2,661 research outputs found
Metal-Poor Stars Observed with the Magellan Telescope I. Constraints on Progenitor Mass and Metallicity of AGB Stars Undergoing s-Process Nucleosynthesis
We present a comprehensive abundance analysis of two newly-discovered
carbon-enhanced metal-poor (CEMP) stars. HE2138-3336 is a s-process-rich star
with [Fe/H] = -2.79, and has the highest [Pb/Fe] abundance ratio measured thus
far, if NLTE corrections are included ([Pb/Fe] = +3.84). HE2258-6358, with
[Fe/H] = -2.67, exhibits enrichments in both s- and r-process elements. These
stars were selected from a sample of candidate metal-poor stars from the
Hamburg/ESO objective-prism survey, and followed up with medium-resolution (R ~
2,000) spectroscopy with GEMINI/GMOS. We report here on derived abundances (or
limits) for a total of 34 elements in each star, based on high-resolution (R ~
30,000) spectroscopy obtained with Magellan-Clay/MIKE. Our results are compared
to predictions from new theoretical AGB nucleosynthesis models of 1.3 Mo with
[Fe/H] = -2.5 and -2.8, as well as to a set of AGB models of 1.0 to 6.0 Mo at
[Fe/H] = -2.3. The agreement with the model predictions suggests that the
neutron-capture material in HE2138-3336 originated from mass transfer from a
binary companion star that previously went through the AGB phase, whereas for
HE2258-6358, an additional process has to be taken into account to explain its
abundance pattern. We find that a narrow range of progenitor masses (1.0 <
M(Mo) < 1.3) and metallicities (-2.8 < [Fe/H] < -2.5) yield the best agreement
with our observed elemental abundance patterns.Comment: Accepted for publication in Ap
Retrieval of lower-order moments of the drop size distribution using CSU-CHILL X-band polarimetric radar: a case study
The lower-order moments of the drop size distribution (DSD) have generally been considered difficult to retrieve accurately from polarimetric radar data because these data are related to higher-order moments. For example, the 4.6th moment is associated with a specific differential phase and the 6th moment with reflectivity and ratio of high-order moments with differential reflectivity. Thus, conventionally, the emphasis has been to estimate rain rate (3.67th moment) or parameters of the exponential or gamma distribution for the DSD. Many double-moment “bulk” microphysical schemes predict the total number concentration (the 0th moment of the DSD, or M0) and the mixing ratio (or equivalently, the 3rd moment M3). Thus, it is difficult to compare the model outputs directly with polarimetric radar observations or, given the model outputs, forward model the radar observables. This article describes the use of double-moment normalization of DSDs and the resulting stable intrinsic shape that can be fitted by the generalized gamma (G-G) distribution. The two reference moments are M3 and M6, which are shown to be retrievable using the X-band radar reflectivity, differential reflectivity, and specific attenuation (from the iterative correction of measured reflectivity Zh using the total Φdp constraint, i.e., the iterative ZPHI method). Along with the climatological shape parameters of the G-G fit to the scaled/normalized DSDs, the lower-order moments are then retrieved more accurately than possible hitherto. The importance of measuring the complete DSD from 0.1 mm onwards is emphasized using, in our case, an optical array probe with 50 µm resolution collocated with a two-dimensional video disdrometer with about 170 µm resolution. This avoids small drop truncation and hence the accurate calculation of lower-order moments. A case study of a complex multi-cell storm which traversed an instrumented site near the CSU-CHILL radar is described for which the moments were retrieved from radar and compared with directly computed moments from the complete spectrum measurements using the aforementioned two disdrometers. Our detailed validation analysis of the radar-retrieved moments showed relative bias of the moments M0 through M2 was 0.9. Both radar measurement and parameterization errors were estimated rigorously. We show that the temporal variation of the radar-retrieved mass-weighted mean diameter with M0 resulted in coherent “time tracks” that can potentially lead to studies of precipitation evolution that have not been possible so far
Impact of Advanced Synoptics and Simplified Checklists During Aircraft Systems Failures
AbstractNatural human capacities are becoming increasingly mismatched to the enormous data volumes, processing capabilities, and decision speeds demanded in todays aviation environment. Increasingly Autonomous Systems (IAS) are uniquely suited to solve this problem. NASA is conducting research and development of IAS - hardware and software systems, utilizing machine learning algorithms, seamlessly integrated with humans whereby task performance of the combined system is significantly greater than the individual components. IAS offer the potential for significantly improved levels of performance and safety that are superior to either human or automation alone. A human-in-the-loop test was conducted in NASA Langleys Integration Flight Deck B-737-800 simulator to evaluate advanced synoptic pages with simplified interactive electronic checklists as an IAS for routine air carrier flight operations and in response to aircraft system failures. Twelve U.S. airline crews flew various normal and non-normal procedures and their actions and performance were recorded in response to failures. These data are fundamental to and critical for the design and development of future increasingly autonomous systems that can better support the human in the cockpit. Synoptic pages and electronic checklists significantly improved pilot responses to non-normal scenarios, but implementation of these aids and other intelligent assistants have barriers to implementation (e.g., certification cost) that must overcome
Determining the Impact of Riparian Wetlands on Nutrient Cycling, Storage and Export in Permeable Agricultural Catchments
The impact of riparian wetlands on the cycling, retention and export of nutrients from land to water varies according to local environmental conditions and is poorly resolved in catchment management approaches. To determine the role a specific wetland might play in a catchment mitigation strategy, an alternative approach is needed to the high-frequency and spatially detailed monitoring programme that would otherwise be needed. Here, we present a new approach using a combination of novel and well-established geochemical, geophysical and isotope ratio methods. This combined approach was developed and tested against a 2-year high-resolution sampling programme in a lowland permeable wetland in the Lambourn catchment, UK. The monitoring programme identified multiple pathways and water sources feeding into the wetland, generating large spatial and temporal variations in nutrient cycling, retention and export behaviours within the wetland. This complexity of contributing source areas and biogeochemical functions within the wetland were effectively identified using the new toolkit approach. We propose that this technique could be used to determine the likely net source/sink function of riparian wetlands prior to their incorporation into any catchment management plan, with relatively low resource implications when compared to a full high-frequency nutrient speciation and isotope geochemistry-based monitoring approach
Searches for Metal-Poor Stars from the Hamburg/ESO Survey using the CH G-band
We describe a new method to search for metal-poor candidates from the
Hamburg/ESO objective-prism survey (HES) based on identifying stars with
apparently strong CH G-band strengths for their colors. The hypothesis we
exploit is that large over-abundances of carbon are common among metal-poor
stars. The selection was made by considering two line indices in the 4300A
region, applied directly to the low-resolution prism spectra. This work also
extends a previously published method by adding bright sources to the sample.
The spectra of these stars suffer from saturation effects, compromising the
index calculations and leading to an undersampling of the brighter candidates.
Visual inspection and classification of the spectra from the HES plates yielded
a list of 5,288 new metal-poor candidates, which are presently being used as
targets for medium-resolution spectroscopic follow-up. Estimates of the stellar
atmospheric parameters, as well as carbon abundances, are now available for 117
of the first candidates, based on follow-up medium-resolution spectra obtained
with the SOAR 4.1m and Gemini 8m telescopes. We demonstrate that our new method
improves the metal-poor star fractions found by our pilot study by up to a
factor of three in the same magnitude range, as compared with our pilot study
based on only one CH G-band index. Our selection scheme obtained roughly a 40%
success rate for identification of stars with [Fe/H] < -1.0; the primary
contaminant is late-type stars with near solar abundances and, often, emission
line cores that filled in the CaII K line on the prism spectrum. Because the
selection is based on carbon, we greatly increase the numbers of known CEMP
stars from the HES with intermediate metallicities -2.0 < [Fe/H] < -1.0, which
previous survey efforts undersampled. There are eight newly discovered stars
with [Fe/H] < -3.0 in our sample, including two with [Fe/H] < -3.5.Comment: 18 pages, 13 figures; accepted for publication in A
Using diffusion tractography to predict cortical connection strength and distance: a quantitative comparison with tracers in the monkey
Tractography based on diffusion MRI offers the promise of characterizing many aspects of long-distance connectivity in the brain, but requires quantitative validation to assess its strengths and limitations. Here, we evaluate tractography's ability to estimate the presence and strength of connections between areas of macaque neocortex by comparing its results with published data from retrograde tracer injections. Probabilistic tractography was performed on high-quality postmortem diffusion imaging scans from two Old World monkey brains. Tractography connection weights were estimated using a fractional scaling method based on normalized streamline density. We found a correlation between log-transformed tractography and tracer connection weights of r = 0.59, twice that reported in a recent study on the macaque. Using a novel method to estimate interareal connection lengths from tractography streamlines, we regressed out the distance dependence of connection strength and found that the correlation between tractography and tracers remains positive, albeit substantially reduced. Altogether, these observations provide a valuable, data-driven perspective on both the strengths and limitations of tractography for analyzing interareal corticocortical connectivity in nonhuman primates and a framework for assessing future tractography methodological refinements objectively
Improving catalyst activity in secondary amine catalysed transformations
The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increases the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed
[O/Fe] Estimates for Carbon-Enhanced Metal-Poor Stars from Near-IR Spectroscopy
We report on oxygen abundances determined from medium-resolution near-IR
spectroscopy for a sample of 57 carbon-enhanced metal-poor (CEMP) stars
selected from the Hamburg/ESO survey. The majority of our program stars exhibit
oxygen-to-iron ratios in the range +0.5 < [O/Fe]< +2.0. The [O/Fe] values for
this sample are statistically compared to available high-resolution estimates
for known CEMP stars, as well as to high-resolution estimates for a set of
carbon-normal metal-poor stars. Carbon, nitrogen, and oxygen abundance patterns
for a sub-sample of these stars are compared to yield predictions for very
metal-poor asymptotic giant-branch abundances in the recent literature. We find
that the majority of our sample exhibit patterns that are consistent with
previously studied CEMP stars having s-process-element enhancements, and thus
have very likely been polluted by carbon- and oxygen-enhanced material
transferred from a metal-poor asymptotic giant-branch companion.Comment: 16 pages, 8 figures, accepted by A
Carbon-Enhanced Metal-Poor Stars in the Inner and Outer Halo Components of the Milky Way
(Abridged) Carbon-enhanced metal-poor (CEMP) stars in the halo components of
the Milky Way are explored, based on accurate determinations of the
carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over
30000 calibration stars from the Sloan Digital Sky Survey (SDSS). Using our
present criterion that low-metallicity stars exhibiting [C/Fe] ratios
("carbonicity") in excess of [C/Fe] are considered CEMP stars, the
global frequency of CEMP stars in the halo system for \feh\ is 8%; for
\feh\ it is 12%; for \feh\ it is 20%. We also confirm a
significant increase in the level of carbon enrichment with declining
metallicity, growing from $\sim +1.0$ at \feh\ $= -1.5$ to
at \feh\ . The nature of the carbonicity
distribution function (CarDF) changes dramatically with increasing distance
above the Galactic plane, Z. For Z kpc, relatively few CEMP
stars are identified. For distances Z kpc, the CarDF exhibits a
strong tail towards high values, up to [C/Fe] +3.0. We also find a clear
increase in the CEMP frequency with Z. For stars with [Fe/H] 1.5, the frequency grows from 5% at Z kpc to 10% at Z
kpc. For stars with [Fe/H] 2.0, the frequency grows from 8% at
Z kpc to 25% at Z kpc. For stars with
[Fe/H] $\sim +1.0$ for 0 kpc $<$
$|$Z$|$ $<$ 10 kpc, with little dependence on $|$Z$|$; for [Fe/H] $< -$2.0,
, again roughly independent of Z.Comment: Accepted for publication in the Astrophysical Journal, 32 pages, 15
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