40,737 research outputs found
Space shuttle plume/simulation application
An analysis of pressure and strain-gage data from space shuttle wind tunnel test IA119 and IA138 was performed to define the influence on aerodynamic characteristics resulting from the main propulsion system and solid rocket booster plumes. Aerodynamic characteristics of each of the elements, the components and total vehicle of the space shuttle vehicle during ascent flight was considered. Pressure data were obtained over the aft portions of the space shuttle wind tunnel model in addition to wing and elevon gage data
Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13
An analysis of the CN and CH band strengths measured in a large sample of M71
and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic
spectra to quantify the underlying C and N abundances. In the case of M71 it is
found that the observed CN and CH band strengths are best matched by the
{\it{identical}} C/N/O abundances which fit the bright giants, implying: 1)
little if any mixing is taking place during red giant branch ascent in M71, and
2) a substantial component of the C and N abundance inhomogeneities is in place
before the main sequence turn-off. The unlikelihood of mixing while on the main
sequence requires an explanation for the abundance variations which lies
outside the present stars (primordial inhomogeneities or intra-cluster self
enrichment). For M13 it is shown that the 3883\AA CN bands are too weak to be
measured in the spectra for any reasonable set of expected compositions. A
similar situation exists for CH as well. However, two of the more luminous
program stars do appear to have C abundances considerably greater than those
found among the bright giants thereby suggesting deep mixing has taken place on
the M13 red giant branch.Comment: 14 pages, 4 figures, accepted for publication by A
Entanglement in a Noninteracting Mesoscopic Structure
We study the time dependent electron-electron and electron-hole correlations
in a mesoscopic device which is splitting an incident current of free fermions
into two spatially separated particle streams. We analyze the appearance of
entanglement as manifested in a Bell inequality test and discuss its origin in
terms of local spin-singlet correlations already present in the initial channel
and the action of post-selection during the Bell type measurement. The time
window over which the Bell inequality is violated is determined in the
tunneling limit and for the general situation with arbitrary transparencies. We
compare our results with alternative Bell inequality tests based on coincidence
probabilities.Comment: 9 pages, 2 figure
Dominant mobility modulation by the electric field effect at the LaAlO_3 / SrTiO_3 interface
Caviglia et al. [Nature (London) 456, 624 (2008)] have found that the
superconducting LaAlO_3 / SrTiO_3 interface can be gate modulated. A central
issue is to determine the principal effect of the applied electric field. Using
magnetotransport studies of a gated structure, we find that the mobility
variation is almost five times as large as the sheet carrier density.
Furthermore, superconductivity can be suppressed at both positive and negative
gate bias. These results indicate that the relative disorder strength strongly
increases across the superconductor-insulator transition.Comment: 4 pages, 4 figure
Large-Scale Structure Shocks at Low and High Redshifts
Cosmological simulations show that, at the present time, a substantial
fraction of the gas in the intergalactic medium (IGM) has been shock-heated to
T>10^5 K. Here we develop an analytic model to describe the fraction of
shocked, moderately overdense gas in the IGM. The model is an extension of the
Press & Schechter (1974) description for the mass function of halos: we assume
that large-scale structure shocks occur at a fixed overdensity during nonlinear
collapse. This in turn allows us to compute the fraction of gas at a given
redshift that has been shock-heated to a specified temperature. We show that,
if strong shocks occur at turnaround, our model provides a reasonable
description of the temperature distribution seen in cosmological simulations at
z~0, although it does overestimate the importance of weak shocks. We then apply
our model to shocks at high redshifts. We show that, before reionization, the
thermal energy of the IGM is dominated by large-scale structure shocks (rather
than virialized objects). These shocks can have a variety of effects, including
stripping ~10% of the gas from dark matter minihalos, accelerating cosmic rays,
and creating a diffuse radiation background from inverse Compton and cooling
radiation. This radiation background develops before the first stars form and
could have measurable effects on molecular hydrogen formation and the spin
temperature of the 21 cm transition of neutral hydrogen. Finally, we show that
shock-heating will also be directly detectable by redshifted 21 cm measurements
of the neutral IGM in the young universe.Comment: 12 pages, 8 figures, submitted to Ap
Excess sub-millimetre emission from GRS 1915+105
We present the first detections of the black hole X-ray binary GRS 1915+105
at sub-millimetre wavelengths. We clearly detect the source at 350 GHz on two
epochs, with significant variability over the 24 hr between epochs.
Quasi-simultaneous radio monitoring indicates an approximately flat spectrum
from 2 - 350 GHz, although there is marginal evidence for a minimum in the
spectrum between 15 - 350 GHz. The flat spectrum and correlated variability
imply that the sub-mm emission arises from the same synchrotron source as the
radio emission. This source is likely to be a quasi-steady partially
self-absorbed jet, in which case these sub-mm observations probe significantly
closer to the base of the jet than do radio observations and may be used in
future as a valuable diagnostic of the disc:jet connection in this source.Comment: 5 pages, 3 figures, accepted for publication in MNRA
C and N Abundances in Stars At the Base of the Red Giant Branch in M5
We present an analysis of a large sample of moderate resolution Keck LRIS
spectra of subgiant (V \sim 17.2) and fainter stars in the Galactic globular
cluster M5 (NGC 5904) with the goal of deriving C and N abundances.
Star-to-star stochastic variations with significant range in both [C/Fe] and
[N/Fe] are found at all luminosities extending to the bottom of the RGB at M_V
\sim +3. Similar variations in CH appear to be present in the main sequence
turnoff spectra. There is no sign of a change in the behavior of C and N with
evolutionary stage over the full range in luminosity of the RGB and SGB. The C
and N abundances appear strongly anti-correlated, as would be expected from the
CN-cycle processing of stellar material. Yet the present stars are considerably
fainter than the RGB bump, the point at which deep mixing is believed to set
in. On this basis, while the observed abundance pattern is consistent with
proton capture nucleosynthesis, we infer that the site of the reactions is
likely not within the present sample, but rather in a population of more
massive (2 -- 5 M(Sun)) now defunct stars. The range of variation of the N
abundances is very large and the sum of C+N increases as C decreases. To
reproduce this requires the incorporation not only of CN but also of
ON-processed material. Furthermore, the existence of this correlation is quite
difficult to reproduce with an external mechanism such as ``pollution'' with
material processed in a more massive AGB star, which mechanism is fundamentally
stochastic in nature. We therefore suggest that although the internal mixing
hypothesis has serious flaws,new theoretical insights are needed and it should
not be ruled out yet. (abridged)Comment: Slightly updated version to conform to that accepted by the A
Wave Function Collapse in a Mesoscopic Device
We determine the non-local in time and space current-current cross correlator
in a mesoscopic conductor with a
scattering center at the origin. Its excess part appearing at finite voltage
exhibits a unique dependence on the retarded variable , with the Fermi
velocity. The non-monotonic dependence of the retardation on and its
absence at the symmetric position is a signature of the wave
function collapse, which thus becomes amenable to observation in a mesoscopic
solid state device.Comment: 7 pages, 2 fugure
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