21,179 research outputs found
Shedding Light on the Matter of Abell 781
The galaxy cluster Abell 781 West has been viewed as a challenge to weak
gravitational lensing mass calibration, as Cook and dell'Antonio (2012) found
that the weak lensing signal-to-noise in three independent sets of observations
was consistently lower than expected from mass models based on X-ray and
dynamical measurements. We correct some errors in statistical inference in Cook
and dell'Antonio (2012) and show that their own results agree well with the
dynamical mass and exhibit at most 2.2--2.9 low compared to the X-ray
mass, similar to the tension between the dynamical and X-ray masses. Replacing
their simple magnitude cut with weights based on source photometric redshifts
eliminates the tension between lensing and X-ray masses; in this case the weak
lensing mass estimate is actually higher than, but still in agreement with, the
dynamical estimate. A comparison of lensing analyses with and without
photometric redshifts shows that a 1--2 chance alignment of
low-redshift sources lowers the signal-to-noise observed by all previous
studies which used magnitude cuts rather than photometric redshifts. The
fluctuation is unexceptional, but appeared to be highly significant in Cook and
dell'Antonio (2012) due to the errors in statistical interpretation.Comment: 7 pages, submitted to MNRA
Graphite ionization vacuum gauge
Triode gauge with electron source, electron collector, and positive ion collector made from either graphite or carbon material extends low-pressure ranges of existing gauges by changing only materials used in construction. Advantages of graphite gauge stem from physical properties of graphite (or carbon)
Study of Umbrella-Type Erectable Paraboloidal Solar Concentrators for Generation of Spacecraft Auxiliary Power
An investigation was made of some aspects of erectable umbrella-type paraboloidal solar-energy concentrators for use in spacecraft auxiliary power systems. An analysis is presented for the design of concentrators employing radial ribs and membrane-type coverings. The rib stiffness necessary to give a desired parabolic curvature upon erection is determined. Rib contour measurements were made in an upright and inverted position on an experimental 10-foot-diameter concentrator giving a focal length of 30.78 inches and 30.6 inches, respectively, as compared with a design focal length of 30 inches. Calorimetric tests employing spherical heat receivers showed a maximum geometrical concentrator efficiency of 75.3 percent with a concentration ratio of 92.2 at a focal length of approximately 30.5 inches. Within the scope of this investigation, it was found that, as the concentrator size increases, the weight per unit projected area increases, time to damp to one-half amplitude increases, and the natural frequency decreases. Umbrella-type solar concentrators appear to be feasible for power conversion systems using low concentration ratios; however, for large concentrators the time required to damp out disturbances may be a problem
Stratifying derived categories of cochains on certain spaces
In recent years, Benson, Iyengar and Krause have developed a theory of
stratification for compactly generated triangulated categories with an action
of a graded commutative Noetherian ring. Stratification implies a
classification of localizing and thick subcategories in terms of subsets of the
prime ideal spectrum of the given ring. In this paper two stratification
results are presented: one for the derived category of a commutative
ring-spectrum with polynomial homotopy and another for the derived category of
cochains on certain spaces. We also give the stratification of cochains on a
space a topological content.Comment: 27 page
Pattern formation in reaction diffusion models with spatially inhomogeneous diffusion coefficients
Reaction-diffusion models for biological pattern formation have been studied extensively in a variety of embryonic and ecological contexts. However, despite experimental evidence pointing to the existence of spatial inhomogeneities in various biological systems, most models have only been considered in a spatially homogeneous environment. The authors consider a two-chemical reaction-diffusion mechanism in one space dimension in which one of the diffusion coefficients depends explicitly on the spatial variable. The model is analysed in the case of a step function diffusion coefficient and the insight gained for this special case is used to discuss pattern generation for smoothly varying diffusion coefficients. The results show that spatial inhomogeneity may be an important biological pattern regulator, and possible applications of the model to chondrogenesis in the vertebrate limb are suggested
- …