806 research outputs found
A Way Around the Divided House Majority
For observers of the U.S. Congress, the inability of Republicans to unite behind a candidate for speaker has been by turn fascinating, exasperating, and frightening. When and if a candidate is finally chosen, Republicans will breathe a sigh of relief, as will many commentators. The real crisis, however, will have just begun. [excerpt
Three-dimensional imaging and detection efficiency performance of orthogonal coplanar CZT strip detectors
We report on recent three-dimensional imaging performance and detection efficiency measurements obtained with 5 mm thick prototype CdZnTe detectors fabricated with orthogonal coplanar anode strips. In previous work, we have shown that detectors fabricated using this design achieve both very good energy resolution and sub-millimeter spatial resolution with fewer electronic channels than are required for pixel detectors. As electron-only devices, like pixel detectors, coplanar anode strip detectors can be fabricated in the thickness required to be effective imagers for photons with energies in excess of 500 keV. Unlike conventional double-sided strip detectors, the coplanar anode strip detectors require segmented contacts and signal processing electronics on only one surface. The signals can be processed to measure the total energy deposit and the photon interaction location in three dimensions. The measurements reported here provide a quantitative assessment of the detection capabilities of orthogonal coplanar anode strip detectors
Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation
Using recent dust continuum data, we generate the intrinsic ellipticity
distribution of dense, starless molecular cloud cores. Under the hypothesis
that the cores are all either oblate or prolate randomly-oriented spheroids, we
show that a satisfactory fit to observations can be obtained with a gaussian
prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we
show that correlations exist between the apparent axis ratio and both the peak
intensity and total flux density of emission from the cores, the sign of which
again favours the prolate hypothesis. The latter result shows that the mass of
a given core depends on its intrinsic ellipticity. Monte Carlo simulations are
performed to find the best-fit power law of this dependence. Finally, we show
how these results are consistent with an evolutionary scenario leading from
filamentary parent clouds to increasingly massive, condensed, and roughly
spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap
Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models
We use 3D numerical MHD simulations to follow the evolution of cold,
turbulent, gaseous systems with parameters representing GMC conditions. We
study three cloud simulations with varying mean magnetic fields, but identical
initial velocity fields. We show that turbulent energy is reduced by a factor
two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically
supercritical cloud models collapse after ~6 Myr, while the subcritical cloud
does not collapse. We compare density, velocity, and magnetic field structure
in three sets of snapshots with matched Mach numbers. The volume and column
densities are both log-normally distributed, with mean volume density a factor
3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4
times the unperturbed value. We use a binning algorithm to investigate the
dependence of kinetic quantities on spatial scale for regions of column density
contrast (ROCs). The average velocity dispersion for the ROCs is only weakly
correlated with scale, similar to the mean size-linewidth relation for clumps
within GMCs. ROCs are often superpositions of spatially unconnected regions
that cannot easily be separated using velocity information; the same difficulty
may affect observed GMC clumps. We analyze magnetic field structure, and show
that in the high density regime, total magnetic field strengths increase with
density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field
strengths vary widely across a projected cloud, and do not correlate with
column density. We compute simulated interstellar polarization maps at varying
orientations, and determine that the Chandrasekhar-Fermi formula multiplied by
a factor ~0.5 yields a good estimate of the plane-of sky magnetic field
strength provided the dispersion in polarization angles is < 25 degrees.Comment: 56 pages, 25 figures; Ap.J., accepte
A Legislative Proposal for Improving Materials Policymaking: Impacts and Issues
The materials data, forecasting, and analysis system is very important to the future materials policies of this country. Adequate, timely, and reliable data, forecasts, and analyses are not sufficient for effective policy to solve complicated national materials problems, but they are necessary for improving the public policymaking process. Under a contract from Congress’ Office of Technology Assessment, the present authors reviewed the existing repositories of materials information and the possibilities for improvement. We interviewed policymakers in government, private industry, and nonprofit organizations. In this Article we describe our principal recommendation resulting from that research, a proposal for a new Bureau of Materials Statistics and Forecasting, and analyze its probable effects on government, free enterprise, and the general public
The nature of the dense core population in the pipe nebula: core and cloud kinematics from C18O observations
We present molecular-line observations of 94 dark cloud cores identified in
the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio
Observatory 12m telescope, we obtained spectra of these cores in the J=1-0
transition of C18O. We use the measured core parameters, i.e., antenna
temperature, linewidth, radial velocity, radius and mass, to explore the
internal kinematics of these cores as well as their radial motions through the
larger molecular cloud. We find that the vast majority of the dark extinction
cores are true cloud cores rather than the superposition of unrelated
filaments. While we identify no significant correlations between the core's
internal gas motions and the cores' other physical parameters, we identify
spatially correlated radial velocity variations that outline two main kinematic
components of the cloud. The largest is a 15pc long filament that is
surprisingly narrow both in spatial dimensions and in radial velocity.
Beginning in the Stem of the Pipe, this filament displays uniformly small C18O
linewidths (dv~0.4kms-1) as well as core to core motions only slightly in
excess of the gas sound speed. The second component outlines what appears to be
part of a large (2pc; 1000 solar mass) ring-like structure. Cores associated
with this component display both larger linewidths and core to core motions
than in the main cloud. The Pipe Molecular Ring may represent a primordial
structure related to the formation of this cloud.Comment: Accepted to ApJ. 14 pages, 11 figures. Complete table at end of
documen
The Mass-Metallicity Relation at z~2
We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean
spectroscopic redshift z=2.26 to study the correlation between metallicity and
stellar mass at high redshift. Using stellar masses determined from SED fitting
to 0.3-8 micron photometry, we divide the sample into six bins in stellar mass,
and construct six composite H-alpha+[NII] spectra from all of the objects in
each bin. We estimate the mean oxygen abundance in each bin from the
[NII]/H-alpha ratio, and find a monotonic increase in metallicity with
increasing stellar mass, from 12+log(O/H) =
2.7e9 Msun to 12+log(O/H) = 8.6 for galaxies with = 1e11 Msun. We use
the empirical relation between star formation rate density and gas density to
estimate the gas fractions of the galaxies, finding an increase in gas fraction
with decreasing stellar mass. These gas fractions combined with the observed
metallicities allow the estimation of the effective yield y_eff as a function
of stellar mass; in constrast to observations in the local universe which show
a decrease in y_eff with decreasing baryonic mass, we find a slight increase.
Such a variation of metallicity with gas fraction is best fit by a model with
supersolar yield and an outflow rate ~4 times higher than the star formation
rate. We conclude that the mass-metallicity relation at high redshift is driven
by the increase in metallicity as the gas fraction decreases through star
formation, and is likely modulated by metal loss from strong outflows in
galaxies of all masses. There is no evidence for preferential loss of metals
from low mass galaxies as has been suggested in the local universe. [Abridged]Comment: 18 pages, 9 figures, 2 tables; accepted for publication in Ap
Update on Local Processing in Iowa: Ethanol and Soybean Processing
Local processing of corn and soybeans in Iowa continues to expand production of food, feed, and fuel products. Opportunities for farmers to sell crops locally to processing plants as well as grain handling facilities are expanding. Iowa farmers are expected to produce in 2007 2,511,000 bu of corn and 443,000 bu of soybeans. With the rapid expansion ofbiofuels production, Iowa has become the national leader in both ethanol and biodiesel production. These two products alone offer markets for 43% of Iowa\u27s corn production and the oil from 42% of Iowa\u27s soybeans. Biodiesel differs from ethanol in that biodiesel uses one of two primary soybean crush products rather than raw grain
In Solidarity
This edition of Next Page is a departure from our usual question and answer format with a featured campus reader. Instead, we asked speakers who participated in the College’s recent Student Solidarity Rally (March 1, 2017) to recommend readings that might further our understanding of the topics on which they spoke
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