2,952 research outputs found
Far Infrared Spectroscopy
Contains a report on a research project.National Science Foundation (Grant G-19637
Hydrothermal alteration at the Roosevelt hot springs thermal area, Utah: Petrographic characterization of the alteration to 2 kilometers depth
Hydrothermal alteration in drill cuttings from Thermal Power drillhole 14-2, Roosevelt Hot Springs Thermal area, has been studied petrographically. The hole is sited in alluvium approximately 1.6 km southeast of the old Resort and was rotary drilled to a depth of 1866.0 m. The exact hole location is 2310 FNL, 350 FWL, Sec. 2, Twp 27S, Rge 9W, elevation 1908.5 m. Core was extracted from 792.5 to 795.5 m. Thin sections were made from samples 15.2 m intervals of drill cuttings collected at 1.5 or 3.0 m intervals during drilling. Thin sections were made of 1.5 or 3.0 m intervals from 274.3 to 304.8 m, 487.9 to 581.2 m, and 868.7 t o 899.2 m. These intervals were chosen for close spaced sampling on the basis of increases in temperature, porosity, conductivity and acoustic velocity shown in geophysical logs. A total of 153 thin sections of cuttings were made, and an additional 9 sections were made from the core. Depths of thin section samples are listed in the appendix. A visual estimate of the percentage of each rock type was made for each thin section
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*
A point-like source of ~TeV gamma-rays has recently been seen towards the
Galactic center by HESS and other air Cerenkov telescopes. In recent work
(Ballantyne et al. 2007), we demonstrated that these gamma-rays can be
attributed to high-energy protons that (i) are accelerated close to the event
horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and
(iii) finally interact to produce gamma-rays. The same hadronic collision
processes will necessarily lead to the creation of electrons and positrons.
Here we calculate the synchrotron emissivity of these secondary leptons in the
same magnetic field configuration through which the initiating protons have
been propagated in our model. We compare this emission with the observed ~GHz
radio spectrum of the inner few pc region which we have assembled from archival
data and new measurements we have made with the Australia Telescope Compact
Array. We find that our model predicts secondary synchrotron emission with a
steep slope consistent with the observations but with an overall normalization
that is too large by a factor of ~ 2. If we further constrain our theoretical
gamma-ray curve to obey the implicit EGRET upper limit on emission from this
region we predict radio emission that is consistent with observations, i.e.,
the hadronic model of gamma ray emission can, simultaneously and without
fine-tuning, also explain essentially all the diffuse radio emission detected
from the inner few pc of the Galaxy.Comment: 11 pages, 2 figures. Two references missing from published version
added and acknowledgements extende
Detecting contaminated birthdates using generalized additive models.
Erroneous patient birthdates are common in health databases. Detection of these errors usually involves manual verification, which can be resource intensive and impractical. By identifying a frequent manifestation of birthdate errors, this paper presents a principled and statistically driven procedure to identify erroneous patient birthdates
Emission-line Helium Abundances in Highly Obscured Nebulae
This paper outlines a way to determine the ICF using only infrared data. We
identify four line pairs, [NeIII] 36\micron/[NeII] 12.8\micron,
[NeIII]~15.6\micron /[NeII] 12.8\micron, [ArIII] 9\micron/[ArII]
6.9\micron, and [ArIII] 21\micron/[ArII] 6.9\micron, that are sensitive
to the He ICF. This happens because the ions cover a wide range of ionization,
the line pairs are not sensitive to electron temperature, they have similar
critical densities, and are formed within the He/H region of the
nebula. We compute a very wide range of photoionization models appropriate for
galactic HII regions. The models cover a wide range of densities, ionization
parameters, stellar temperatures, and use continua from four very different
stellar atmospheres.
The results show that each line pair has a critical intensity ratio above
which the He ICF is always small. Below these values the ICF depends very
strongly on details of the models for three of the ratios, and so other
information would be needed to determine the helium abundance. The [Ar III]
9\micron/[ArII] 6.9\micron ratio can indicate the ICF directly due to the
near exact match in the critical densities of the two lines. Finally, continua
predicted by the latest generation of stellar atmospheres are sufficiently hard
that they routinely produce significantly negative ICFs.Comment: Accepted by PASP. Scheduled for the October 1999 issue. 11 pages, 5
figure
Advanced photovoltaic power systems using tandem GaAs/GaSb concentrator modules
In 1989, Boeing announced the fabrication of a tandem gallium concentrator solar cell with an energy conversion efficiency of 30 percent. This research breakthrough has now led to panels which are significantly smaller, lighter, more radiation resistant, and potentially less expensive than the traditional silicon flat plate electric power supply. The new Boeing tandem concentrator (BTC) module uses an array of lightweight silicone Fresnel lenses mounted on the front side of a light weight aluminum honeycomb structure to focus sunlight onto small area solar cells mounted on a thin back plane. This module design is shown schematically. The tandem solar cell in this new module consists of a gallium arsenide light sensitive cell with a 24 percent energy conversion efficiency stacked on top of a gallium antimonide infrared sensitive cell with a conversion efficiency of 6 percent. This gives a total efficiency 30 percent for the cell-stack. The lens optical efficiency is typically 85 percent. Discounting for efficiency losses associated with lens packing, cell wiring, and cell operating temperature still allows for a module efficiency of 22 percent which leads to a module power density of 300 Watts/sq. m. This performance provides more than twice the power density available from a single crystal silicon flat plate module and at least four times the power density available from amorphous silicon modules. The fact that the lenses are only 0.010 ft. thick and the aluminum foil back plane is only 0.003 ft. thick leads to a very lightweight module. Although the cells are an easy to handle thickness of 0.020 ft., the fact that they are small, occupying one-twenty-fifth of the module area, means that they add little to the module weight. After summing all the module weights and given the high module power, we find that we are able to fabricate BTC modules with specific power of 100 watts/kg
Achromatic late-time variability in thermonuclear X-ray bursts - an accretion disk disrupted by a nova-like shell?
An unusual Eddington-limited thermonuclear X-ray burst was detected from the
accreting neutron star in 2S 0918-549 with the Rossi X-ray Timing Explorer. The
burst commenced with a brief (40 ms) precursor and maintained near-Eddington
fluxes during the initial 77 s. These characteristics are indicative of a
nova-like expulsion of a shell from the neutron star surface. Starting 122 s
into the burst, the burst shows strong (87 +/- 1% peak-to-peak amplitude)
achromatic fluctuations for 60 s. We speculate that the fluctuations are due to
Thompson scattering by fully-ionized inhomogeneities in a resettling accretion
disk that was disrupted by the effects of super-Eddington fluxes. An expanding
shell may be the necessary prerequisite for the fluctuations.Comment: 7 pages, 4 figures. Submitted to A&
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation
We present results from photoionization models of low-metallicity HII
regions. These nebulae form the basis for measuring the primordial helium
abundance. Our models show that the helium ionization correction factor (ICF)
can be non-negligible for nebulae excited by stars with effective temperatures
larger than 40,000 K. Furthermore, we find that when the effective temperature
rises to above 45,000 K, the ICF can be significantly negative. This result is
independent of the choice of stellar atmosphere. However, if an HII region has
an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that,
regardless of its metallicity, it will have a negligibly small ICF. A similar,
but metallicity dependent, result was found using the [O III] 5007/H
ratio. These two results can be used as selection criteria to remove nebulae
with potentially non-negligible ICFs. Using our metallicity independent
criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of
the rms scatter about the best fit line. A fit to the selected data
results in a slight increase of the value of the primordial helium abundance.Comment: 10 pages, 5 figures, accepted by the Ap
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