5,981 research outputs found
Abundances in Stars from the Red Giant Branch Tip to the Near Main Sequence in M71: II. Iron Abundance
We present [Ffe/H] abundance results that involve a sample of stars with a
wide range in luminosity from luminous giants to stars near the turnoff in a
globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more
luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous
than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II
lines in high dispersion spectra observed with HIRES at the W. M. Keck
Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines
agree with each other and with earlier determinations. Also the [Fe/H] obtained
from Fe I and Fe II lines is constant within the rather small uncertainties for
this group of stars over the full range in Teff and luminosity, suggesting that
NLTE effects are negligible in our iron abundance determination. In this
globular cluster, there is no difference among the mean [Fe/H] of giant stars
located at or above the RHB, RHB stars, giant stars located below the RHB and
stars near the turnoff.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 2 of a pair
Magnetic and electrical properties of (Pu,Lu)Pd3
We present measurements of the magnetic susceptibility, heat capacity and
electrical resistivity of PuLuPd, with =0, 0.1, 0.2, 0.5,
0.8 and 1. PuPd is an antiferromagnetic heavy fermion compound with
~K. With increasing Lu doping, both the Kondo and RKKY interaction
strengths fall, as judged by the Sommerfeld coefficient and N\'eel
temperature . Fits to a crystal field model of the resistivity also
support these conclusions. The paramagnetic effective moment
increases with Lu dilution, indicating a decrease in the
Kondo screening. In the highly dilute limit, approaches
the value predicted by intermediate coupling calculations. In conjunction with
an observed Schottky peak at 60~K in the magnetic heat capacity,
corresponding to a crystal field splitting of 12~meV, a mean-field
intermediate coupling model with nearest neighbour interactions has been
developed.Comment: 13 pages, 13 figure
Abundances of Baade's Window Giants from Keck/HIRES Spectra: I. Stellar Parameters and [Fe/H] Values
We present the first results of a new abundance survey of the Milky Way bulge
based on Keck/HIRES spectra of 27 K-giants in the Baade's Window (, ) field. The spectral data used in this study are of much higher resolution
and signal-to-noise than previous optical studies of Galactic bulge stars. The
[Fe/H] values of our stars, which range between -1.29 and , were used to
recalibrate large low resolution surveys of bulge stars. Our best value for the
mean [Fe/H] of the bulge is . This mean value is similar to the
mean metallicity of the local disk and indicates that there cannot be a strong
metallicity gradient inside the solar circle. The metallicity distribution of
stars confirms that the bulge does not suffer from the so-called ``G-dwarf''
problem. This paper also details the new abundance techniques necessary to
analyze very metal-rich K-giants, including a new Fe line list and regions of
low blanketing for continuum identification.Comment: Accepted for publication in January 2006 Astrophysical Journal. Long
tables 3--6 withheld to save space (electronic tables in journal paper). 53
pages, 10 figures, 9 table
Phase Response Curves of Coupled Oscillators
Many real oscillators are coupled to other oscillators and the coupling can
affect the response of the oscillators to stimuli. We investigate phase
response curves (PRCs) of coupled oscillators. The PRCs for two weakly coupled
phase-locked oscillators are analytically obtained in terms of the PRC for
uncoupled oscillators and the coupling function of the system. Through
simulation and analytic methods, the PRCs for globally coupled oscillators are
also discussed.Comment: 5 pages 4 figur
Activation Energy of Hydrogen Adsorption on Pt(111) in Alkaline Media: An Impedance Spectroscopy Study at Variable Temperatures
The hydrogen evolution reaction is one of the most studied processes in electrochemistry, and platinum is by far the best catalyst for this reaction. Despite the importance of this reaction on platinum, detailed and accurate kinetic measurements of the steps that lead to the main reaction are still lacking, particularly because of the fast rate of the reaction. Hydrogen adsorption on Pt(111) has been taken as a benchmark system in a large number of computational studies, but reliable experimental data to compare with the computational studies is very scarce. To gain further knowledge on this matter, a temperature study of the hydrogen adsorption reaction has been carried out to obtain kinetic information for this process on Pt(111) in alkaline solution. This was achieved by measuring electrochemical impedance spectra and cyclic voltammograms in the range of 278 †T †318 (K) to obtain the corresponding surface coverage by adsorbed species and the faradaic charge transfer resistance. From this data, the standard rate constant has been extracted with a kinetic model assuming a Frumkin-type isotherm, resulting in values of 2.60 Ă 10â7 †k0 †1.68 Ă 10â6 (sâ1). The Arrehnius plot gives an activation energy of 32 kJ molâ1. Comparisons are made with values calculated by computational methods and reported values for the overall HER, giving a reference frame to support future studies on hydrogen catalysis.This work was financially supported by the MINECO (Spain) project no. CTQ2016-76221-P. L.E.B. thanks the Generalitat Valenciana for funding from the Santiago Grisolia Program (no. GRISOLIAP/2017/181)
The type II phase resetting curve is optimal for stochastic synchrony
The phase-resetting curve (PRC) describes the response of a neural oscillator
to small perturbations in membrane potential. Its usefulness for predicting the
dynamics of weakly coupled deterministic networks has been well characterized.
However, the inputs to real neurons may often be more accurately described as
barrages of synaptic noise. Effective connectivity between cells may thus arise
in the form of correlations between the noisy input streams. We use constrained
optimization and perturbation methods to prove that PRC shape determines
susceptibility to synchrony among otherwise uncoupled noise-driven neural
oscillators. PRCs can be placed into two general categories: Type I PRCs are
non-negative while Type II PRCs have a large negative region. Here we show that
oscillators with Type II PRCs receiving common noisy input sychronize more
readily than those with Type I PRCs.Comment: 10 pages, 4 figures, submitted to Physical Review
Modeling seismic wave propagation and amplification in 1D/2D/3D linear and nonlinear unbounded media
To analyze seismic wave propagation in geological structures, it is possible
to consider various numerical approaches: the finite difference method, the
spectral element method, the boundary element method, the finite element
method, the finite volume method, etc. All these methods have various
advantages and drawbacks. The amplification of seismic waves in surface soil
layers is mainly due to the velocity contrast between these layers and,
possibly, to topographic effects around crests and hills. The influence of the
geometry of alluvial basins on the amplification process is also know to be
large. Nevertheless, strong heterogeneities and complex geometries are not easy
to take into account with all numerical methods. 2D/3D models are needed in
many situations and the efficiency/accuracy of the numerical methods in such
cases is in question. Furthermore, the radiation conditions at infinity are not
easy to handle with finite differences or finite/spectral elements whereas it
is explicitely accounted in the Boundary Element Method. Various absorbing
layer methods (e.g. F-PML, M-PML) were recently proposed to attenuate the
spurious wave reflections especially in some difficult cases such as shallow
numerical models or grazing incidences. Finally, strong earthquakes involve
nonlinear effects in surficial soil layers. To model strong ground motion, it
is thus necessary to consider the nonlinear dynamic behaviour of soils and
simultaneously investigate seismic wave propagation in complex 2D/3D geological
structures! Recent advances in numerical formulations and constitutive models
in such complex situations are presented and discussed in this paper. A crucial
issue is the availability of the field/laboratory data to feed and validate
such models.Comment: of International Journal Geomechanics (2010) 1-1
Spurious Shear in Weak Lensing with LSST
The complete 10-year survey from the Large Synoptic Survey Telescope (LSST)
will image 20,000 square degrees of sky in six filter bands every few
nights, bringing the final survey depth to , with over 4 billion
well measured galaxies. To take full advantage of this unprecedented
statistical power, the systematic errors associated with weak lensing
measurements need to be controlled to a level similar to the statistical
errors.
This work is the first attempt to quantitatively estimate the absolute level
and statistical properties of the systematic errors on weak lensing shear
measurements due to the most important physical effects in the LSST system via
high fidelity ray-tracing simulations. We identify and isolate the different
sources of algorithm-independent, \textit{additive} systematic errors on shear
measurements for LSST and predict their impact on the final cosmic shear
measurements using conventional weak lensing analysis techniques. We find that
the main source of the errors comes from an inability to adequately
characterise the atmospheric point spread function (PSF) due to its high
frequency spatial variation on angular scales smaller than in the
single short exposures, which propagates into a spurious shear correlation
function at the -- level on these scales. With the large
multi-epoch dataset that will be acquired by LSST, the stochastic errors
average out, bringing the final spurious shear correlation function to a level
very close to the statistical errors. Our results imply that the cosmological
constraints from LSST will not be severely limited by these
algorithm-independent, additive systematic effects.Comment: 22 pages, 12 figures, accepted by MNRA
Rubidium in Metal-Deficient Disk and Halo Stars
We report the first extensive study of stellar Rb abundances. High-resolution
spectra have been used to determine, or set upper limits on, the abundances of
this heavy element and the associated elements Y, Zr, and Ba in 44 dwarfs and
giants with metallicities spanning the range -2.0 <[Fe/H] < 0.0. In
metal-deficient stars Rb is systematically overabundant relative to Fe; we find
an average [Rb/Fe] of +0.21 for the 32 stars with [Fe/H] < -0.5 and measured
Rb. This behavior contrasts with that of Y, Zr, and Ba, which, with the
exception of three new CH stars (HD 23439A and B and BD +5 3640), are
consistently slightly deficient relative to Fe in the same stars; excluding the
three CH stars, we find the stars with [Fe/H] < -0.5 have average [Y/Fe],
[Zr/Fe], and [Ba/Fe] of --0.19 (24 stars), --0.12 (28 stars), and --0.06 (29
stars), respectively. The different behavior of Rb on the one hand and Y, Zr,
and Ba on the other can be attributed in part to the fact that in the Sun and
in these stars Rb has a large r-process component while Y, Zr, and Ba are
mostly s-process elements with only small r-process components. In addition,
the Rb s-process abundance is dependent on the neutron density at the
s-processing site. Published observations of Rb in s-process enriched red
giants indicate a higher neutron density in the metal-poor giants. These
observations imply a higher s-process abundance for Rb in metal-poor stars. The
calculated combination of the Rb r-process abundance, as estimated for the
stellar Eu abundances, and the s-process abundance as estimated for red giants
accounts satisfactorily for the observed run of [Rb/Fe] with [Fe/H].Comment: 23 pages, 5 tables, 7 figure
Methodological approaches to determining the marine radiocarbon reservoir effect
The marine radiocarbon reservoir effect is an offset in 14C age between contemporaneous organisms from the terrestrial environment and organisms that derive their carbon from the marine environment. Quantification of this effect is of crucial importance for correct calibration of the <sup>14</sup>C ages of marine-influenced samples to the calendrical timescale. This is fundamental to the construction of archaeological and palaeoenvironmental chronologies when such samples are employed in <sup>14</sup>C analysis. Quantitative measurements of temporal variations in regional marine reservoir ages also have the potential to be used as a measure of process changes within Earth surface systems, due to their link with climatic and oceanic changes. The various approaches to quantification of the marine radiocarbon reservoir effect are assessed, focusing particularly on the North Atlantic Ocean. Currently, the global average marine reservoir age of surface waters, R(t), is c. 400 radiocarbon years; however, regional values deviate from this as a function of climate and oceanic circulation systems. These local deviations from R(t) are expressed as +R values. Hence, polar waters exhibit greater reservoir ages (δR = c. +400 to +800 <sup>14</sup>C y) than equatorial waters (δR = c. 0 <sup>14</sup>C y). Observed temporal variations in δR appear to reflect climatic and oceanographic changes. We assess three approaches to quantification of marine reservoir effects using known age samples (from museum collections), tephra isochrones (present onshore/offshore) and paired marine/terrestrial samples (from the same context in, for example, archaeological sites). The strengths and limitations of these approaches are evaluated using examples from the North Atlantic region. It is proposed that, with a suitable protocol, accelerator mass spectrometry (AMS) measurements on paired, short-lived, single entity marine and terrestrial samples from archaeological deposits is the most promising approach to constraining changes over at least the last 5 ky BP
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