11,661 research outputs found
Frequency stability of maser oscillators operated with cavity Q
The short term frequency stability of masers equipped with an external feedback loop to increase the cavity quality factor was studied. The frequency stability of a hydrogen and a rubidium maser were measured and compared with theoretical evaluation. It is shown that the frequency stability passes through an optimum when the cavity Q is varied. Long term fluctuations are discussed and the optimum mid term frequency stability achievably by small size active and passive H-masers is considered
A Possible Divot in the Size Distribution of the Kuiper Belt's Scattering Objects
Via joint analysis of a calibrated telescopic survey, which found scattering
Kuiper Belt objects, and models of their expected orbital distribution, we
measure the form of the scattering object's size distribution. Ruling out a
single power-law at greater than 99% confidence, we constrain the form of the
size distribution and find that, surprisingly, our analysis favours a very
sudden decrease (a divot) in the number distribution as diameters decrease
below 100 km, with the number of smaller objects then rising again as expected
via collisional equilibrium. Extrapolating at this collisional equilibrium
slope produced enough kilometer-scale scattering objects to supply the nearby
Jupiter-Family comets. Our interpretation is that this divot feature is a
preserved relic of the size distribution made by planetesimal formation, now
"frozen in" to portions of the Kuiper Belt sharing a "hot" orbital inclination
distribution, explaining several puzzles in Kuiper Belt science. Additionally,
we show that to match today's scattering-object inclination distribution, the
supply source that was scattered outward must have already been vertically
heated to of order 10 degrees.Comment: accepted 2013 January 8; published 2013 January 22 21 pages, 4
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Intestinal absorption of macromolecules during viral enteritis: an experimental study on rotavirus-infected conventional and germ-free mice.
Epithelial transport and degradation of horseradish peroxidase (HRP), a macromolecular tracer, was studied in conventional and germ-free suckling mice following an experimental infection with rotavirus. Conventional and germ-free mice developed diarrhea from days 2 to 8 postinfection (pi), with growth failure. In mucosal homogenates, infectious virus detected by immunofluorescence on MA 104 cells was present from day 2 through day 8 pi in germ-free mice, but persisted longer (day 13 pi) in conventional mice. Only mild histological lesions were observed during diarrhea, but obvious macrovacuolation of epithelial cells and increased cellular density occurred during the convalescence period (days 9 to 13 pi). Intact and degraded HRP fluxes from mucosa to serosa were measured in vitro on segments of jejunum mounted in Ussing chambers. Both groups of mice developed increased HRP permeability during the experimental period, but at different times after inoculation: during the diarrheal period (days 2 and 3 pi) conventional mouse epithelium absorbed five times more HRP than noninfected controls and during the convalescence period (days 9 to 13 pi) HRP absorption in germ-free mice rose 10-fold as compared to its level before infection. In both cases, this increase in HRP permeability was entirely due to an increase in intact HRP absorption, probably via a transcellular route, and occurred without any alteration in degraded HRP transport. These results indicate that in mice, rotavirus infection causes a transient rise in gut permeability to undegraded proteins. The intestinal microflora seems to affect the timing, magnitude, and duration of this increased permeability
The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae
Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a
convective envelope. Fast rotators on the main sequence, or Ap star
descendants, are expected to become magnetic active subgiants during this
evolutionary phase. Aims: We compare the surface magnetic fields and activity
indicators of two active, fast rotating red giants with similar masses and
spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com
(Prot=6.8 d) - to address the question of the origin of their magnetism and
high activity.
Methods: Observations were carried out with the Narval spectropolarimeter in
2008 and 2013.We used the least squares deconvolution technique to extract
Stokes V and I profiles to detect Zeeman signatures of the magnetic field of
the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring,
and a precise estimation of stellar parameters. We use stellar evolutionary
models to infer the evolutionary status and the initial rotation velocity on
the main sequence.
Results: The detected magnetic field of OU And is a strong one. Its
longitudinal component Bl reaches 40 G and presents an about sinusoidal
variation with reversal of the polarity. The magnetic topology of OU And is
dominated by large scale elements and is mainly poloidal with an important
dipole component, and a significant toroidal component. The detected magnetic
field of 31 Com is weaker, with a magnetic map showing a more complex field
geometry, and poloidal and toroidal components of equal contributions. The
evolutionary models show that the progenitors of OU And and 31 Com must have
been rotat
Conclusions: OU And appears to be the probable descendant of a magnetic Ap
star, and 31 Com the descendant of a relatively fast rotator on the main
sequence.Comment: 16 pages, 12 figure
Revisiting the chemistry of graphite oxides and its effect on ammonia adsorption
Graphite oxide (GO) was synthesized using two different methods: one with sulfuric acid as part of the oxidizing mixture (Hummers-Offeman method) and another one without the sulfur-containing compound involved in the oxidation process (Brodie method). They were both tested for ammonia adsorption in dynamic conditions, at ambient temperature, and characterized before and after exposure to ammonia by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, potentiometric titration, energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Analyses of the initial materials showed that besides epoxy, hydroxyl and carboxylic groups, a significant amount of sulfur is incorporated as sulfonic group for GO prepared by the Hummers-Offeman method. The process of ammonia adsorption seems to be strongly related to the type of GO. For GO prepared by the Brodie method, ammonia is mainly retained via intercalation in the interlayer space of GO and by reaction with the carboxylic groups present at the edges of the graphene layers. On the contrary, when GO prepared by the Hummers method is used, the ways of retention are different: not only is the intercalation of ammonia observed but its reaction with the epoxy, carboxylic and sulfonic groups present is also observed. In particular, during the ammonia adsorption process, sulfonic groups are converted to sulfates in the presence of superoxide anions O2-*. These sulfates can then react with ammonia to form ammonium sulfates. For both GOs, an incorporation of a significant part of the ammonia adsorbed as amines in their structure is observed as a result of reactive adsorption. © 2009 The Royal Society of Chemistry
The Kuiper Belt Luminosity Function from m(R)=21 to 26
We have performed an ecliptic imaging survey of the Kuiper belt with our
deepest and widest field achieving a limiting flux of m(g') = 26.4, with a sky
coverage of 3.0 square-degrees. This is the largest coverage of any other
Kuiper belt survey to this depth. We detect 72 objects, two of which have been
previously observed. We have improved the Bayesian maximum likelihood fitting
technique presented in Gladman et al. (1998) to account for calibration and sky
density variations and have used this to determine the luminosity function of
the Kuiper belt. Combining our detections with previous surveys, we find the
luminosity function is well represented by a single power-law with slope alpha
= 0.65 +/- 0.05 and an on ecliptic sky density of 1 object per square-degree
brighter than m(R)=23.42 +/- 0.13. Assuming constant albedos, this slope
suggests a differential size-distribution slope of 4.25 +/- 0.25, which is
steeper than the Dohnanyi slope of 3.5 expected if the belt is in a state of
collisional equilibrium. We find no evidence for a roll-over or knee in the
luminosity function and reject such models brightward of m(R) ~ 24.6.Comment: 50 Pages, 8 Figure
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