11,810 research outputs found
Secondary and compound concentrators for parabolic dish solar thermal power systems
A secondary optical element may be added to a parabolic dish solar concentrator to increase the geometric concentration ratio attainable at a given intercept factor. This secondary may be a Fresnel lens or a mirror, such as a compound elliptic concentrator or a hyperbolic trumpet. At a fixed intercept factor, higher overall geometric concentration may be obtainable with a long focal length primary and a suitable secondary matched to it. Use of a secondary to increase the geometric concentration ratio is more likely to e worthwhile if the receiver temperature is high and if errors in the primary are large. Folding the optical path with a secondary may reduce cost by locating the receiver and power conversion equipment closer to the ground and by eliminating the heavy structure needed to support this equipment at the primary focus. Promising folded-path configurations include the Ritchey-Chretien and perhaps some three element geometries. Folding the optical path may be most useful in systems that provide process heat
Testing modified gravity with globular cluster velocity dispersions
Globular clusters (GCs) in the Milky Way have characteristic velocity
dispersions that are consistent with the predictions of Newtonian gravity, and
may be at odds with Modified Newtonian Dynamics (MOND). We discuss a modified
gravity (MOG) theory that successfully predicts galaxy rotation curves, galaxy
cluster masses and velocity dispersions, lensing, and cosmological
observations, yet produces predictions consistent with Newtonian theory for
smaller systems, such as GCs. MOG produces velocity dispersion predictions for
GCs that are independent of the distance from the galactic center, which may
not be the case for MOND. New observations of distant GCs may produce strong
criteria that can be used to distinguish between competing gravitational
theories.Comment: 4 pages, 2 figures; accepted for publication in Ap
Casimir forces beyond the proximity approximation
The proximity force approximation (PFA) relates the interaction between
closely spaced, smoothly curved objects to the force between parallel plates.
Precision experiments on Casimir forces necessitate, and spur research on,
corrections to the PFA. We use a derivative expansion for gently curved
surfaces to derive the leading curvature modifications to the PFA. Our methods
apply to any homogeneous and isotropic materials; here we present results for
Dirichlet and Neumann boundary conditions and for perfect conductors. A Pad\'e
extrapolation constrained by a multipole expansion at large distance and our
improved expansion at short distances, provides an accurate expression for the
sphere-plate Casimir force at all separations.Comment: 4 pages, 1 figur
The Connection between Supernova Remnants and the Galactic Magnetic Field: A Global Radio Study of the Axisymmetric Sample
The study of supernova remnants (SNRs) is fundamental to understanding the
chemical enrichment and magnetism in galaxies, including our own Milky Way. In
an effort to understand the connection between the morphology of SNRs and the
Galactic magnetic field (GMF), we have examined the radio images of all known
SNRs in our Galaxy and compiled a large sample that have an "axisymmetric"
morphology, which we define to mean SNRs with a "bilateral" or "barrel"-shaped
morphology, in addition to one-sided shells. We selected the cleanest examples
and model each of these at their appropriate Galactic position using two GMF
models, those of Jansson & Farrar (2012a), which includes a vertical halo
component, and Sun et al. (2008) that is oriented entirely parallel to the
plane. Since the magnitude and relative orientation of the magnetic field
changes with distance from the sun, we analyse a range of distances, from 0.5
to 10 kpc in each case. Using a physically motivated model of a SNR expanding
into the ambient GMF, we find the models using Jansson & Farrar (2012a) are
able to reproduce observed morphologies of many SNRs in our sample. These
results strongly support the presence of an off-plane, vertical component to
the GMF, and the importance of the Galactic field on SNR morphology. Our
approach also provides a potential new method for determining distances to
SNRs, or conversely, distances to features in the large-scale GMF if SNR
distances are known.Comment: 24 pages, 8 figures plus one 5-page appendix figure, 3 tables,
accepted to A&
The 100 and 160 micron maps of the dust reemission from the nucleus and inner-arm regions of NGC 6946
Dust reemission from the Scd galaxy NGC 6946 has been measured at 100 and 160 microns with the 32-channel University of Chicago Far-Infrared Camera. Researchers present fully sampled maps of the nucleus and inner spiral arms at 45 seconds resolution. The far-infrared morphology of the galaxy is a bright peak centered on a diffuse disk, where the peak occurs about 24 seconds NE of the Dressel and Condon optical center. The 100/160 micron color temperature is correlated with the H alpha surface brightness. Assuming the distance from Earth to the galaxy is 10.1 Mpc, researchers determine that Tc is 32 K at the nucleus and at radius 5.4 kpc, where there is a concentration of H II regions. In the intermediate annulus of relatively low H alpha surface brightness, the temperature drops to a local minimum of 25 K at radius 3 kpc. The ratio of reradiated to transmitted stellar luminosity is approx. 3.0 at the nucleus and approx. 0.9 for the disk. The optical depth at 100 micron increases from .0005 at the edges of our map to .0035 at the far infrared radiation (FIR) peak. Combining our observations with a fully sampled map of similar spatial extent in CO(1 greater than 0), researchers determine that the ratio F sub IR/I sub CO at the center of the galaxy is almost twice that for the disk, where the value is more or less constant
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