45,621 research outputs found
From actinides to zinc: Using the full abundance pattern of the brightest star in Reticulum II to distinguish between different r-process sites
The ultra-faint dwarf galaxy Reticulum II was enriched by a rare and prolific
r-process event, such as a neutron star merger. To investigate the nature of
this event, we present high-resolution Magellan/MIKE spectroscopy of the
brightest star in this galaxy. The high signal-to-noise allows us to determine
the abundances of 41 elements, including the radioactive actinide element Th
and first ever detections of third r-process peak elements (Os and Ir) in a
star outside the Milky Way. The observed neutron-capture element abundances
closely match the solar r-process component, except for the first r-process
peak which is significantly lower than solar but matches other r-process
enhanced stars. The ratio of first peak to heavier r-process elements implies
the r-process site produces roughly equal masses of high and low electron
fraction ejecta, within a factor of 2. We compare the detailed abundance
pattern to predictions from nucleosynthesis calculations of neutron star
mergers and magneto-rotationally driven jet supernovae, finding that nuclear
physics uncertainties dominate over astrophysical uncertainties. We measure
\log\mbox{Th/Eu} = -0.84 \pm 0.06\,\text{(stat)} \pm 0.22\,\text{(sys)},
somewhat lower than all previous Th/Eu observations. The youngest age we derive
from this ratio is Gyr,
indicating that current initial production ratios do not well describe the
r-process event in Reticulum II. The abundance of light elements up to Zn are
consistent with extremely metal-poor Milky Way halo stars. They may eventually
provide a way to distinguish between neutron star mergers and
magneto-rotationally driven jet supernovae, but this would require more
detailed knowledge of the chemical evolution of Reticulum II.Comment: 23 pages, 7 figures, accepted to Ap
The Kuratowski convergence of medial axes and conflict sets
This paper consists of two parts. In the first one we study the behaviour of
medial axes (skeletons) of closed, definable (in some o-minimal structure) sets
in {\Rz}^n under deformations. The second one is devoted to a similar study
of conflict sets in definable families. We apply a new approach to the
deformation process. Instead of seeing it as a `jump' from the initial to the
final state, we perceive it as a continuous process, expressed using the
Kuratowski convergence of sets (hence, unlike other authors, we do not require
any regularity of the deformation). Our main `medial axis inner
semi-continuity' result has already proved useful, as it was used to compute
the tangent cone of the medial axis with application in singularity theory.Comment: The preprint has been extended to include also the study of the
behaviour of the conflict set of a continuous family of definable sets
performed with a new co-author. Therefore the title has slightly been
changed, too. Besides that, the references have also been updated and in the
last version we strengthened the statement of Theorem 5.1
Racial and Ethnic Attitudes and Individual Relatedness Among Greek-Americans
The article looks at the self-identity of Greek immigrants in the U.S. and incorporation of American racial ideologies into their racial repertoires. It recognizes Greek Americans for creating a national and racial framework that blends elements of both home and host society institutions and ideologies. It recalls the arrival of thousands of Greek immigrants in the U.S. in the 20th century. The increasing inter-marriage rates between Greek immigrants and Greek Americans are also noted
A Presheaf Interpretation of the Generalized Freyd Conjecture
We give a generalized version of the Freyd conjecture and a way to think
about a possible proof. The essential point is to describe an elementary formal
reduction of the question that holds in any triangulated category. There are no
new results, but at least one known example drops out quite trivially.Comment: 8 pages; formerly titled "Thinking about the Freyd conjecture
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The Chemical Imprint Of Silicate Dust On The Most Metal-Poor Stars
We investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (<1 M-circle dot) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming that the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H](crit)). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H](crit) = -4.5 +/- 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H](crit) = -5.3 +/- 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine-structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].NSF AST-1255160, AST-1009928NASA ATFP NNX09-AJ33GAstronom
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The use of cues to convergence and accommodation in naive, uninstructed participants
A remote haploscopic video refractor was used to assess vergence and accommodation responses in a
group of 32 emmetropic, orthophoric, symptom free, young adults naïve to vision experiments in a minimally
instructed setting. Picture targets were presented at four positions between 2 m and 33 cm. Blur,
disparity and looming cues were presented in combination or separately to asses their contributions to
the total near response in a within-subjects design.
Response gain for both vergence and accommodation reduced markedly whenever disparity was
excluded, with much smaller effects when blur and proximity were excluded. Despite the clinical homogeneity
of the participant group there were also some individual differences
Online Sorting via Searching and Selection
In this paper, we present a framework based on a simple data structure and
parameterized algorithms for the problems of finding items in an unsorted list
of linearly ordered items based on their rank (selection) or value (search). As
a side-effect of answering these online selection and search queries, we
progressively sort the list. Our algorithms are based on Hoare's Quickselect,
and are parameterized based on the pivot selection method.
For example, if we choose the pivot as the last item in a subinterval, our
framework yields algorithms that will answer q<=n unique selection and/or
search queries in a total of O(n log q) average time. After q=\Omega(n) queries
the list is sorted. Each repeated selection query takes constant time, and each
repeated search query takes O(log n) time. The two query types can be
interleaved freely. By plugging different pivot selection methods into our
framework, these results can, for example, become randomized expected time or
deterministic worst-case time. Our methods are easy to implement, and we show
they perform well in practice
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