26,176 research outputs found
The national debt: a secular perspective
An examination of the various factors that have determined the level and growth of the federal debt over the past 40 years, with some perspective on future levels of federal debt.Debts, Public ; Budget deficits
Probing the wind-wind collision in Gamma Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization
We present a new analysis of an archived Chandra HETGS X-ray spectrum of the
WR+O colliding wind binary Gamma Velorum. The spectrum is dominated by emission
lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a
combination of broad-band spectral analysis and an analysis of line flux ratios
we infer a wide range of temperatures in the X-ray emitting plasma (~4-40 MK).
As in the previously published analysis, we find the X-ray emission lines are
essentially unshifted, with a mean FWHM of 1240 +/- 30 km/s. Calculations of
line profiles based on hydrodynamical simulations of the wind-wind collision
predict lines that are blueshifted by a few hundred km/s. The lack of any
observed shift in the lines may be evidence of a large shock-cone opening
half-angle (> 85 degrees), and we suggest this may be evidence of sudden
radiative braking. From the R and G ratios measured from He-like
forbidden-intercombination-resonance triplets we find evidence that the Mg XI
emission originates from hotter gas closer to the O star than the Si XIII
emission, which suggests that non-equilibrium ionization may be present.Comment: 22 pages, 14 figures. Accepted for publication in MNRA
Separation of bacterial spores from flowing water in macro-scale cavities by ultrasonic standing waves
The separation of micron-sized bacterial spores (Bacillus cereus) from a
steady flow of water through the use of ultrasonic standing waves is
demonstrated. An ultrasonic resonator with cross-section of 0.0254 m x 0.0254 m
has been designed with a flow inlet and outlet for a water stream that ensures
laminar flow conditions into and out of the resonator section of the flow tube.
A 0.01905-m diameter PZT-4, nominal 2-MHz transducer is used to generate
ultrasonic standing waves in the resonator. The acoustic resonator is 0.0356 m
from transducer face to the opposite reflector wall with the acoustic field in
a direction orthogonal to the water flow direction. At fixed frequency
excitation, spores are concentrated at the stable locations of the acoustic
radiation force and trapped in the resonator region. The effect of the
transducer voltage and frequency on the efficiency of spore capture in the
resonator has been investigated. Successful separation of B. cereus spores from
water with typical volume flow rates of 40-250 ml/min has been achieved with
15% efficiency in a single pass at 40 ml/min.Comment: 11 pages, 6 figure
Absolute Calibration of the Radio Astronomy Flux Density Scale at 22 to 43 GHz Using Planck
The Planck mission detected thousands of extragalactic radio sources at
frequencies from 28 to 857 GHz. Planck's calibration is absolute (in the sense
that it is based on the satellite's annual motion around the Sun and the
temperature of the cosmic microwave background), and its beams are well
characterized at sub-percent levels. Thus Planck's flux density measurements of
compact sources are absolute in the same sense. We have made coordinated VLA
and ATCA observations of 65 strong, unresolved Planck sources in order to
transfer Planck's calibration to ground-based instruments at 22, 28, and 43
GHz. The results are compared to microwave flux density scales currently based
on planetary observations. Despite the scatter introduced by the variability of
many of the sources, the flux density scales are determined to 1-2% accuracy.
At 28 GHz, the flux density scale used by the VLA runs 3.6% +- 1.0% below
Planck values; at 43 GHz, the discrepancy increases to 6.2% +- 1.4% for both
ATCA and the VLA.Comment: 16 pages, 4 figures and 4 table
Performance of the SERT 2 spacecraft after 4 and one-half years in space
The SERT 2 satellite, launched in February, 1970, has recently been reactivated after being dormant for a year. The satellite orientation was changed to spin stabilization in 1973 and the satellite is now coning such that the active side of the solar arrays move into and out of the sun every 23 days. There is sufficient power to operate the ion thrusters for seven days of this cycle. The 2 ion thruster system was successfully operated; the electrical short in the accelerating grids had cleared during the year long dormant phase
Neutron scattering measurements of phonons in nickel at elevated temperatures
Measurements of elastic and inelastic neutron scatterings from elemental nickel were made at 10, 300, 575, 875, and 1275 K. The phonon densities of states (DOSs) were calculated from the inelastic scattering and were fit with Born–von Kármán models of the lattice dynamics. With ancillary data on thermal expansion and elastic moduli, we found a small, negative anharmonic contribution to the phonon entropy at high temperature. We used this to place bounds on the magnetic entropy of nickel. A significant broadening of the phonon DOS at elevated temperatures, another indication of anharmonicity, was also measured and quantified
The elements of the thermodynamic structure of the tropical atmosphere
Understanding of the tropical atmosphere is elaborated around two elementary ideas, one being that density is homogenized on isobars, which is referred to as the weak temperature gradient (WTG), and the other being that the vertical thermal structure follows a moist-adiabatic lapse rate. This study uses simulations from global storm-resolving models to investigate the accuracy of these ideas. Our results show that horizontally, the density temperature appears to be homogeneous, but only in the mid- and lower troposphere (between 400 hPa and 800 hPa). To achieve a homogeneous density temperature, the horizontal absolute temperature structure adjusts to balance the horizontal moisture difference. Thus, water vapor plays an important role in the horizontal temperature distribution. Density temperature patterns in the mid- and lower troposphere vary by about 0.3 K on the scale of individual ocean basins but differ by 1 K among basins. We use equivalent potential temperature to explore the vertical structure of the tropical atmosphere, and we compare the results assuming the temperature following pseudo-adiabat and reversible-adiabat (isentropic) with the effect of condensate loading. Our results suggest that the tropical atmosphere in saturated convective regions tends to adopt a thermal structure that is isentropic below the zero-degree isotherm and pseudo-adiabatic above it. However, the tropical mean temperature is substantially colder and is set by the bulk of convection, which is affected by entrainment in the lower troposphere
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