlation between I_100
excess and W_CO: firstly, there are regions with enhanced infrared emissivity
without CO, and secondly, dust grains associated with molecular gas have a low
infrared emissivity. In one region, these two factors completely hide the
presence of molecular gas in the infrared. In the second region, we could
identify the area with molecular gas, but I_100 excess significantly
underestimates the column density of molecular hydrogen because of the second
factor mentioned above. We therefore conclude that tau_100 excess, rather than
I_100 excess, is an accurate indicator of molecular content along the line of
sight. We derive tau_100/N(H)=(1.00+-0.02)*10^-5~(10^20 cm^-2)^-1, and
X=N(H_2)/W_CO=~0.7*10^20 cm^-2 (K km s^-1)^-1. Our results suggest that I_100
excess could still be used to estimate the molecular content if the result is
multiplied by a correction factor xi_c=_HI/_H_2 (~2 in
the second region), which accounts for the different infrared emissivities of
atomic and molecular gas. We also discuss some limitations of this work.Comment: 10 pages, 9 postscript figures, uses aas2pp4.sty to be published in
Astrophyslcal Journa
Faint HI 21-cm Emission Line Wings at Forbidden-Velocities
We present the results of a search for faint HI 21-cm emission line wings at
velocities forbidden by Galactic rotation in the Galactic plane using the
Leiden/Dwingeloo HI Survey data and the HI Southern Galactic Plane Survey data.
These ``forbidden-velocity wings (FVWs)'' appear as protruding excessive
emission in comparison with their surroundings in limited (< 2 deg) spatial
regions over velocity extent more than ~20 km/s in large-scale (l-v) diagrams.
Their high-velocities imply that there should be some dynamical phenomena
associated. We have identified 87 FVWs. We present their catalog, and discuss
their distribution and statistical properties. We found that 85% of FVWs are
not coincident with known supernova remnants (SNRs), galaxies, or high-velocity
clouds. Their natures are currently unknown. We suspect that many of them are
fast-moving HI shells and filaments associated with the oldest SNRs that are
essentially invisible except via their HI line emission. We discuss other
possible origins.Comment: 41 pages, 14 figures, to be published in apj
Detection of thermal X-ray emission in the halo of the plerionic supernova remnant G21.5-0.9
The detection of a soft thermal X-ray component in the spectrum of a bright
knot in the halo of the plerion G21.5-0.9 is reported. Using a collisional
ionization equilibrium model for an hot optically thin plasma, a temperature
kev, a mass of 0.3--1.0 M and a density of 1.6-6
cm is derived. The spectral analysis suggests a possible overabundance
of Silicon with respect to the solar value in the knot; if this will be
confirmed this object may be a clump of shocked ejecta.Comment: 6 pages, 4 figures, Adv.Sp.Res. in press, proc of COSPAR Session E1.4
"Young Neutron Stars and Supernova Remnants",
http://www.astropa.unipa.it/Library/OAPA_preprints/ns.ps.g
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