1,052 research outputs found
Fluorine in the solar neighborhood - is it all produced in AGB-stars?
The origin of 'cosmic' fluorine is uncertain, but there are three proposed
production sites/mechanisms: AGB stars, nucleosynthesis in Type II
supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of
these production sites has not been established even for the solar
neighborhood, leading to uncertainties in stellar evolution models of these
stars as well as uncertainties in the chemical evolution models of stellar
populations.
We determine the fluorine and oxygen abundances in seven bright, nearby
giants with well-determined stellar parameters. We use the 2.3 m
vibrational-rotational HF line and explore a pure rotational HF line at 12.2
m. The latter has never been used before for an abundance analysis. To be
able to do this we have calculated a line list for pure rotational HF lines. We
find that the abundances derived from the two diagnostics agree.
Our derived abundances are well reproduced by chemical evolution models only
including fluorine production in AGB-stars and therefore we draw the conclusion
that this might be the main production site of fluorine in the solar
neighborhood. Furthermore, we highlight the advantages of using the 12 m
HF lines to determine the possible contribution of the -process to the
fluorine budget at low metallicities where the difference between models
including and excluding this process is dramatic
A Case History of Chemical Attack of a Clay Shale
This paper describes the results of an extensive investigation made to determine the cause of excessive settlements of two 800 MN steam turbine units located at the Four Corners Steam Electric Station near Farmington, New Mexico. The units are located on a clay-shale formation with numerous gypsum seams. The settlement was originally attributed to the solutioning of these gypsum seams. It was found that the construction of a large unlined cooling pond raised the ground water table in the vicinity of the power plant. In this saturated environment, a chemical attack of the existing clay shales began. It is believed that the chemical reaction causing the degradation of the clay-shale has been identified and that a chemical solution to the problem has been found. The reaction involves the removal of the exchangeable cation from the clay lattice, which goes into solution. As the cations are leached out, the clays tend to weather toward the montmorillonite end of the transformation series
A New Approach to Swine Shade Construction
A project was initiated by the Agricultural Engineering Department at the South Dakota Agricultural Experiment Station to design a type of swine shade which would not require all the heavy timber under-framing of the current swine shade plans, and which would be actually portable. The need for such swine shelters has been established; research results from other stations have shown that some shading device, either natural or artificial, is necessary to protect swine, especially the heavier ones, from the direct heat of the sun. Extensive work conducted at California shows the effects of different types of surfaces and shade materials on the radiant heat load beneath the shade. Another report, one from Purdue University on the effectiveness of various types of shade on the growth of swine in a normal summer environment, concluded that there were no differences between materials used and no measurable effects of roof color on rate of gain of swine. While these studies may not agree on the effectiveness of different types of shading materials, their findings support the need for some type of shade for swine to maintain or improve rate of gain
A computer program for hydrostatic bearings including the effects of non-uniform film thickness and relative velocity for various methods of lubricant supply final technical report
Computer program for hydrostatic bearing - effects of nonuniform film thickness and lubricant suppl
Water vapor on supergiants. The 12 micron TEXES spectra of mu Cephei
Several recent papers have argued for warm, semi-detached, molecular layers
surrounding red giant and supergiant stars, a concept known as a MOLsphere.
Spectroscopic and interferometric analyses have often corroborated this general
picture. Here, we present high-resolution spectroscopic data of pure rotational
lines of water vapor at 12 microns for the supergiant mu Cephei. This star has
often been used to test the concept of molecular layers around supergiants.
Given the prediction of an isothermal, optically thick water-vapor layer in
Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12
micron lines to be in emission or at least in absorption but filled in by
emission from the molecular layer around the star. Our data, however, show the
contrary; we find definite absorption. Thus, our data do not easily fit into
the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore,
put new, strong constraints on the MOLsphere concept and on the nature of water
seen in signatures across the spectra of early M supergiants. We also find that
the absorption is even stronger than that calculated from a standard,
spherically symmetric model photosphere without any surrounding layers. A cool
model photosphere, representing cool outer layers is, however, able to
reproduce the lines, but this model does not account for water vapor emission
at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be
lacking. It does seem necessary to model the underlying photospheres of these
supergiants in their whole complexity. The strong water vapor lines clearly
reveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa
GaAs interfacial self-cleaning by atomic layer deposition
The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial "self-cleaning" is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements
Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation
The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined
Fluorine Abundances in the Orion Nebula Cluster
This study uses cool dwarfs as sources with which to probe fluorine
abundances via HF. This molecule is detected for the first time in young K-M
dwarf members of an OB association. Fluorine, oxygen, and carbon abundances
were derived from the HF(1--0) R9 line along with samples of OH and CO
vibration-rotation lines present in high-resolution infrared spectra observed
with the Phoenix spectrograph on the Gemini South Telescope. The fluorine and
oxygen results obtained for these targets, still in the pre-main-sequence stage
of evolution, agree well with the general trend defined for the Milky Way disk;
the latter being deduced from observations of more evolved giant stars. In
addition, the carbon and oxygen abundances obtained for the studied stars
overlap results from previous studies of the more massive OB stars and FG dwarf
members of the Orion Nebula Cluster. We conclude from this agreement that the
fluorine abundances derived for the Orion K-M dwarfs (when there is no
conspicuous evidence of disks) can be considered a good representation of the
current fluorine abundance value for the Milky Way disk.Comment: 18 pages, including 2 tables and 3 figures. Accepted for publication
in The Astrophysical Journa
Towards More Precise Survey Photometry for PanSTARRS and LSST: Measuring Directly the Optical Transmission Spectrum of the Atmosphere
Motivated by the recognition that variation in the optical transmission of
the atmosphere is probably the main limitation to the precision of ground-based
CCD measurements of celestial fluxes, we review the physical processes that
attenuate the passage of light through the Earth's atmosphere. The next
generation of astronomical surveys, such as PanSTARRS and LSST, will greatly
benefit from dedicated apparatus to obtain atmospheric transmission data that
can be associated with each survey image. We review and compare various
approaches to this measurement problem, including photometry, spectroscopy, and
LIDAR. In conjunction with careful measurements of instrumental throughput,
atmospheric transmission measurements should allow next-generation imaging
surveys to produce photometry of unprecedented precision. Our primary concerns
are the real-time determination of aerosol scattering and absorption by water
along the line of sight, both of which can vary over the course of a night's
observations.Comment: 41 pages, 14 figures. Accepted PAS
Mira's wind explored in scattering infrared CO lines
We have observed the intermediate regions of the circumstellar envelope of
Mira (o Ceti) in photospheric light scattered by three vibration-rotation
transitions of the fundamental band of CO, from low-excited rotational levels
of the ground vibrational state, at an angular distance of beta = 2"-7" away
from the star. The data were obtained with the Phoenix spectrometer mounted on
the 4 m Mayall telescope at Kitt Peak. The spatial resolution is approximately
0.5" and seeing limited. Our observations provide absolute fluxes, leading to
an independent new estimate of the mass-loss rate of approximately 3e-7
Msun/yr, as derived from a simple analytic wind model. We find that the
scattered intensity from the wind of Mira for 2" < beta < 7" decreases as
beta^-3, which suggests a time constant mass-loss rate, when averaged over 100
years, over the past 1200 years.Comment: accepted for publication in the Astrophysical Journa
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