14,730 research outputs found
The crystal and molecular structure of Hydridotetrakis(diethyl phenylphosphonite)cobalt(I)
An X-ray structure determination of the title compound shows that the co-ordination about the cobalt atom is approximately trigonal bipyramidal; n.m.r. data indicate that the complex is non-rigid in solution
Ground-runup tests of acoustically treated inlets and fan ducts
Ground performance tests on noise intensity of turbofan engine ducts modified by acoustic lining
Conceptual design study of advanced acoustic-composite nacelles
Conceptual studies were conducted to assess the impact of incorporating advanced technologies in the nacelles of a current wide-bodied transport and an advanced technology transport. The improvement possible in the areas of fuel consumption, flyover noise levels, airplane weight, manufacturing costs, and airplane operating cost were evaluated for short and long-duct nacelles. Use of composite structures for acoustic duct linings in the fan inlet and exhaust ducts was considered as well as for other nacelle components. For the wide-bodied transport, the use of a long-duct nacelle with an internal mixer nozzle in the primary exhaust showed significant improvement in installed specific fuel consumption and airplane direct operating costs compared to the current short-duct nacelle. The long-duct mixed-flow nacelle is expected to achieve significant reductions in jet noise during takeoff and in turbo-machinery noise during landing approach. Recommendations were made of the technology development needed to achieve the potential fuel conservation and noise reduction benefits
Admission Diagnosis of Cerebral Malaria in Adults in an Endemic Area of Tanzania:Implications and Clinical Description
Cerebralmalariais commonly diagnosed in adults in endemic areas in Africa, both in hospitals and in the community. This presents a paradox inconsistent with the epidemiological understanding that the development of immunity during childhood confers protection against severe disease in adult life. To establish the contribution of Plasmodium falciparum infection in adults admitted with neurological dysfunction in an endemic area, to assess the implications of an admission clinical diagnosis of ‘cerebral malaria’ on the treatment and clinical outcome, and to describe the clinical features of patients with malaria parasitaemia. Prospective observational study. We studied adult patients admitted with neurological dysfunction to Muhimbili National Hospital, Dar-es-Salaam, Tanzania from October 2000 to July 2001. A full blood count was done and serum creatinine, blood glucose and P. falciparum parasite load were measured. Of 199 patients (median age 34.5 years), 38% were diagnosed as ‘cerebral malaria’ on admission, but only 7.5% had detectable parasitaemia, giving a positive predictive value of 13.3%. Only 1% fulfilled the WHO criteria for cerebral malaria. The prevalence of parasitaemia (7.5%) was less than that observed in a group of asymptomatic controls (9.3%), but distribution of parasite densities was higher in the patients. Mortality was higher in patients with no parasitaemia (22.3%) than in those with parasitaemia (13%). Cerebral malaria was grossly overdiagnosed, resulting in unnecessary treatment and insufficient investigation of other possible diagnoses, which could lead to higher mortality. Extension of this misperception to the assessment of cause of death in community surveys may lead to an overestimation of the impact of malaria in adults.\u
Tracing the spiral arms in IP Pegasi
We report the analysis of time-resolved spectroscopy of IP Pegasi in outburst with eclipse mapping techniques to investigate the location and geometry of the observed spiral structures. We were able to obtain an improved view of the spiral structures with the aid of light curves extracted in velocity bins matching the observed range of velocities of the spiral arms combined with a double default map tailored for reconstruction of asymmetric structures. Two-armed spiral structures are clearly seen in all eclipse maps. The arms are located at different distances from the disc centre. The “blue” arm is farther out in the disc (R = 0.55 ± 0.05 R L1 ) than the “red” arm (R = 0.30 ± 0.05 R L1 ). There is evidence that
the velocity of the emitting gas along the spiral pattern is lower than the Keplerian velocity for the same disc radius. The discrepancy is smaller in the outer arm (measured velocities 10–15 per cent lower than Keplerian) and is more significant in the inner arm (observed velocities up to 40 per cent lower than Keplerian). We measured the opening angle of the spirals from the azimuthal intensity distribution of the eclipse maps to
be φ = 25◦ ± 3◦ . A comparison with similar measurements on data at different outburst stages reveals that the opening angle of the spiral arms in IP Peg decreases while the outbursting accretion disc cools and shrinks, in agreement with the expected evolution of a tidally driven spiral
wave. The sub-Keplerian velocities along the spiral pattern and the clear correlation between the opening angle of the spirals and the outburst stage favors the interpretation of these asymmetric structures as tidally-induced spiral shocks
New results on GP Com
We present high resolution optical and UV spectra of the 46 min orbital
period, helium binary, GP Com. Our data contains simultaneous photometric
correction which confirms the flaring behaviour observed in previous optical
and UV data. In this system all lines show a triple peaked structure where the
outer two peaks are associated with the accretion disc around the compact
object. The main aim of this paper is to constrain the origin of the central
peak, also called ``central spike''. We find that the central spike contributes
to the flare spectra indicating that its origin is probably the compact object.
We also detect that the central spike moves with orbital phase following an
S-wave pattern. The radial velocity semiamplitude of the S-wave is ~10 km/s
indicating that its origin is near the centre of mass of the system, which in
this case lies very close to the white dwarf. Our resolution is higher than
that of previous data which allows us to resolve structure in the central peak
of the line. The central spike in three of the HeI lines shows another peak
blueshifted with respect to the main peak. We propose that one of the peaks is
a neutral helium forbidden transition excited in a high electron density
region. This forbidden transition is associated with the permitted one (the
stronger peak in two of the lines). The presence of a high electron density
region again favours the white dwarf as their origin.Comment: 14 pages, 16 figures. Accepted for publication in A&
Low Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey for low mass
stellar and substellar companions to white dwarfs. A wide field proper motion
survey of 261 white dwarfs was capable of directly detecting companions at
orbital separations between and 5000 AU with masses as low as 0.05
, while a deep near field search of 86 white dwarfs was capable of
directly detecting companions at separations between and 1100 AU with
masses as low as 0.02 . Additionally, all white dwarf targets were
examined for near-infrared excess emission, a technique capable of detecting
companions at arbitrarily close separations down to masses of 0.05 .
No brown dwarf candidates were detected, which implies a brown dwarf
companion fraction of % for white dwarfs. In contrast, the stellar
companion fraction of white dwarfs as measured by this survey is 22%,
uncorrected for bias. Moreover, most of the known and suspected stellar
companions to white dwarfs are low mass stars whose masses are only slightly
greater than the masses of brown dwarfs. Twenty previously unknown stellar
companions were detected, five of which are confirmed or likely white dwarfs
themselves, while fifteen are confirmed or likely low mass stars.
Similar to the distribution of cool field dwarfs as a function of spectral
type, the number of cool unevolved dwarf companions peaks at mid-M type. Based
on the present work, relative to this peak, field L dwarfs appear to be roughly
2-3 times more abundant than companion L dwarfs. Additionally, there is no
evidence that the initial companion masses have been altered by post main
sequence binary interactions.Comment: 149 pages, 59 figures, 11 tables, accepted to ApJ Supplement
Ultrafast harmonic mode-locking of monolithic compound-cavity laser diodes incorporating photonic-bandgap reflectors
We present the first demonstration of reproducible harmonic mode-locked operation from a novel design of monolithic semiconductor laser comprising a compound cavity formed by a 1-D photonic-bandgap (PBG) mirror. Mode-locking (ML) is achieved at a harmonic of the fundamental round-trip frequency with pulse repetition rates from 131 GHz up to a record high frequency of 2.1 THz. The devices are fabricated from GaAs-Al-GaAs material emitting at a wavelength of 860 nm and incorporate two gain sections with an etched PBG reflector between them, and a saturable absorber section. Autocorrelation studies are reported which allow the device behavior for different ML frequencies, compound cavity ratios, and type and number of intra-cavity reflectors to be analyzed. The highly reflective PBG microstructures are shown to be essential for subharmonic-free ML operation of the high-frequency devices. We have also demonstrated that the single PBG reflector can be replaced by two separate features with lower optical loss. These lasers may find applications in terahertz; imaging, medicine, ultrafast optical links, and atmospheric sensing
Spiral shocks in the accretion disc of IP Peg during outburst maximum
In response to our recent discovery of spiral arms in the accretion disc of
IP Peg during rise to outburst, we have obtained time-resolved
spectrophotometry of IP Peg during outburst maximum. In particular, indirect
imaging of HeII 4686, using Doppler tomography, shows a two-arm spiral pattern
on the disc image, which confirms repeatability over different outbursts. The
jump in HeII intensity (a factor of more than two) and in velocity (~200--300
km/s) clarifies the shock nature of the spiral structure. The HeII shocks show
an azimuthal extent of ~90 degrees, a shallow power-law emissivity ~V^{-1}, an
upper limit of 30 degrees in opening angle, and a flux contribution of 15 per
cent of the total disc emission. We discuss the results in view of recent
simulations of accretion discs which show that spiral shocks can be raised in
the accretion disc by the secondary star.Comment: 5 pages, 4 figures, MNRAS journal paper. in pres
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