3,835 research outputs found
Adhesive for aluminum withstands cryogenic temperatures
Polyurethane adhesive mixed to various proportions with milled glass fibers match the thermal characteristics of 2014-T6 aluminum at cryogenic temperatures
How Saturated are Absorption Lines in the Broad Absorption Line Quasar PG 1411+442 ?
Recently, convincing evidence was found for extremely large X-ray absorption
by column densities in broad absorption line quasars. One
consequence of this is that any soft X-ray emission from these QSOs would be
the scattered light or leaked light from partially covering absorbing material.
A detection of the unabsorbed soft X-ray and absorbed hard X-ray compo nent
will allow to determine the total column density as well as the effective
covering factor of the absorbing material, which can be hardly obtained from
the UV absorption lines. Brinkmann et al. (1999) showed that both the
unabsorbed and absorbed components are detected in the nearby very bright broad
absorption line quasar PG 1411+442. In this letter, we make a further analysis
of the broad band X-ray spectrum and the UV spectrum from HST, and demonstrate
that broad absorption lines are completely saturated at the bottom of
absorption troughs.Comment: 6 pages, 3 postscript figures. to appear in Astrophy. J. Letter
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
Relationship between low-frequency electric-field fluctuations and ion conics around the cusp/cleft region
We investigated the relationship between low-frequency (0.2-4.0 Hz) electric-field fluctuations (LEFs) and ion conics around the dayside cusp/cleft region in the altitude range from 5000 to 10000km from observations made by the Akebono satellite. Ion conics were generally associated with intense LEFs. We found a significant correlation between the power spectral density of LEFs at any frequency and the energy of simultaneously observed ion conics. Ion conics with a conic angle near 90 deg and those more aligned with magnetic field lines both had an equivalent correlation with the local intensity of the LEFs. The LEFs associated with near-perpendicular ion conics were, however, generally more intense than those associated with folded conics. The difference was clearer for low-energy conics. These results are in agreement with a scenario of height-integrated heating of ions and energization of ions by electromagnetic energy supplied by local LEFs. Ions generally stay in the energization region during their upward motion along the field line, so that more folded ion conics with weak energization reach the same energy level as near-perpendicular conics with strong energization, due to the difference in integration time. The limit on residence time in the intense heating region causes the clearer difference for low-energy conics. We set up a simple model to examine the relationship between the energization rate and the evolution of ion conics along the field lines, and obtained good agreement with the observation results
A lack of classical Cepheids in the inner part of the Galactic disk
Recent large-scale infrared surveys have been revealing stellar populations
in the inner Galaxy seen through strong interstellar extinction in the disk. In
particular, classical Cepheids with their period-luminosity and period-age
relations are useful tracers of Galactic structure and evolution. Interesting
groups of Cepheids reported recently include four Cepheids in the Nuclear
Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga
et al. and those spread across the inner part of the disk reported by Dekany
and collaborators. We here report our discovery of nearly thirty classical
Cepheids towards the bulge region, some of which are common with Dekany et al.,
and discuss the large impact of the reddening correction on distance estimates
for these objects. Assuming that the four Cepheids in the NSD are located at
the distance of the Galactic Centre and that the near-infrared extinction law,
i.e. wavelength dependency of the interstellar extinction, is not
systematically different between the NSD and other bulge lines-of-sight, most
of the other Cepheids presented here are located significantly further than the
Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region
of the Galactic disk except the NSD. Recent radio observations show a similar
distribution of star-forming regions.Comment: 8 pages, 4 figures, accepted for publication in MNRA
Multiscale modeling and simulation for polymer melt flows between parallel plates
The flow behaviors of polymer melt composed of short chains with ten beads
between parallel plates are simulated by using a hybrid method of molecular
dynamics and computational fluid dynamics. Three problems are solved: creep
motion under a constant shear stress and its recovery motion after removing the
stress, pressure-driven flows, and the flows in rapidly oscillating plates. In
the creep/recovery problem, the delayed elastic deformation in the creep motion
and evident elastic behavior in the recovery motion are demonstrated. The
velocity profiles of the melt in pressure-driven flows are quite different from
those of Newtonian fluid due to shear thinning. Velocity gradients of the melt
become steeper near the plates and flatter at the middle between the plates as
the pressure gradient increases and the temperature decreases. In the rapidly
oscillating plates, the viscous boundary layer of the melt is much thinner than
that of Newtonian fluid due to the shear thinning of the melt. Three different
rheological regimes, i.e., the viscous fluid, visco-elastic liquid, and
visco-elastic solid regimes, form over the oscillating plate according to the
local Deborah numbers. The melt behaves as a viscous fluid in a region for
, and the crossover between the liquid-like and
solid-like regime takes place around (where
is the angular frequency of the plate and and
are Rouse and relaxation time, respectively).Comment: 13pages, 12figure
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