6,675 research outputs found
Study of the technique of stellar occultation
The results are reported of a study of the stellar occultation technique for measuring the composition of the atmosphere. The intensity of starlight was monitored during the occultation using the Wisconsin stellar ultraviolet photometers aboard the Orbiting Astronomical Observatory (OAO-A2). A schematic diagram of an occultation is shown where the change in intensity at a given wavelength is illustrated. The vertical projection of the attenuation region is typically 60 km deep for molecular oxygen and 30 km deep for ozone. Intensity profiles obtained during various occultations were analyzed by first determining the tangential columm density of the absorbing gases, and then Abel inverting the column densities to obtain the number density profile. Errors are associated with each step in the inversion scheme and have been considered as an integral part of this study
Millisecond accuracy video display using OpenGL under Linux
To measure people’s reaction times to the nearest millisecond, it is necessary to know exactly when
a stimulus is displayed. This article describes how to display stimuli with millisecond accuracy on a
normal CRT monitor, using a PC running Linux. A simple C program is presented to illustrate how this
may be done within X Windows using the OpenGL rendering system. A test of this system is reported
that demonstrates that stimuli may be consistently displayed with millisecond accuracy. An algorithm
is presented that allows the exact time of stimulus presentation to be deduced, even if there are relatively
large errors in measuring the display time
Aerodynamic Characteristics of Several Hypersonic Boost-Glide-Type Configurations at Mach Numbers from 2.30 to 4.63
An investigation has been conducted at Mach numbers from 2.30 to 4.63 to determine the static aerodynamic characteristics of several configurations designed for flight at hypersonic Mach numbers. Two all-wing and three wing-body configurations were tested through an angle-of-attack range from about -4 degrees to 33 degrees and an angle-of-sideslip range from about -4 degrees to 8 degrees at a Reynolds number of 3 times 10 (sup 6) per foot (9.84 times 10 (sup 6) per meter). The results of the investigation indicated that the wing-body configurations produced higher values of maximum lift-drag ratio than those produced by the all-wing models. The high wing-body configurations tend to have a self-trimming capability as opposed to that for the low wing-body configurations. Each of the configurations produced a positive dihedral effect that increased with increasing angle of attack and decreased with increasing Mach number. The high wing-body models produced decreasing values of directional stability with increase in angle of attack, whereas the low wing-body models provided increasing values of directional stability with increase in angle of attack
Synthesis And Characterization Of (pyNO−)2GaCl: A Redox-Active Gallium Complex
We report the synthesis of a gallium complex incorporating redox-active pyridyl nitroxide ligands. The (pyNO−)2GaCl complex was prepared in 85% yield via a salt metathesis route and was characterized by 1H and 13C NMR spectroscopies, X-ray diffraction, and theory. UV–Vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties of the complex, respectively. Our discussion focuses primarily on a comparison of the gallium complex to the corresponding aluminum derivative and shows that although the complexes are very similar, small differences in the electronic structure of the complexes can be correlated to the identity of the metal
Ages and Abundances of Red Sequence Galaxies as a Function of LINER Emission Line Strength
Although the spectrum of a prototypical early-type galaxy is assumed to lack
emission lines, a substantial fraction (likely as high as 30%) of nearby red
sequence galaxy spectra contain emission lines with line ratios characteristic
of low ionization nuclear emission-line regions (LINERs). We use spectra of
~6000 galaxies from the Sloan Digital Sky Survey (SDSS) in a narrow redshift
slice (0.06 < z < 0.08) to compare the stellar populations of red sequence
galaxies with and without LINER-like emission. The spectra are binned by
internal velocity dispersion and by emission properties to produce high S/N
stacked spectra. The recent stellar population models of R. Schiavon (2007)
make it possible to measure ages, [Fe/H], and individual elemental abundance
ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We
find that red sequence galaxies with strong LINER-like emission are
systematically 2-3.5 Gyr (10-40%) younger than their emission-free counterparts
at the same velocity dispersion. This suggests a connection between the
mechanism powering the emission (whether AGN, post-AGB stars, shocks, or
cooling flows) and more recent star formation in the galaxy. We find that mean
stellar age and [Fe/H] increase with velocity dispersion for all galaxies.
Elemental abundance [Mg/Fe] increases modestly with velocity dispersion in
agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly
with velocity dispersion than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar
for all galaxies. At fixed velocity dispersion, galaxies with fainter r-band
luminosities have lower [Fe/H] and older ages but similar abundance ratios
compared to brighter galaxies.Comment: 25 pages, 17 figures, Accepted for publication in ApJ as of 16 July
2007; acceptance status updated, paper unchange
Cell interactions between histoincompatible T and B lymphocytes. VII. Cooperative responses between lymphocytes are controlled by genes in the I region of the H-2 complex
The results of this study provide compelling evidence for the existence of the gene or genes controlling optimal T-B-cell cooperative interactions in the designated I region of the H-2 gene complex. Previously, we have speculated that the relevant gene(s) involved may well be located in this region based on several observations from our earlier work in this area (3, 5, 6). Thus, in the preceding paper, we showed that T and B cells from B10.BR and A strain mice developed effective cooperative interactions in vitro to DNP-KLH in a system identical to the one reported herein. Since these mice differ for genes in the S and D regions of H-2 but are identical for K and I region genes, we were able to localize the critical genes to the K-end of H-2
Pressure control of magnetic order and excitations in the pyrochlore antiferromagnet MgCrO
MgCrO is one of the best-known realizations of the
pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic
exchange interactions are perturbed by small further-neighbor exchanges such
that this compound may in principle realize a spiral spin liquid (SSL) phase in
the zero-temperature limit. However, a spin Jahn-Teller transition below
K yields a complicated long-range magnetic order with
multiple coexisting propagation vectors. We present neutron scattering and
thermo-magnetic measurements of MgCrO samples under applied
hydrostatic pressure up to GPa demonstrating the existence of multiple
close-lying nearly degenerate magnetic ground states. We show that the
application of hydrostatic pressure increases the ordering temperature by
around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by
around 0.5 meV per GPa. We also evidence a strong tendency for the preferential
occupation of a subset of magnetic domains under pressure. In particular, we
show that the magnetic phase, which is almost
negligible at ambient pressure, dramatically increases in spectral weight under
pressure. This modifies the spectrum of magnetic excitations, which we
interpret unambiguously as spin waves from multiple magnetic domains. Moreover,
we report that the application of pressure reveals a feature in the magnetic
susceptibility above the magnetostructural transition. We interpret this as the
onset of a short-range ordered phase associated with ,
previously not observed in magnetometry measurements.Comment: 19 pages including supplementary information, 10 main figures 6
supplementary figure
Predictors of complications in acute type B aortic dissection
Objectives: Medical treatment is generally advocated for patients with acute type B aortic dissection without complications. The objective of this retrospective analysis was to determine whether there are any initial findings that can help predict the long-term course of the disease. Methods: Case records of the 130 patients treated for type B aortic dissection between 1988 and 1997 were reviewed; 41 (31%) were operated on in the acute phase (≪14 days), 31 (24%) were operated on in the chronic phase and 58 (45%) were treated medically. Results: Overall acute mortality was 10.8%; 22% for patients operated on in the early phase and 5.6% for medically treated patients. Age (P=0.002), persistent pain (P=0.01) and malperfusion (P=0.001) were significant independent predictors of the need for surgery. Paraplegia/para paresis (P=0.0001), leg ischaemia (P=0.003), pleural effusion (P=0.003), rupture (P=0.0001), shock (P=0.0001), age (P=0.003), cardiac failure (P=0.002) and aortic diameter ≫4.5cm (P=0.002) were significant predictors of poor survival. Age and shock also emerged as independent risk factors. Patients without malperfusion (P=0.0001), pleural effusion (P=0.003), rupture (P=0.0001) and shock (P=0.0001) had a significantly better event-free survival (freedom from repeat surgery and death). The actuarial survival rate for high-risk patients (malperfusion, rupture, shock) was 62% at 1 year and 40% at 5 years; the corresponding values for low-risk patients were 94 and 84%, respectively. Conclusions: Rupture, shock and malperfusion are significant predictors of poor survival in patients with acute type B aortic dissectio
Polar methane accumulation and rainstorms on Titan from simulations of the methane cycle
Titan has a methane cycle akin to Earth's water cycle. It has lakes in polar regions, preferentially in the north; dry low latitudes with fluvial features and occasional rainstorms; and tropospheric clouds mainly (so far) in southern middle latitudes and polar regions. Previous models have explained the low-latitude dryness as a result of atmospheric methane transport into middle and high latitudes. Hitherto, no model has explained why lakes are found only in polar regions and preferentially in the north; how low-latitude rainstorms arise; or why clouds cluster in southern middle and high latitudes. Here we report simulations with a three-dimensional atmospheric model coupled to a dynamic surface reservoir of methane. We find that methane is cold-trapped and accumulates in polar regions, preferentially in the north because the northern summer, at aphelion, is longer and has greater net precipitation than the southern summer. The net precipitation in polar regions is balanced in the annual mean by slow along-surface methane transport towards mid-latitudes, and subsequent evaporation. In low latitudes, rare but intense storms occur around the equinoxes, producing enough precipitation to carve surface features. Tropospheric clouds form primarily in middle and high latitudes of the summer hemisphere, which until recently has been the southern hemisphere. We predict that in the northern polar region, prominent clouds will form within about two (Earth) years and lake levels will rise over the next fifteen years
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