29,812 research outputs found
Walk or wait?: An empirical analysis of street crossing decisions
We examine the behaviour of pedestrians wishing to cross a stream of trasffic at signalized intersections. We model each padestrian as making a desccrete crossing choice by comparing the gaps between vehicles in traffic to an individual-specific "critical gap" that characterizes the individual's minimal acceptable gap. We propose both parametric and nonparametric approaches to estimate the distribution of critical gaps in the population of pedestrians. To estimate the model, we gather field data on crossing decisions and vehicle flows at three intersections in New Delhi. The estimates provide information about heterogeneity in critical gaps across pedestrians and intersections and permits simulation of the effect of changes in traffic light sequence (flows)on pedestran crossing behavior and waiting times.
On General Off-Shell Representations of Worldline (1D) Supersymmetry
Every finite-dimensional unitary representation of the N-extended worldline
supersymmetry without central charges may be obtained by a sequence of
differential transformations from a direct sum of minimal Adinkras, simple
supermultiplets that are identifiable with representations of the Clifford
algebra. The data specifying this procedure is a sequence of subspaces of the
direct sum of Adinkras, which then opens an avenue for classification of the
continuum of so constructed off-shell supermultiplets.Comment: 21 pages, 5 illustrations; references update
The Infrared Extinction Law at Extreme Depth in a Dark Cloud Core
We combined sensitive near-infrared data obtained with ground-based imagers
on the ESO NTT and VLT telescopes with space mid-infrared data acquired with
the IRAC imager on the Spitzer Space Telescope to calculate the extinction law
A_\lambda/A_K as a function of \lambda between 1.25 and 7.76 micron to an
unprecedented depth in Barnard 59, a star forming, dense core located in the
Pipe Nebula. The ratios A_\lambda/A_K were calculated from the slopes of the
distributions of sources in color-color diagrams \lambda-K vs. H-K. The
distributions in the color-color diagrams are fit well with single slopes to
extinction levels of A_K ~ 7 (A_V ~ 59 mag). Consequently, there appears to be
no significant variation of the extinction law with depth through the B59 line
of sight. However, when slopes are translated into the relative extinction
coefficients A_\lambda/A_K, we find an extinction law which departs from the
simple extrapolation of the near-infrared power law extinction curve, and
agrees more closely with a dust extinction model for a cloud with a total to
selective absorption R_V=5.5 and a grain size distribution favoring larger
grains than those in the diffuse ISM. Thus, the difference we observe could be
possibly due to the effect of grain growth in denser regions. Finally, the
slopes in our diagrams are somewhat less steep than those from the study of
Indebetouw et al. (2005) for clouds with lower column densities, and this
indicates that the extinction law between 3 and 8 micron might vary slightly as
a function of environment.Comment: 22 pages manuscript, 4 figures (2 multipart), 1 tabl
Thermal Decomposition of Cross-linked Polybutadiene and its Copolymers
Polybutadiene and two copolymers containing butadiene and styrene have been cross-linked by thermal processes, by the use of initiators in solution, and by intimately mixing an initiator with the polymer and then heating this blend. The most efficient cross-linking process is the use of an intimate blend of the initiator and polymer. Most cross-linking processes lower the onset temperature of degradation, presumably because chain scission reactions occur simultaneously with cross-linking, while also increasing the fraction of non-volatile residue which is produced. It is believed that the density of cross-links is responsible for the increased yield of non-volatile residue
HST Observations of the Central-Cusp Globular Cluster NGC 6752. The Effect of Binary Stars on the Luminosity Function in the Core
We consider the effect of binary stars on the main-sequence luminosity
functions observed in the core of globular clusters, with specific reference to
NGC 6752. We find that mass segregation results in an increased binary fraction
at fainter magnitudes along the main-sequence. If this effect is not taken into
account when analyzing luminosity functions, erroneous conclusions can be drawn
regarding the distribution of single stars, and the dynamical state of the
cluster. In the core of NGC 6752, our HST data reveal a flat luminosity
function, in agreement with previous results. However, when we correct for the
increasing binary fraction at faint magnitudes, the LF begins to fall
immediately below the turn-off. This effect appears to be confined to the inner
core radius of the cluster.Comment: 10 pages, 3 figures Accepted to ApJ Lett Vol 513 Number
The Formation Rate of Blue Stragglers in 47 Tucanae
We investigate the effects of changes in the blue straggler formation rate in
globular clusters on the blue straggler distribution in the color-magnitude
diagram. We find that the blue straggler distribution is highly sensitive to
the past formation rate. Comparing our models to new UBV observations of a
region close to the core of 47 Tucanae suggests that this cluster may have
stopped forming blue straggler formation several Gyr ago. This cessation of
formation can be associated with an epoch of primordial binary burning which
has been invoked in other clusters to infer the imminence of core collapse.Comment: 17 pages, 9 figures, submitted to the Astrophysical Journa
On-chip cavity quantum phonodynamics with an acceptor qubit in silicon
We describe a chip-based, solid-state analogue of cavity-QED utilizing
acoustic phonons instead of photons. We show how long-lived and tunable
acceptor impurity states in silicon nanomechanical cavities can play the role
of a matter non-linearity for coherent phonons just as, e.g., the Josephson
qubit plays in circuit-QED. Both strong coupling (number of Rabi oscillations ~
100) and strong dispersive coupling (0.1-2 MHz) regimes can be reached in
cavities in the 1-20 GHz range, enabling the control of single phonons,
phonon-phonon interactions, dispersive phonon readout of the acceptor qubit,
and compatibility with other optomechanical components such as phonon-photon
translators. We predict explicit experimental signatures of the acceptor-cavity
system.Comment: 6 pages, 2 figures, PDFLaTeX. New version improves clarit
Aerobic stability of heat and orchardgrass round-bale silage
In Arkansas, silage is typically stored as balage in long rows of round bales wrapped in plastic film. It is important to evaluate the aerobic stability of this fermented forage when it is exposed to air, especially during the winter months when most of it is fed to livestock or sold as a cash crop. Two types of forage, orchardgrass (Dactylis glomerata) and wheat (Triticum aestivum), were harvested in May 2002 and stored as balage. Twenty-one bales of each balage type were unwrapped and exposed to air on 10 Dec. 2002 for 0, 2, 4, 8, 16, 24, or 32 d to evaluate aerobic stability. For both orchardgrass and wheat balage, final bale weight, dry matter (DM) content, and pH were not affected (P \u3e 0.05) by exposure time. Across both balage types, DM recoveries were â„97% for all bales, indicating that both types of balage were very stable when exposed to air. Concentrations of neutral detergent fiber (NDF) and 48-h ruminal in situ digestibility were not affected (P \u3e 0.05) by exposure time for either balage type. Concentrations of N were greater (P = 0.045) for orchardgrass balage exposed to air for 16 d or longer compared to balage sampled at exposure (d 0), but this response was not observed (P \u3e 0.05) for wheat balage. These results suggest that the balage evaluated in this trial was very stable after exposure to air for up to 32 d. This should allow for considerable flexibility with respect to feeding, transport, and marketing of balage during winter months without significant aerobic deterioration
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