26,886 research outputs found
Asteroid selection for mission opportunities
A study to assess the present state of knowledge of asteroids as well as the rate of change of that knowledge to better identify the mission and target priorities for advanced planning of asteroidal flights in the 1980's is presented. Topics discussed include; the present state of asteroid knowledge, the scientific goals and priorities attached to asteroid exploration, the anticipated advances in knowledge over the current decade, asteroid mission consideration, and asteroid selection. Data sheets for 118 asteroids are contained. These are asteroids for which some data is available over and above orbital parameters and magnitude
Communications of the lunar planetary laboratory, volume 3, no. 40- the system of lunar craters, quadrant ii
Observable properties of craters in second lunar quadran
Modulation of individual components of gastric motor response to duodenal glucose
AIM: To evaluate individual components of the antro-pyloro-duodenal (APD) motor response to graded small intestinal glucose infusions in healthy humans. METHODS: APD manometry was performed in 15 healthy subjects (12 male; 40 ± 5 years, body mass index 26.5 ± 1.6 kg/m2) during four 20-min intraduodenal infusions of glucose at 0, 0.5, 1.0 and 1.5 kcal/min, in a randomised double-blinded fashion. Glucose solutions were infused at a rate of 1 mL/min and separated by 40-min “wash-out” period. Data are mean ± SE. Inferential analyses are repeated measure analysis of variance with Bonferroni post-hoc testing. RESULTS: At 0 kcal/min frequency of pressure waves were: antrum (7.5 ± 1.8 waves/20 min) and isolated pyloric pressure waves (IPPWs) (8.0 ± 2.3 waves/20 min) with pyloric tone (0.0 ± 0.9 mmHg). Intraduodenal glucose infusion acutely increased IPPW frequency (P < 0.001) and pyloric tone (P = 0.015), and decreased antral wave frequency (P = 0.007) in a dose-dependent fashion. A threshold for stimulation was observed at 1.0 kcal/min for pyloric phasic pressure waves (P = 0.002) and 1.5 kcal/min for pyloric tone and antral contractility. CONCLUSION: There is hierarchy for the activation of gastrointestinal motor responses to duodenal glucose infusion. An increase in IPPWs is the first response observed.Adam M Deane, Laura K Besanko, Carly M Burgstad, Marianne J Chapman, Michael Horowitz, Robert JL Frase
Chemical abundances in the nucleus of the Sagittarius dwarf spheroidal galaxy
We present Iron, Magnesium, Calcium, and Titanium abundances for 235 stars in
the central region of the Sagittarius dwarf spheroidal galaxy (within 9.0
arcmin ~70 pc from the center) from medium-resolution Keck/DEIMOS spectra. All
the considered stars belong to the massive globular cluster M54 or to the
central nucleus of the galaxy (Sgr,N). In particular we provide abundances for
109 stars with [Fe/H] > -1.0, more than doubling the available sample of
spectroscopic metallicity and alpha-elements abundance estimates for Sgr dSph
stars in this metallicity regime. Also, we find the first confirmed member of
the Sagittarius dwarf spheroidal with [Fe/H]< -2.0 based on analysis of iron
lines. We find for the first time a metallicity gradient in the Sgr,N
population, whose peak iron abundance goes from [Fe/H]=-0.38 for R < 2.5 arcmin
to [Fe/H]=-0.57 for 5.0 < R < 9.0 arcmin. On the other hand the trends of
[Mg/Fe], [Ca/Fe], and [Ti/Fe] with [Fe/H] are the same over the entire region
explored by our study. We reproduce the observed chemical patterns of the
Sagittarius dwarf spheroidal as a whole with a chemical evolution model
implying a high mass progenitor ( M_(DM)=6 X 10^{10} Msun ) and a significant
event of mass-stripping occurred a few Gyr ago, presumably starting at the
first peri-Galactic passage after infall.Comment: Accepted for publication to A&A, 12 pages, 14 figures, 1 tabl
A sandpile model with tokamak-like enhanced confinement phenomenology
Confinement phenomenology characteristic of magnetically confined plasmas
emerges naturally from a simple sandpile algorithm when the parameter
controlling redistribution scalelength is varied. Close analogues are found for
enhanced confinement, edge pedestals, and edge localised modes (ELMs), and for
the qualitative correlations between them. These results suggest that tokamak
observations of avalanching transport are deeply linked to the existence of
enhanced confinement and ELMs.Comment: Manuscript is revtex (latex) 1 file, 7 postscript figures Revised
version is final version accepted for publication in PRL Revisions are mino
Surfatron and stochastic acceleration of electrons in astrophysical plasmas
Electron acceleration by large amplitude electrostatic waves in astrophysical plasmas is studied using particle-in-cell (PIC) simulations. The waves are excited initially at the electron plasma frequency by a Buneman instability driven by ion beams: the parameters of the ion beams are appropriate for high Mach number astrophysical shocks, such as those associated with supernova remnants (SNRs). If is much higher than the electron cyclotron frequency , the linear phase of the instability does not depend on the magnitude of the magnetic field. However, the subsequent time evolution of particles and waves depends on both and the size of the simulation box . If is equal to one wavelength, , of the Buneman-unstable mode, electrons trapped by the waves undergo acceleration via the surfatron mechanism across the wave front. This occurs most efficiently when : in this case electrons are accelerated to speeds of up where is the speed of light. In a simulation with and , it is found that sideband instabilities give rise to a broad spectrum of wavenumbers, with a power law tail. Some stochastic electron acceleration is observed in this case, but not the surfatron process. Direct integration of the electron equations of motion, using parameters approximating to those of the wave modes observed in the simulations, suggests that the surfatron is compatible with the presence of a broad wave spectrum if . It is concluded that a combination of stochastic and surfatron acceleration could provide an efficient generator of mildly relativistic electrons at SNR shocks
Squeezing Uncertainty from Saccadic Compression
Brief visual stimuli presented before and during a saccade are often mislocalized due to spatial compression. This saccadic compression effect is thought to have a perceptual basis, and results in visual objects being squeezed together and their number underestimated. Here we show that observers are also uncertain about their visual experiences just before and during a saccade. It is known that responses tend to be biased away from extreme values under conditions of uncertainty. Thus, a plausible alternative explanation of compression is that it reflects the uncertainty-bias to underestimate the number of items that were presented. We test this hypothesis and find that saccadic compression is independent of certainty, and is significantly modulated by orientation, with larger effects for stimuli oriented horizontally, in the direction of the saccade. These findings confirm that saccadic compression is a perceptual phenomenon that may enable seamless perceptual continuity across saccades
Further multiwavelength observations of the SSA22 Ly_alpha emitting `blob'
We present new follow-up observations of the sub-mm luminous
Ly_alpha-emitting object in the SSA22 z=3.09 galaxy overdensity, referred to as
`Blob 1' by Steidel et al.(2000). In particular we discuss high resolution
Hubble Space Telescope optical imaging, Owens Valley Radio Observatory spectral
imaging, Keck spectroscopy, VLA 20cm radio continuum imaging, and Chandra X-ray
observations. We also present a more complete analysis of the existing James
Clerk Maxwell Telescope sub-mm data. We detect several optical continuum
components which may be associated with the core of the submillimeter emitting
region. A radio source at the position of one of the HST components
(22:17:25.94, +00:12:38.9) identifies it as the likely counterpart to the
submillimeter source. We also tentatively detect the CO(4-3) molecular line,
centered on the radio position. We use the CO(4-3) intensity to estimate a
limit on the gas mass for the system. The optical morphology of sources within
the Ly_alpha cloud appears to be filamentary, while the optical source
identified with the radio source has a dense knot which may be an AGN or
compact starburst. We obtain a Keck-LRIS spectrum of this object, despite its
faintness (R=26.8). The spectrum reveals weak Ly_alpha emission, but no other
obvious features, suggesting that the source is not an energetic AGN (or that
it is extremely obscured). We use non-detections in deep Chandra X-ray images
to constrain the nature of the `Blob'. Although conclusive evidence regarding
the nature of the object remains hard to obtain at this redshift, the evidence
presented here is at least consistent with a dust-obscured AGN surrounded by a
starburst situated at the heart of this giant Ly_alpha cloud.Comment: 8 pages, 9figs (low res), to appear in ApJ, for higher res figures,
http://www.submm.caltech.edu/~schapman/sa22_sept4.ps.g
Superconductivity in Ca-doped graphene
Graphene, a zero-gap semimetal, can be transformed into a metallic,
semiconducting or insulating state by either physical or chemical modification.
Superconductivity is conspicuously missing among these states despite
considerable experimental efforts as well as many theoretical proposals. Here,
we report superconductivity in calcium-decorated graphene achieved by
intercalation of graphene laminates that consist of well separated and
electronically decoupled graphene crystals. In contrast to intercalated
graphite, we find that Ca is the only dopant that induces superconductivity in
graphene laminates above 1.8 K among intercalants used in our experiments such
as potassium, caesium and lithium. Ca-decorated graphene becomes
superconducting at ~ 6 K and the transition temperature is found to be strongly
dependent on the confinement of the Ca layer and the induced charge carrier
concentration. In addition to the first evidence for superconducting graphene,
our work shows a possibility of inducing and studying superconductivity in
other 2D materials using their laminates
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