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Comet Bennett (1970 II) was observed with the ultraviolet photometers on OAO-2 from April 13.39 to May 13.88, 1970, while its heliocentric distance increased from R = 0.77 to 1.26 a.u. An analysis of the photometer data for the emission features of OH λ 3090 and H λ1216 indicates the production rates of OH and H were 2.0 x 1029 molecule sec-1 and 5.4 x 1029 atom sec-1, respectively, at R = 1 a.u During this period the production rates of H and OH varied as R-2.3 . This is consistent with the assumption that water vaporization controls the production rate of gas in comets at small heliocentric distances.
Comet Bennett (1970II) was observed with an ultraviolet spectrometer on the OAO-2 spacecraft during April 1970 The instrument consisted of a plane grating, a parabolic mirror, a slit, and a photomultiplier tube. The exit slit provided a 2′ x 8′ rectangular field-of-view and a 23 A bandpass. At this resolution emission features such as OH λ3090 and CN λ3883 were essentially monochromatic. Since the instrument had no entrance slit, movement of the grating produced a shift in the field of view, with 1 arc min of spatial offset corresponding to a 10 A wavelength shift.
Time series spectra of the F5IV star Procyon (α CMi) were obtained at the Kitt Peak National Observatory during a 35-night observing run in January-February 1997. The observations were obtained as part of an international collaboration to detect and study acoustic p-mode oscillations in solar-type stars. Spectra covered the wavelength range from 4000 to 5300 Å, with a resolving power of approximately 3500 (1.3 Å resolution). The sampling rate was one observation per minute, and the typical S/N ratio per pixel after averaging along columns is in excess of 1000. We obtained 12,888 spectra. A sample spectrum is shown in Figure 1
We present results of an inversion procedure that derives the turbulent velocity, the magnetic field strength, and the temperature stratification of the photospheric layers of solar magnetic fluxtubes from 10 FeI and FeII Stokes V line profiles around 5250 Å and from the continuum contrast. The free parameters of two-dimensional magnetohydrostatic fluxtube models are determined by minimizing the difference between observed and calculated Stokes V parameters in an iterative manner. Results of this inversion procedure applied to observations of a plage and a network region at disk center indicate a temperature deficit (at equal geometrical height) of the fluxtubes at the level of continuum formation and a temperature excess at the highest levels of line formation in general agreement with the latest theoretical fluxtube models.
Endophenotypes are laboratory-based measures hypothesized to lie in the causal chain between genes and clinical disorder, and to serve as a more powerful way to identify genes associated with the disorder. One promise of endophenotypes is that they may assist in elucidating the neurobehavioral mechanisms by which an associated genetic polymorphism affects disorder risk in complex traits. We evaluated this promise by testing the extent to which variants discovered to be associated with schizophrenia through large-scale meta-analysis show associations with psychophysiological endophenotypes.
We genome-wide genotyped and imputed 4905 individuals. Of these, 1837 were whole-genome-sequenced at 11× depth. In a community-based sample, we conducted targeted tests of variants within schizophrenia-associated loci, as well as genome-wide polygenic tests of association, with 17 psychophysiological endophenotypes including acoustic startle response and affective startle modulation, antisaccade, multiple frequencies of resting electroencephalogram (EEG), electrodermal activity and P300 event-related potential.
Using single variant tests and gene-based tests we found suggestive evidence for an association between contactin 4 (CNTN4) and antisaccade and P300. We were unable to find any other variant or gene within the 108 schizophrenia loci significantly associated with any of our 17 endophenotypes. Polygenic risk scores indexing genetic vulnerability to schizophrenia were not related to any of the psychophysiological endophenotypes after correction for multiple testing.
The results indicate significant difficulty in using psychophysiological endophenotypes to characterize the genetically influenced neurobehavioral mechanisms by which risk loci identified in genome-wide association studies affect disorder risk.
The Commission is intended to study the origin of the solar wind, the heliosphere, i.e., the region dominated by the supersonic solar wind, and the heliospheric interface with the interstellar medium. The activities of the Commission cover both theoretical and observational aspects of these three regions.
The Dawn spacecraft orbited Asteroid (4) Vesta for a year, and returned disk-resolved images and spectra covering visible and near-infrared wavelengths at scales as high as 20 m/pix. The visible geometric albedo of Vesta is ~ 0.36. The disk-integrated phase function of Vesta in the visible wavelengths derived from Dawn approach data, previous ground-based observations, and Rosetta OSIRIS observations is consistent with an IAU H-G phase law with H=3.2 mag and G=0.28. Hapke's modeling yields a disk-averaged single-scattering albedo of 0.50, an asymmetry factor of -0.25, and a roughness parameter of ~20 deg at 700 nm wavelength. Vesta's surface displays the largest albedo variations observed so far on asteroids, ranging from ~0.10 to ~0.76 in geometric albedo in the visible wavelengths. The phase function of Vesta displays obvious systematic variations with respect to wavelength, with steeper slopes within the 1- and 2-micron pyroxene bands, consistent with previous ground-based observations and laboratory measurement of HED meteorites showing deeper bands at higher phase angles. The relatively high albedo of Vesta suggests significant contribution of multiple scattering. The non-linear effect of multiple scattering and the possible systematic variations of phase function with albedo across the surface of Vesta may invalidate the traditional algorithm of applying photometric correction on airless planetary surfaces.
In this study, we use a quantum well (QW) probe structure to explore the size dependent effects of sidewall recombination in GaN. Mesas 0.8-7 μm in width with pitches of 4 μm, 8 μm, and 12 μm were etched into the QW probe structure, exposing the QW at the sidewalls. Several etch conditions were investigated. Room temperature photoluminescence (PL) measurements, using a He-Cd laser as an excitation source and laser spot size of approximately 230 μm, were taken before and after the mesas were etched. The effects of sidewall formation were quantified by comparing the maximum PL intensity of the QW before and after etch. Higher remaining PL intensity was observed for etch conditions which used both Ar ions and Cl2 gas instead of only Ar ions. The fraction of remaining PL decreased with decreasing mesa width, however the remaining PL intensity was relatively large even for small features. The preliminary data suggested that GaN is relatively insensitive to sidewall damage.
Temperature-variable Hall and Shubnikov- de Haas effects have been used to study persistent photoconductivity in an AlGaN/GaN heterojunction. At liquid helium temperatures, the mobility in this structure was close to 55000 cm2/Vs. A blue GaN-based light emitting diode was used to illuminate the sample. This illumination resulted in a persistent photocurrent, which allowed us to vary the carrier density and study the dependence of the mobility on the carrier concentration. Exposing the sample to this light resulted in an increase in the carrier density. For small increases in the density, the mobility also increased. However, unlike in previous reports by other authors, extended illumination resulted in an increase in the density and a decrease in the mobility. The initial increase in the mobility is attributed to increased screening due to the increase in the carrier density, while the decrease in the mobility may be attributed to alloy scattering.
We present the scanning tunneling microscope-induced luminescence (STL) imaging of defects in optoelectronic materials. Resolution is first discussed using cross-sectional images of InGaAs/GaAs quantum dots. Proof of concept is then provided through the nanometer-scale imaging of GaN layers and quantum wells. The expected λ=356±25 nm range dominates the low temperature STL of GaN. Mapping of luminescence shows circular non-emitting areas around threading dislocations. Extent of dark areas suggests a hole diffusion length of Ld=30–55 nm, in agreement with reported values. The expected λ=450±35 nm range dominates the STL from a buried InGaN/GaN multiple quantum well. Imaging reveals 30–100 nm wide smooth fluctuations of luminescence.
The optical properties of (In, Al) GaN thin films and heterostructures have been compared under the conditions of strong nanosecond excitation. The stimulated emission (SE) threshold from AIGaN epilayers was found to increase with increasing Al content compared to GaN, in contrast to InGaN epilayers, where an order of magnitude decrease is observed. Optically pumped SE has been observed from AIGaN films with aluminum concentrations as high as 26%. Room temperature SE at wavelengths as low as 327 nm has been achieved. In contrast to the increase of SE threshold seen for AlGaN films, we found that AlGaN/GaN heterostructures which utilize carrier confinement and optical waveguiding drastically enhance the lasing characteristics. We demonstrate that AIGaN/GaN heterostructures are suitable for the development of deep ultraviolet laser diodes.
We report the results of nondegenerate optical pump-probe absorption experiments performed on GaN and InGaN thin films and quantum wells under the conditions of strong optical band to band excitation. The evolution of the band edge in these materials was monitored as the number of photoexcited free carriers was increased beyond that required to achieve population inversion and observe stimulated emission. The band edge of InGaN is shown to exhibit markedly different high excitation behavior than that of GaN, explaining in part the reduction in stimulated emission threshold that typically accompanies the incorporation of indium into GaN to form InGaN. A comparison of the band edge absorption changes observed in pump-probe experiments to the gain spectra measured in variable-stripe gain experiments is also given.
Image sensors in TFA (Thin Film on ASIC) technology have been successfully fabricated and tested. This paper provides a survey of TFA research results so far and outlines future perspectives. The properties of different a-Si:H b/w and color thin film detectors are evaluated, including spectral sensitivity, dark current, temperature influence and transient behavior. Furthermore several TFA prototypes and emerging concepts are presented, ranging from a simple one-transistor cell design to a locally autoadaptive sensor.
Temperature-dependent photoluminescence (PL) studies have been performed on InGaN epilayers and InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous temperature dependent emission behavior (specifically an S-shaped decrease-increase-decrease) of the peak energy (EpL) of the InGaN-related PL emission with increasing temperature. In the case of the InGaN epilayer, EPL decreases in the temperature range of 10 - 50 K, increases for 50 - 110 K, and decreases again for 110 - 300 K with increasing temperature. For the InGaN/GaN MQWs, EPL decreases from 10 - 70 K, increases from 70 - 150 K, then decreases again for 150 - 300 K. The actual temperature dependence of the PL emission was estimated with respect to the bandgap energy determined by photoreflectance spectra. We observed that the PL peak emission shift has an excellent correlation with a change in carrier lifetime with temperature. We demonstrate that the temperature-induced S-shaped PL shift is caused by the change in carrier recombination dynamics with increasing temperature due to inhomogeneities in the InGaN structures.
Mercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.
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