4,423 research outputs found
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.Comment: 41 pages, 7 Postscript figures. Accepted for publication in the A
Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants
We summarize a selection process to identify red giants in the direction of
the southern warp of the Galactic disk, employing VI_C photometry and
multi-object spectroscopy. We also present results from follow-up
high-resolution, high-S/N echelle spectroscopy of three field red giants,
finding [Fe/H] values of about -0.5. The field stars, with Galactocentric
distances estimated at 10 to 15 kpc, support the conclusion of Yong, Carney, &
de Almeida (2005) that the Galactic metallicity gradient disappears beyond R_GC
values of 10 to 12 kpc for the older stars and clusters of the outer disk. The
field and cluster stars at such large distances show very similar abundance
patterns, and, in particular, all show enhancements of the "alpha" elements O,
Mg, Si, Ca, and Ti and the r-process element Eu. These results suggest that
Type II supernovae have been significant contributors to star formation in the
outer disk relative to Type Ia supernovae within the past few Gyrs. We also
compare our results with those available for much younger objects. The limited
results for the H II regions and B stars in the outer disk also suggest that
the radial metallicity gradient in the outer disk is shallow or absent. The
much more extensive results for Cepheids confirm these trends, and that the
change in slope of the metallicity gradient may occur at a larger
Galactocentric distance than for the older stars and clusters. However, the
younger stars also show rising alpha element enhancements with increasing R_GC,
at least beyond 12 kpc. These trends are consistent with the idea of a
progressive growth in the size of the Galactic disk with time, and episodic
enrichment by Type II supernovae as part of the disk's growth. [Abridged]Comment: Accepted for publication in A
High Reynolds number test of a NACA 651-213, a equals 0.5 airfoil at transonic speeds
Wind-Tunnel tests were conducted in the Lockheed-Georgia Company's compressible flow facility to determine the transonic two-dimensional aerodynamic characteristics of a NACA 65 sub 1-213 a = 0.50 airfoil. The results are correlated with data obtained in the NASA-Langley 8-foot transonic pressure tunnel and the NAE high Reynolds number 15x60-inch two-dimensional test facility. The tests were conducted over a Mach number range from 0.60 to 0.80 and an angle of attack range from -1 deg to 8 deg. Reynolds numbers, based on the airfoil chord, were varied
Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars
Branching fraction measurements from Fourier transform spectra in conjunction
with published radiative lifetimes are used to determine transition
probabilities for 263 lines of neutral chromium. These laboratory values are
employed to derive a new photospheric abundance for the Sun: log (Cr
I) = 5.640.01 (). These Cr I solar abundances do
not exhibit any trends with line strength nor with excitation energy and there
were no obvious indications of departures from LTE. In addition, oscillator
strengths for singly-ionized chromium recently reported by the FERRUM Project
are used to determine: log (Cr II) = 5.770.03 (). Transition probability data are also applied to the spectra of three
stars: HD 75732 (metal-rich dwarf), HD 140283 (metal-poor subgiant), and CS
22892-052 (metal-poor giant). In all of the selected stars, Cr I is found to be
underabundant with respect to Cr II. The possible causes for this abundance
discrepancy and apparent ionization imbalance are discussed.Comment: 44 pages, 6 figure
Ideal MHD theory of low-frequency Alfven waves in the H-1 Heliac
A part analytical, part numerical ideal MHD analysis of low-frequency Alfven
wave physics in the H-1 stellarator is given. The three-dimensional,
compressible ideal spectrum for H-1 is presented and it is found that despite
the low beta (approx. 10^-4) of H-1 plasmas, significant Alfven-acoustic
interactions occur at low frequencies. Several quasi-discrete modes are found
with the three-dimensional linearised ideal MHD eigenmode solver CAS3D,
including beta-induced Alfven eigenmode (BAE)- type modes in beta-induced gaps.
The strongly shaped, low-aspect ratio magnetic geometry of H-1 causes CAS3D
convergence difficulties requiring the inclusion of many Fourier harmonics for
the parallel component of the fluid displacement eigenvector even for shear
wave motions. The highest beta-induced gap reproduces large parts of the
observed configurational frequency dependencies in the presence of hollow
temperature profiles
SHCal04 Southern Hemisphere Calibration, 0–11.0 cal kyr BP
Recent measurements on dendrochronologically-dated wood from the Southern Hemisphere have shown that there are differences between the structural form of the radiocarbon calibration curves from each hemisphere. Thus, it is desirable, when possible, to use calibration data obtained from secure dendrochronologically-dated wood from the corresponding hemisphere. In this paper, we outline the recent work and point the reader to the internationally recommended data set that should be used for future calibration of Southern Hemisphere ¹⁴C dates
Model Data Fusion: developing Bayesian inversion to constrain equilibrium and mode structure
Recently, a new probabilistic "data fusion" framework based on Bayesian
principles has been developed on JET and W7-AS. The Bayesian analysis framework
folds in uncertainties and inter-dependencies in the diagnostic data and signal
forward-models, together with prior knowledge of the state of the plasma, to
yield predictions of internal magnetic structure. A feature of the framework,
known as MINERVA (J. Svensson, A. Werner, Plasma Physics and Controlled Fusion
50, 085022, 2008), is the inference of magnetic flux surfaces without the use
of a force balance model. We discuss results from a new project to develop
Bayesian inversion tools that aim to (1) distinguish between competing
equilibrium theories, which capture different physics, using the MAST spherical
tokamak; and (2) test the predictions of MHD theory, particularly mode
structure, using the H-1 Heliac.Comment: submitted to Journal of Plasma Fusion Research 10/11/200
Oxygen Abundances in Two Metal-Poor Subgiants from the Analysis of the 6300 A Forbidden O I Line
Recent LTE analyses (Israelian et al. 1998 and Bosegaard et al. 1999) of the
OH bands in the optical-ultraviolet spectra of nearby metal-poor subdwarfs
indicate that oxygen abundances are generally higher than those previously
determined. The difference increases with decreasing metallicity and reaches
delta([O/Fe]) ~ +0.6 dex as [Fe/H] approaches -3.0.
Employing high resolution (R = 50000), high S/N (~ 250) echelle spectra of
the two stars found by Israelian et al. (1998) to have the highest
[O/Fe]-ratios, viz, BD +23 3130 and BD +37 1458, we conducted abundance
analyses based on about 60 Fe I and 7-9 Fe II lines. We determined from Kurucz
LTE models the values of the stellar parameters, as well as abundances of Na,
Ni, and the traditional alpha-elements, independent of the calibration of color
vs scales. We determined oxygen abundances from spectral synthesis of
the stronger line (6300 A) of the [O I] doublet.
The syntheses of the [O I] line lead to smaller values of [O/Fe], consistent
with those found earlier among halo field and globular cluster giants. We
obtain [O/Fe] = +0.35 +/- 0.2 for BD +23 3130 and +0.50 +/- 0.2 for BD +37
1458. In the former, the [O I] line is very weak (~ 1 mA), so that the quoted
[O/Fe] value may in reality be an upper limit.
Therefore in these two stars a discrepancy exists between the [O/Fe]- ratios
derived from [O I] and the OH feature, and the origin of this difference
remains unclear. Until the matter is clarified, we suggest it is premature to
conclude that the ab initio oxygen abundances of old, metal-poor stars need to
be revised drastically upward.Comment: 38 pages, 5 tables, 14 figures To appear in July 1999 AJ Updated
April 16, 1999. Fixed typo
Are young stars always associated with cold massive disks? A CO and millimeter interferometric continuum survey
The results of a combined millimeter-spectral-line and continuum survey of cold far-infrared sources selected to favor embedded young stars in the Galaxy are presented. The spectral-line observations were performed with the 5 meter antenna of the University of Texas Millimeter-Wave Observatory. High resolution continuum observations were obtained with the Owens Valley (OVRO) Millimeter-Wave Interferometer. The goal of the survey was to gain insight into the mass, temperature, and distribution of cold dust which envelopes stars during the earliest stages of their evolution. The first phase of our survey involved 1.2 arcmin resolution observations of CO-12 and CO-13 emission lines toward each source. All but two sources had detectable CO emission. We found that 40% of the sources appear to be associated with star formation as evidenced by the presence of enhanced CO-12 line widths or broad wings. At least five of these objects are associated with bipolar molecular outflows. The second phase of our survey involves high resolution 2.7 mm continuum observations with 3 interferometer baselines ranging from 15 to 55 m in length. Preliminary results indicate that about 25% of the sources in our sample have detectable continuum emission on scales less than 30 arcsec. The high percentage of sources with enhanced CO-12 line widths or broad wings indicates that a significant fraction of our samples, 40%, are likely to be young stars. The lower detection percentage in the continuum observations, 25%, suggest that such objects are not always surrounded by large concentrations of gas and dust. The continuum detection percentage for actual dust emission could be lower than that given above since emission from ionized gas could be responsible for the observed 2.7 mm emission in some objects. To get an understanding of the type of object detected in our survey, a map of one of the survey sources, L1689N, has been made using the OVRO mm interferometer
Improved parameters for extrasolar transiting planets
We present refined values for the physical parameters of transiting
exoplanets, based on a self-consistent and uniform analysis of transit light
curves and the observable properties of the host stars. Previously it has been
difficult to interpret the ensemble properties of transiting exoplanets,
because of the widely different methodologies that have been applied in
individual cases. Furthermore, previous studies often ignored an important
constraint on the mean stellar density that can be derived directly from the
light curve. The main contributions of this work are 1) a critical compilation
and error assessment of all reported values for the effective temperature and
metallicity of the host stars; 2) the application of a consistent methodology
and treatment of errors in modeling the transit light curves; and 3) more
accurate estimates of the stellar mass and radius based on stellar evolution
models, incorporating the photometric constraint on the stellar density. We use
our results to revisit some previously proposed patterns and correlations
within the ensemble. We confirm the mass-period correlation, and we find
evidence for a new pattern within the scatter about this correlation: planets
around metal-poor stars are more massive than those around metal-rich stars at
a given orbital period. Likewise, we confirm the proposed dichotomy of planets
according to their Safronov number, and we find evidence that the systems with
small Safronov numbers are more metal-rich on average. Finally, we confirm the
trend that led to the suggestion that higher-metallicity stars harbor planets
with a greater heavy-element content.Comment: To appear in The Astrophysical Journal. 23 pages in emulateapj
format, including figures and tables. Figures 7, 8, and 9 are low resolution;
higher resolution versions will be available from the journal when published.
Acknowledgement added, and minor changes made to TrES-3 and TrES-4 in the
Appendi
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