1,305 research outputs found
Zirconium, Barium, Lanthanum and Europium Abundances in Open Clusters
We present an analysis of the s-process elements Zr, Ba, and La and the
r-process element Eu in a sample of 50 stars in 19 open clusters. Stellar
abundances of each element are based on measures of a minimum of two lines per
species via both equivalent width and spectrum synthesis techniques. We
investigate cluster mean neutron-capture abundance trends as a function of
cluster age and location in the Milky Way disk and compare them to results
found in other studies in the literature. We find a statistically significant
trend of increasing cluster [Ba/Fe] as a function of decreasing cluster age, in
agreement with recent findings for other open cluster samples, supporting the
increased importance of low-mass asymptotic giant branch stars to the
generation of s-process elements. However, the other s-process elements,
[La/Fe] and [Zr/Fe], do not show similar dependences, in contrast to
theoretical expectations and the limited observational data from other studies.
Conversely, cluster [Eu/Fe] ratios show a slight increase with increasing
cluster age, although with marginal statistical significance. Ratios of
[s/r]-process abundances, [Ba/Eu] and [La/Eu], however, show more clearly the
increasing efficiency of s-process relative to r-process enrichment in open
cluster chemical evolution, with significant increases among younger clusters.
Last, cluster neutron-capture element abundances appear to be independent of
Galactocentric distance. We conclude that a homogeneous analysis of a larger
sample of open clusters is needed to resolve the apparent discrepant
conclusions between different studies regarding s-process element abundance
trends with age to better inform models of galactic chemical evolution.Comment: 24 pages, 13 figures, 10 tables; published in The Astronomical
Journa
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
Identification of key residues that regulate the interaction of kinesins with microtubule ends
Kinesins are molecular motors that use energy derived from ATP turnover to walk along microtubules, or when at the microtubule end, regulate growth or shrinkage. All kinesins that regulate microtubule dynamics have long residence times at microtubule ends, whereas those that only walk have short end‐residence times. Here, we identify key amino acids involved in end binding by showing that when critical residues from Kinesin‐13, which depolymerises microtubules, are introduced into Kinesin‐1, a walking kinesin with no effect on microtubule dynamics, the end‐residence time is increased up to several‐fold. This indicates that the interface between the kinesin motor domain and the microtubule is malleable and can be tuned to favour either lattice or end binding
A Chemical Abundance Study of 10 Open Clusters Based on WIYN-Hydra Spectroscopy
We present a detailed chemical abundance study of evolved stars in 10 open
clusters based on Hydra multi-object echelle spectra obtained with the WIYN
3.5m telescope. From an analysis of both equivalent widths and spectrum
synthesis, abundances have been determined for the elements Fe, Na, O, Mg, Si,
Ca, Ti, Ni, Zr, and for two of the 10 clusters, Al and Cr. To our knowledge,
this is the first detailed abundance analysis for clusters NGC 1245, NGC 2194,
NGC 2355 and NGC 2425. These 10 clusters were selected for analysis because
they span a Galactocentric distance range Rgc~9-13 kpc, the approximate
location of the transition between the inner and outer disk. Combined with
cluster samples from our previous work and those of other studies in the
literature, we explore abundance trends as a function of cluster Rgc, age, and
[Fe/H]. The [Fe/H] distribution appears to decrease with increasing Rgc to a
distance of ~12 kpc, and then flattens to a roughly constant value in the outer
disk. Cluster average element [X/Fe] ratios appear to be independent of Rgc,
although the picture for [O/Fe] is more more complicated by a clear trend of
[O/Fe] with [Fe/H] and sample incompleteness. Other than oxygen, no other
element [X/Fe] exhibits a clear trend with [Fe/H]; likewise, there does not
appear to be any strong correlation between abundance and cluster age. We
divided clusters into different age bins to explore temporal variations in the
radial element distributions. The radial metallicity gradient appears to have
flattened slightly as a function of time, as found by other studies. There is
also indication that the transition from the inner disk to the outer disk
occurs at different Galactocentric radii for different age bins. (Abridged.)Comment: 35 pages, 12 figures, 18 tables; published in The Astronomical
Journal (http://stacks.iop.org/1538-3881/142/59
NGC 7789: An Open Cluster Case Study
We have obtained high-resolution spectra of 32 giants in the open cluster NGC
7789 using the Wisconsin-Indiana-Yale-NOAO Hydra spectrograph. We explore
differences in atmospheric parameters and elemental abundances caused by the
use of the linelist developed for the Gaia-ESO Survey (GES) compared to one
based on Arcturus used in our previous work. [Fe/H] values decrease when using
the GES linelist instead of the Arcturus-based linelist; these differences are
probably driven by systematically lower (~ -0.1 dex) GES surface gravities.
Using the GES linelist we determine abundances for 10 elements - Fe, Mg, Si,
Ca, Ti, Na, Ni, Zr, Ba, and La. We find the cluster's average metallicity
[Fe/H] = 0.03 +/- 0.07 dex, in good agreement with literature values, and a
lower [Mg/Fe] abundance than has been reported before for this cluster (0.11
+/- 0.05 dex). We also find the neutron-capture element barium to be highly
enhanced - [Ba/Fe] = +0.48 +/- 0.08 - and disparate from cluster measurements
of neutron-capture elements La and Zr (-0.08 +/- 0.05 and 0.08 +/- 0.08,
respectively). This is in accordance with recent discoveries of supersolar Ba
enhancement in young clusters along with more modest enhancement of other
neutron-capture elements formed in similar environments.Comment: 15 pages, 9 figures, Table 1 typo fixe
Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651
This is the first of a series of papers devoted to derive the metallicity of
old open clusters in order to study the time evolution of the chemical
abundance gradient in the Galactic disk. We present detailed iron abundances
from high resolution (R~40000) spectra of several red clump and bright giant
stars in the open clusters IC 4651, NGC 2506 and NGC 6134. We observed 4 stars
of NGC 2506, 3 stars of NGC 6134, and 5 stars of IC 4651 with the FEROS
spectrograph at the ESO 1.5 m telescope; moreover, 3 other stars of NGC 6134
were observed with the UVES spectrograph on Kueyen (VLT UT2). After excluding
the cool giants near the red giant branch tip (one in IC 4651 and one in NGC
2506), we found overall [Fe/H] values of -0.20 +/- 0.01, rms = 0.02 dex (2
stars) for NGC 2506, +0.15 +/- 0.03, rms = 0.07 dex (6 stars) for NGC 6134, and
+0.11 +/- 0.01, rms = 0.01 dex (4 stars) for IC 4651. The metal abundances
derived from line analysis for each star were extensively checked using
spectrum synthesis of about 30 to 40 Fe I lines and 6 Fe II lines. Our
spectroscopic temperatures provide reddening values in good agreement with
literature data for these clusters, strengthening the reliability of the
adopted temperature and metallicity scale. Also, gravities from the Fe
equilibrium of ionization agree quite well with expectations based on cluster
distance moduli and evolutionary masses.Comment: 13 pages, 7 figures, uses aa.cls, accepted for publication on
Astronomy & Astrophysic
Enabling internal electronic circuitry within additively manufactured metal structures - The effect and importance of inter-laminar topography
Purpose: This paper aims to explore the potential of ultrasonic additive manufacturing (UAM) to incorporate the direct printing of electrical materials and arrangements (conductors and insulators) at the interlaminar interface of parts during manufacture to allow the integration of functional and optimal electrical circuitries inside dense metallic objects without detrimental effect on the overall mechanical integrity. This holds promise to release transformative device functionality and applications of smart metallic devices and products. Design/methodology/approach: To ensure the proper electrical insulation between the printed conductors and metal matrices, an insulation layer with sufficient thickness is required to accommodate the rough interlaminar surface which is inherent to the UAM process. This in turn increases the total thickness of printed circuitries and thereby adversely affects the integrity of the UAM part. A specific solution is proposed to optimise the rough interlaminar surface through deforming the UAM substrates via sonotrode rolling or UAM processing. Findings: The surface roughness (Sa) could be reduced from 4.5 to 4.1 µm by sonotrode rolling and from 4.5 to 0.8 µm by ultrasonic deformation. Peel testing demonstrated that sonotrode-rolled substrates could maintain their mechanical strength, while the performance of UAM-deformed substrates degraded under same welding conditions ( approximately 12 per cent reduction compared with undeformed substrates). This was attributed to the work hardening of deformation process which was identified via dual-beam focussed ion beam–scanning electron microscope investigation. Originality/value: The sonotrode rolling was identified as a viable methodology in allowing printed electrical circuitries in UAM. It enabled a decrease in the thickness of printed electrical circuitries by ca. 25 per cent
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