182 research outputs found
Lifetimes, transition probabilities, and level energies in Fe I
We use time-resolved laser-induced fluorescence to measure the lifetime of 186 Fe levels with energies between 25 900 and 60 758 cm . Measured emission branching fractions for these levels yield transition probabilities for 1174 transitions in the range 225-2666 nm. We find another 640 Fe transition probabilities by interpolating level populations in the inductively coupled plasma spectral source. We demonstrate the reliability of the interpolation method by comparing our transition probabilities with absorption oscillator strengths measured by the Oxford group [Blackwell et al., Mon. Not. R. Astron. Soc. 201, 595-602 (1982)]. We derive precise Fe level energies to support the automated method that is used to identify transitions in our spectra
Boron in Very Metal-Poor Stars
We have observed the B I 2497 A line to derive the boron abundances of two
very metal-poor stars selected to help in tracing the origin and evolution of
this element in the early Galaxy: BD +23 3130 and HD 84937. The observations
were conducted using the Goddard High Resolution Spectrograph on board the
Hubble Space Telescope. A very detailed abundance analysis via spectral
synthesis has been carried out for these two stars, as well as for two other
metal-poor objects with published spectra, using both Kurucz and OSMARCS model
photospheres, and taking into account consistently the NLTE effects on the line
formation. We have also re-assessed all published boron abundances of old disk
and halo unevolved stars. Our analysis shows that the combination of high
effective temperature (Teff > 6000 K, for which boron is mainly ionized) and
low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of
boron abundances from the B I 2497 A line. This is the case of HD 84937 and
three other published objects (including two stars with [Fe/H] ~ -3), for which
only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and
logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a
firm measurement of the boron abundance presently exists. The evolution of the
boron abundance with metallicity that emerges from the seven remaining stars
with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived
using the same stellar parameters, shows a linear increase with a slope ~ 1.
Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the
range -3<[Fe/H]<-1. These results point to spallation reactions of ambient
protons and alpha particles with energetic particles enriched in CNO as the
origin of boron and beryllium in halo stars.Comment: 38 pages, 11 Encapsulated Postscript figures (included), uses
aaspp4.sty. Accepted for publication in The Astrophysical Journal. The
preprint is also available at: http://www.iac.es/publicaciones/preprints.htm
Center-to-Limb Variation of Solar 3-D Hydrodynamical Simulations
We examine closely the solar Center-to-Limb variation of continua and lines
and compare observations with predictions from both a 3-D hydrodynamic
simulation of the solar surface (provided by M. Asplund and collaborators) and
1-D model atmospheres. Intensities from the 3-D time series are derived by
means of the new synthesis code ASSET, which overcomes limitations of
previously available codes by including a consistent treatment of scattering
and allowing for arbitrarily complex line and continuum opacities. In the
continuum, we find very similar discrepancies between synthesis and observation
for both types of model atmospheres. This is in contrast to previous studies
that used a ``horizontally'' and time averaged representation of the 3-D model
and found a significantly larger disagreement with observations. The presence
of temperature and velocity fields in the 3-D simulation provides a significant
advantage when it comes to reproduce solar spectral line shapes. Nonetheless, a
comparison of observed and synthetic equivalent widths reveals that the 3-D
model also predicts more uniform abundances as a function of position angle on
the disk. We conclude that the 3-D simulation provides not only a more
realistic description of the gas dynamics, but, despite its simplified
treatment of the radiation transport, it also predicts reasonably well the
observed Center-to-Limb variation, which is indicative of a thermal structure
free from significant systematic errors.Comment: 18 pages, 9 figures; accepted for publication in the Astrophysical
Journal (June 1, 2008
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
Realistic model atmosphere and revised abundances of the coolest Ap star HD101065
Among the known Ap stars, HD101065 is probably one of the most interesting
objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly
peculiar photometric parameters of this star that can not be fully reproduced
by any modern theoretical calculations, even those accounting for realistic
chemistry of its atmosphere. In this study we investigate a role of missing REE
line opacity and construct a self-consistent atmospheric model based on
accurate abundance and chemical stratification analysis. We employed the
LLmodels stellar model atmosphere code together with DDAFit and SynthMag
software packages to derive homogeneous and stratified abundances for 52
chemical elements and to construct a self-consistent model of HD101065
atmosphere. The opacity in REE lines is accounted for in details, by using
up-to-date extensive theoretical calculations. We show that REE elements play a
key role in the radiative energy balance in the atmosphere of HD101065, leading
to the strong suppression of the Balmer jump and energy redistribution very
different from that of normal stars. Introducing new line lists of REEs allowed
us to reproduce, for the first time, spectral energy distribution of HD101065
and achieve a better agreement between the unusually small observed Str\"omgren
c1 index and the model predictions. Using combined photometric and
spectroscopic approaches and based on the iterative procedure of abundance and
stratification analysis we find effective temperature of HD101065 to be
Teff=6400K.Comment: Accepted by A&A, 13 pages (3 of Online Material), 10 figures, 4
table
Mapping Vesta: First Results from Dawn’s Survey Orbit
The geologic objectives of the Dawn Mission [1] are
to derive Vesta’s shape, map the surface geology,
understand the geological context and contribute to
the determination of the asteroids’ origin and
evolution.Geomorphology and distribution of surface features
will provide evidence for impact cratering, tectonic activity, volcanism, and regolith processes. Spectral
measurements of the surface will provide evidence of
the compositional characteristics of geological units.
Age information, as derived from crater sizefrequency
distributions, provides the stratigraphic
context for the structural and compositional mapping
results, thus revealing the geologic history of Vesta.
We present here the first results of the Dawn mission
from data collected during the approach to Vesta, and
its first discrete orbit phase – the Survey Orbit, which
lasts 21 days after the spacecraft had established a
circular polar orbit at a radius of ~3000 km with a
beta angle of 10°-15°
Abundances of Extremely Metal-Poor Star Candidates
We present chemical abundances for 110 stars identified in objective-prism
surveys as candidates to be very metal-poor. The abundances are derived from
high S/N, intermediate-resolution spectra obtained with the Keck Observatory
Echelle Spectrometer and Imager. An additional 25 stars with well-determined
abundances ranging from [Fe/H] and -3.2 were observed and the results
used to help calibrate our analysis and determine the accuracy of our abundance
determinations. Abundances for the program stars were measured for Fe, Mg, Ca,
Ti, Cr and Ba with an accuracy of approximately 0.3 dex. 53 of the stars in our
sample have [Fe/H]<= -2, 22 have [Fe/H]<= -2.5 and 13 stars have [Fe/H]<= -2.9.
Surprisingly, approximately one third of the sample is relatively metal rich
with [Fe/H]>-1.5. In addition to identifying a number of extremely metal-poor
stars, this study also shows that moderate-resolution spectra obtained with the
Keck Echelle Spectrometer and Imager yield relatively accurate abundances for
stars as faint as V=14 in modest exposure time (~20 minutes). This capability
will prove useful if the so-far elusive stars at [Fe/H]<-4 turn out to be
mostly fainter than V=15.Comment: 60 pages, 10 figures, Accepted for publication in the A
Oxygen and Magnesium Abundance in the Ultra-Metal-Poor Giants CS22949-037 and CS29498-043: Challenges in Models of Atmospheres
We report the results of a non-LTE Fe, O and Mg abundance analysis of the
carbon-nitrogen-rich ultra-metal-poor giants CS29498--043 and CS22949--037. The
abundance of oxygen has been derived from measurements of the oxygen triplet at
7771-5 A in high resolution spectra obtained with KeckI/HIRES and the forbidden
line [OI] 6300 A detected in the TNG/SARG spectra of CS29498-043. Detailed
non-LTE analysis of Fe lines has provided reliable stellar parameters which,
however, do not resolve the oxygen abundance conflict as derived from the OI
7771-5 A triplet and the [OI] 6300 A forbidden lines. We obtained the following
oxygen abundance: for CS22949-037 [O/Fe] =3.13, 1.95; and for CS29498-043;
[O/Fe]=3.02, 2.49, based on the OI 7771-5 A triplet and the [OI] 6300 A
forbidden line, respectively. A similar conflict appears to exist between the
forbidden resonance line MgI 4571 A and several subordinate lines, such as MgI
5172 and 5183 A. Our analysis demonstrates the failure of standard
plane--parallel atmosphere models to describe the physical conditions in the
line-forming regions of these ultra-metal-poor giants.Comment: 16 pages, 15 figures, accepted by Astronomy and Astrophysic
Systems Analysis Unfolds the Relationship between the Phosphoketolase Pathway and Growth in Aspergillus nidulans
Background: Aspergillus nidulans is an important model organism for studies on fundamental eukaryotic cell biology and on industrial processes due to its close relation to A. niger and A. oryzae. Here we identified the gene coding for a novel metabolic pathway in A. nidulans, namely the phosphoketolase pathway, and investigated the role of an increased phosphoketolase activity. Methodology/Principal Findings: Over-expression of the phosphoketolase gene (phk) improved the specific growth rate on xylose, glycerol and ethanol. Transcriptome analysis showed that a total of 1,222 genes were significantly affected by overexpression of the phk, while more than half of the affected genes were carbon source specific. During growth on glucose medium, the transcriptome analysis showed that the response to phk over-expression is targeted to neutralize the effect of the over-expression by regulating the acetate metabolism and initiate a growth dampening response. Conclusions/Significance: Metabolic flux analysis using 13C-labelled glucose, showed that over-expression of phosphoketolase added flexibility to the central metabolism. Our findings further suggests that A. nidulans is not optimized for growth on xylose, glycerol or ethanol as the sole carbon sources. © 2008 Panagiotou et al.published_or_final_versio
A quantitative investigation for deployment of mobile collaborative robots in high-value manufacturing
Component inspection is often the bottleneck in high-value manufacturing, driving industries like aerospace toward automated inspection technologies. Current systems often employ fixed arm robots, but they lack the flexibility in adapting to new components or orientations Advanced mobile robotic platforms with updated sensor technologies and algorithms have improved localization and path planning capabilities, making them ideal for bringing inspection processes directly to parts. However, mobile platforms introduce challenges in localization and maneuverability, leading to potential errors. Their positional uncertainty is higher than fixed systems due to the lack of a fixed calibrated location, posing challenges for position-sensitive inspection sensors. Therefore, it's essential to assess the positional accuracy and repeatability of mobile manipulator platforms. The KUKA KMR iiwa was chosen for its collaborative features, robust build, and scalability within the KUKA product range. The accuracy and repeatability of the mobile platform were evaluated through a series of tests to evaluate the performance of its integrated feature mapping, the effect of various speeds on positional accuracy, and the efficiency of the omnidirectional wheels for a range of translation orientations. Experimental evaluation revealed that enabling feature mapping substantially improves the KUKA KMR iiwa's performance, with accuracy gains and error reductions exceeding 90%. Repeatability errors were under 7 mm with mapping activated and around 2.5 mm in practical scenarios, demonstrating that mobile manipulators, incorporating both the manipulator and platform, can fulfil the precise requirements of industries with high precision needs. Providing a highly diverse alternative to traditional fixed-base industrial manipulators
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