12,270 research outputs found
Quantitative Determination of the Adiabatic Condition Using Force-Detected Nuclear Magnetic Resonance
The adiabatic condition governing cyclic adiabatic inversion of proton spins
in a micron-sized ammonium chloride crystal was studied using room temperature
nuclear magnetic resonance force microscopy. A systematic degradation of
signal-to-noise was observed as the adiabatic condition became violated. A
theory of adiabatic following applicable to cyclic adiabatic inversion is
reviewed and implemented to quantitatively determine an adiabaticity threshold
from our experimental results.Comment: 5 pages, 3 fig
Structure and magnetic interactions in the solid solution Ba3-xSrxCr2O8
Solid solutions of the magnetic insulators Ba3Cr2O8 and Sr3Cr2O8
(Ba3-xSrxCr2O8) have been prepared in polycrystalline form for the first time.
Single crys- talline material was obtained using a mirror image floating zone
technique. X-ray diffraction data taken at room temperature indicate that the
space group of Ba3-xSrxCr2O8 remains unchanged for all values of x, while the
cell parameters depend on the chemical composition, as expected. Magnetization
data, measured from 300 K down to 2 K, suggests that the interaction constant
Jd within the Cr5+ dimers varies in a peculiar way as a function of x, starting
at Jd = 25K for x = 0, then first slightly dropping to Jd = 18K for x = 0.75,
before reaching Jd = 62K for x = 3
Toxicity of materials in fire situations: Laboratory data obtained at the University of San Francisco
Approximately 300 materials were evaluated using a specific set of test conditions. Materials tested included wood, fibers, fabrics and synthetic polymers. Data obtained using 10 different sets of test conditions are presented
Reduced-rank spatio-temporal modeling of air pollution concentrations in the Multi-Ethnic Study of Atherosclerosis and Air Pollution
There is growing evidence in the epidemiologic literature of the relationship
between air pollution and adverse health outcomes. Prediction of individual air
pollution exposure in the Environmental Protection Agency (EPA) funded
Multi-Ethnic Study of Atheroscelerosis and Air Pollution (MESA Air) study
relies on a flexible spatio-temporal prediction model that integrates land-use
regression with kriging to account for spatial dependence in pollutant
concentrations. Temporal variability is captured using temporal trends
estimated via modified singular value decomposition and temporally varying
spatial residuals. This model utilizes monitoring data from existing regulatory
networks and supplementary MESA Air monitoring data to predict concentrations
for individual cohort members. In general, spatio-temporal models are limited
in their efficacy for large data sets due to computational intractability. We
develop reduced-rank versions of the MESA Air spatio-temporal model. To do so,
we apply low-rank kriging to account for spatial variation in the mean process
and discuss the limitations of this approach. As an alternative, we represent
spatial variation using thin plate regression splines. We compare the
performance of the outlined models using EPA and MESA Air monitoring data for
predicting concentrations of oxides of nitrogen (NO)-a pollutant of primary
interest in MESA Air-in the Los Angeles metropolitan area via cross-validated
. Our findings suggest that use of reduced-rank models can improve
computational efficiency in certain cases. Low-rank kriging and thin plate
regression splines were competitive across the formulations considered,
although TPRS appeared to be more robust in some settings.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS786 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Active Carbon and Oxygen Shell Burning Hydrodynamics
We have simulated 2.5 s of the late evolution of a star with full hydrodynamic behavior. We present the first simulations
of a multiple-shell burning epoch, including the concurrent evolution and
interaction of an oxygen and carbon burning shell. In addition, we have evolved
a 3D model of the oxygen burning shell to sufficiently long times (300 s) to
begin to assess the adequacy of the 2D approximation. We summarize striking new
results: (1) strong interactions occur between active carbon and oxygen burning
shells, (2) hydrodynamic wave motions in nonconvective regions, generated at
the convective-radiative boundaries, are energetically important in both 2D and
3D with important consequences for compositional mixing, and (3) a spectrum of
mixed p- and g-modes are unambiguously identified with corresponding adiabatic
waves in these computational domains. We find that 2D convective motions are
exaggerated relative to 3D because of vortex instability in 3D. We discuss the
implications for supernova progenitor evolution and symmetry breaking in core
collapse.Comment: 5 pages, 4 figures in emulateapj format. Accepted for publication in
ApJ Letters. High resolution figure version available at
http://spinach.as.arizona.ed
Peering beyond IRAS: The 100 to 350 micron dust emission from galaxies
Several arguments can be made to study the continuum emission from dust in galaxies at wavelengths between the cutoff of the Infrared Astronomy Satellite (IRAS) survey (about 100 microns) and the shortest wavelength that is commonly accessible from the ground (about 350 microns). Some theoretical work (see the summary by Cox and Mezger 1989) indicates that there are very cool (T sub d less than or equal to 25 K) components to the dust emission that emit primarily at wavelengths between 100 and 250 microns. In fact, a significant fraction of the total luminosity, representing a large fraction of the dust mass in some types of galaxies, is emitted at long far-infrared wavelengths. In such cases, the cool dust must play a major role in regulation of the energy balance of the Interstellar Medium (ISM) and in shielding the cores of neutral clouds
The Impact of Hydrodynamic Mixing on Supernova Progenitors
Recent multidimensional hydrodynamic simulations have demonstrated the
importance of hydrodynamic motions in the convective boundary and radiative
regions of stars to transport of energy, momentum, and composition. The impact
of these processes increases with stellar mass. Stellar models which
approximate this physics have been tested on several classes of observational
problems. In this paper we examine the implications of the improved treatment
on supernova progenitors. The improved models predict substantially different
interior structures. We present pre-supernova conditions and simple explosion
calculations from stellar models with and without the improved mixing treatment
at 23 solar masses. The results differ substantially.Comment: 12 pages, 2 figures, accepted for publication in the Astrophysical
Journal Letter
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