16,979 research outputs found
A failure diagnosis and impact assessment prototype for Space Station Freedom
NASA is investigating the use of advanced automation to enhance crew productivity for Space Station Freedom in numerous areas, one being failure management. A prototype is described that diagnoses failure sources and assesses the future impacts of those failures on other Freedom entities
New urea-absorbing polymers for artificial kidney machines
Etherified polymer is made from modified cellulose derivative which is reacted with periodate. It will absorb 2 grams of urea per 100 grams of polymer. Indications are that polymers could be used to help remove uremic wastes in artificial kidneys, or they could be administered orally as therapy for uremia
Aldehyde-containing urea-absorbing polysaccharides
A novel aldehyde containing polymer (ACP) is prepared by reaction of a polysaccharide with periodate to introduce aldehyde groups onto the C2 - C3 carbon atoms. By introduction of ether and ester groups onto the pendant primary hydroxyl solubility characteristics are modified. The ACP is utilized to absorb nitrogen bases such as urea in vitro or in vivo
On the orbital periods of the AM CVn stars HP Librae and V803 Centauri
We analyse high-time-resolution spectroscopy of the AM CVn stars HP Librae
and V803 Centauri, taken with the New Technology Telescope (NTT) and the Very
Large Telescope (VLT) of the European Southern Observatory, Chile.
We present evidence that the literature value for V803 Cen's orbital period
is incorrect, based on an observed `S-wave' in the binary's spectrogram. We
measure a spectroscopic period P=1596.4+/-1.2s of the S-wave feature, which is
significantly shorter than the 1611-second periods found in previous
photometric studies. We conclude that the latter period likely represents a
`superhump'. If one assumes that our S-wave period is the orbital period, V803
Cen's mass ratio can be expected to be much less extreme than previously
thought, at q~0.07 rather than q~0.016. This relaxes the constraints on the
masses of the components considerably: the donor star does then not need to be
fully degenerate, and the mass of the accreting white dwarf no longer has to be
very close to the Chandrasekhar limit.
For HP Lib, we similarly measure a spectroscopic period P=1102.8+/-0.2s. This
supports the identification of HP Lib's photometric periods found in the
literature, and the constraints upon the masses derived from them.Comment: Accepted for publication in MNRA
The Grassmannian Sigma Model in SU(2) Yang-Mills Theory
Spin-charge separation in pure SU(2) Yang-Mills theory was recently found to
involve the dynamics of an O(3) non-linear sigma model and, seemingly, a
Grassmannian non-linear sigma model. In this article we explicitly construct
the Grassmannian sigma model of the form appearing in the the spin-charge
separated SU(2) theory through a quaternionic decomposition of the manifold,
thus verifying its relevance in this context. The coupling between this model
and the O(3) non-linear sigma model is further commented upon.Comment: 11 pages, undergraduate research project; version published in J.
Phys.
A failure management prototype: DR/Rx
This failure management prototype performs failure diagnosis and recovery management of hierarchical, distributed systems. The prototype, which evolved from a series of previous prototypes following a spiral model for development, focuses on two functions: (1) the diagnostic reasoner (DR) performs integrated failure diagnosis in distributed systems; and (2) the recovery expert (Rx) develops plans to recover from the failure. Issues related to expert system prototype design and the previous history of this prototype are discussed. The architecture of the current prototype is described in terms of the knowledge representation and functionality of its components
UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya
We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and
V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow
central components of the dominant helium lines and determine radial velocity
semi-amplitudes of km s for GP Com and
km s for V396 Hya. The mean velocities of
the narrow central components show variations from line to line. Compared to
calculated line profiles that include Stark broadening we are able to explain
the displacements, and the appearance of forbidden helium lines, by additional
Stark broadening of emission in a helium plasma with an electron density
cm. More than nitrogen and more than
neon lines emission lines were detected in both systems. Additionally,
nitrogen absorption lines are only seen in GP Com. The radial velocity
variations of these lines show the same phase and velocity amplitude as the
central helium emission components. The small semi-amplitude of the central
helium emission component, the consistency of phase and amplitude with the
absorption components in GP Com as well as the measured Stark effect shows that
the central helium emission component, the so-called central-spike, is
consistent with an origin on the accreting white dwarf. We use the dynamics of
the bright spot and the central spike to constrain the binary parameters for
both systems and find a donor mass of - M for GP
Com and - M for V396 Hya. We find an upper limit
for the rotational velocity of the accretor of km s for
GP Com and km s for V396 Hya which excludes a fast
rotating accretor in both systems.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table
Detailed gravimetric geoid confirmation of short wavelength features of sea surface topography detected by the Skylab S-193 altimeter in the Atlantic Ocean
A detailed gravimetric geoid was computed for the Northwest Atlantic Ocean and Caribbean Sea area in support of the calibration and evaluation of the GEOS-C altimeter. This geoid, computed on a 15 ft. x 15 ft. grid was based upon a combination of surface gravity data with the GSFC GEM-6 satellite derived gravity data. A comparison of this gravimetric geoid with 10 passes of SKYLAB altimeter data is presented. The agreement of the two data types is quite good with the differences generally less than 2 meters. Sea surface manifestations of numerous short wavelength (approximately 100 km) oceanographic features are now indicated in the gravimetric geoid and are also confirmed by the altimetry data
Kinematics of the ultracompact helium accretor AM canum venaticorum
We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma.
We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA).
In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated
The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
A model is presented in the philosophy of the "String Axiverse" of Arvanitaki
et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight
axions to their corresponding moduli through the mass term. The light fields
roll in their potentials at late times and contribute to the dark sector energy
densities in the cosmological expansion. The addition of a coupling and extra
field greatly enrich the possible phenomenology of the axiverse. There are a
number of interesting phases where the axion and modulus components behave as
Dark Matter or Dark Energy and can have considerable and distinct effects on
the expansion history of the universe by modifying the equation of state in the
past or causing possible future collapse of the universe. In future such a
coupling may help to alleviate fine tuning problems for cosmological axions. We
motivate and present the model, and briefly explore its cosmological
consequences numerically.Comment: 13 pages, 17 figures, published in PRD. v3: corrected SUSY
interpretation of axion potential scal
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