7,299 research outputs found
Characterizing the Heavy Elements in Globular Cluster M22 and an Empirical s-process Abundance Distribution Derived from the Two Stellar Groups
We present an empirical s-process abundance distribution derived with
explicit knowledge of the r-process component in the low-metallicity globular
cluster M22. We have obtained high-resolution, high signal-to-noise spectra for
6 red giants in M22 using the MIKE spectrograph on the Magellan-Clay Telescope
at Las Campanas Observatory. In each star we derive abundances for 44 species
of 40 elements, including 24 elements heavier than zinc (Z=30) produced by
neutron-capture reactions. Previous studies determined that 3 of these stars
(the "r+s group") have an enhancement of s-process material relative to the
other 3 stars (the "r-only group"). We confirm that the r+s group is moderately
enriched in Pb relative to the r-only group. Both groups of stars were born
with the same amount of r-process material, but s-process material was also
present in the gas from which the r+s group formed. The s-process abundances
are inconsistent with predictions for AGB stars with M =< 3 Msun and suggest an
origin in more massive AGB stars capable of activating the Ne-22(alpha,n)Mg-25
reaction. We calculate the s-process "residual" by subtracting the r-process
pattern in the r-only group from the abundances in the r+s group. In contrast
to previous r- and s-process decompositions, this approach makes no assumptions
about the r- and s-process distributions in the solar system and provides a
unique opportunity to explore s-process yields in a metal-poor environment.Comment: Accepted for publication in the Astrophysical Journal. 18 pages, 8
figure
NGC 1866: First Spectroscopic Detection of Fast Rotating Stars in a Young LMC Cluster
High-resolution spectroscopic observations were taken of 29 extended main
sequence turn-off (eMSTO) stars in the young (200 Myr) LMC cluster, NGC
1866 using the Michigan/Magellan Fiber System and MSpec spectrograph on the
Magellan-Clay 6.5-m telescope. These spectra reveal the first direct detection
of rapidly rotating stars whose presence has only been inferred from
photometric studies. The eMSTO stars exhibit H-alpha emission (indicative of
Be-star decretion disks), others have shallow broad H-alpha absorption
(consistent with rotation 150 km s), or deep H-alpha core
absorption signaling lower rotation velocities (150 km s ).
The spectra appear consistent with two populations of stars - one rapidly
rotating, and the other, younger and slowly rotating.Comment: 9 pages, 4 figures, Accepted for publication in ApJ Letter
An empirical analysis of smart contracts: platforms, applications, and design patterns
Smart contracts are computer programs that can be consistently executed by a
network of mutually distrusting nodes, without the arbitration of a trusted
authority. Because of their resilience to tampering, smart contracts are
appealing in many scenarios, especially in those which require transfers of
money to respect certain agreed rules (like in financial services and in
games). Over the last few years many platforms for smart contracts have been
proposed, and some of them have been actually implemented and used. We study
how the notion of smart contract is interpreted in some of these platforms.
Focussing on the two most widespread ones, Bitcoin and Ethereum, we quantify
the usage of smart contracts in relation to their application domain. We also
analyse the most common programming patterns in Ethereum, where the source code
of smart contracts is available.Comment: WTSC 201
Van der Waals effect in weak adsorption affecting trends in adsorption, reactivity, and the view of substrate nobility
The ubiquitous van der Waals (vdW) force, particularly discernible in weak adsorption, is studied on noble and transition metals. In calculations with the vdW density functional (DF) [ M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], the atomic structure near the adsorption site is systematically varied, including dense fcc(111) surface, adatom, pyramid, and step defects. In weak adsorption the vdW force (i) is shown necessary to account for, (ii) is sizable, (iii) has a strong spatial variation, relevant for adsorption on surface defects, (iv) changes reaction rules, and (v) changes adsorption trends in agreement with experimental data. Traditional physisorption theory is also given support and interpretation
The State-of-the-Art HST Astro-Photometric Analysis of the core of \omega Centauri. III. The Main Sequence's Multiple Populations Galore
We take advantage of the exquisite quality of the Hubble Space Telescope
26-filter astro-photometric catalog of the core of Omega Cen presented in the
first paper of this series and the empirical differential-reddening correction
presented in the second paper in order to distill the main sequence into its
constituent populations. To this end, we restrict ourselves to the five most
useful filters: the magic "trio" of F275W, F336W, and F438W, along with F606W
and F814W. We develop a strategy for identifying color systems where different
populations stand out most distinctly, then we isolate those populations and
examine them in other filters where their sub-populations also come to light.
In this way, we have identified at least 15 sub-populations, each of which has
a distinctive fiducial curve through our 5-dimensional photometric space. We
confirm the MSa to be split into two subcomponents, and find that both the bMS
and the rMS are split into three subcomponents. Moreover, we have discovered
two additional MS groups: the MSd (which has three subcomponents) shares
similar properties with the bMS, and the MSe (which has four subcomponents),
has properties more similar to those of the rMS. We examine the fiducial curves
together and use synthetic spectra to infer relative heavy-element,
light-element, and Helium abundances for the populations. Our findings show
that the stellar populations and star formation history of Omega Cen are even
more complex than inferred previously. Finally, we provide as a supplement to
the original catalog a list that identifies for each star which population it
most likely is associated with.Comment: 22 pages, 17 figures (most in lower res), 2 tables, accepted for
publication in Ap
Quantum skyrmions and the destruction of long-range antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x)
We study the destruction of the antiferromagnetic order in the high-Tc
superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x) in the framework of
the CP1-nonlinear sigma model formulation of the 2D quantum Heisenberg
antiferromagnet. The dopants are introduced as independent fermions with
appropriate dispersion relations determined by the shape of the Fermi surface.
The energy of skyrmion topological defects, which are shown to be introduced by
doping, is used as an order parameter for antiferromagnetic order. We obtain
analytic expressions for this as a function of doping which allow us to plot
the curves T_N(x_c)\times x_c and M(x)\times x, for both YBCO and LSCO, in good
quantitative agreement with the experimental data.Comment: 4 pages, revtex, 5 embeeded figure
Testing the chemical tagging technique with open clusters
Context. Stars are born together from giant molecular clouds and, if we
assume that the priors were chemically homogeneous and well-mixed, we expect
them to share the same chemical composition. Most of the stellar aggregates are
disrupted while orbiting the Galaxy and most of the dynamic information is
lost, thus the only possibility of reconstructing the stellar formation history
is to analyze the chemical abundances that we observe today.
Aims. The chemical tagging technique aims to recover disrupted stellar
clusters based merely on their chemical composition. We evaluate the viability
of this technique to recover co-natal stars that are no longer gravitationally
bound.
Methods. Open clusters are co-natal aggregates that have managed to survive
together. We compiled stellar spectra from 31 old and intermediate-age open
clusters, homogeneously derived atmospheric parameters, and 17 abundance
species, and applied machine learning algorithms to group the stars based on
their chemical composition. This approach allows us to evaluate the viability
and efficiency of the chemical tagging technique.
Results. We found that stars at different evolutionary stages have distinct
chemical patterns that may be due to NLTE effects, atomic diffusion, mixing,
and biases. When separating stars into dwarfs and giants, we observed that a
few open clusters show distinct chemical signatures while the majority show a
high degree of overlap. This limits the recovery of co-natal aggregates by
applying the chemical tagging technique. Nevertheless, there is room for
improvement if more elements are included and models are improved.Comment: accepted for publication in Astronomy and Astrophysics. Corrected
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