15,845 research outputs found
Extreme-Energy Cosmic Rays: Puzzles, Models, and Maybe Neutrinos
The observation of twenty cosmic-ray air-showers at and above 10^{20} eV
poses fascinating problems for particle astrophysics: how the primary particles
are accelerated to these energies, how the primaries get here through the 2.7K
microwave background filling the Universe, and how the highest-energy events
exhibit clustering on few-degree angular scales on the sky when charged
particles are expected be bent by cosmic magnetic fields. An overview of the
puzzles is presented, followed by a brief discussion of many of the models
proposed to solve these puzzles. Emphasis is placed on (i) the signatures by
which cosmic ray experiments in the near future will discriminate among the
many proposed models, and (ii) the role neutrino primaries may play in
resolving the observational issues. It is an exciting prospect that
highest-energy cosmic rays may have already presented us with new physics not
accessible in terrestrial accelerator searches.Comment: 12 pages, RevTeX, 4 figures, Expansion of talks given at NU2000
(Canada); Metepec, Mexico; RADHEP2000 (UCLA
Asteroseismology and evolution of EHB stars
The properties of the Extreme Horizontal Branch stars are quite well
understood, but much uncertainty surrounds the many paths that bring a star to
this peculiar configuration. Asteroseismology of pulsating EHB stars has been
performed on a number of objects, bringing us to the stage where comparisons of
the inferred properties with evolutionary models becomes feasible. In this
review I outline our current understanding of the formation and evolution of
these stars, with emphasis on recent progress. The aim is to show how the
physical parameters derived by asteroseismology can enable the discrimination
between different evolutionary models.Comment: 13 pages, 6 figures, invited review to appear in Communications in
Asteroseismology vol.159, "Proceedings of the JENAM 2008 Symposium No 4:
Asteroseismology and Stellar Evolution
Seed particle formation for silicate dust condensation by SiO nucleation
Clustering of the abundant SiO molecules has been discussed as a possible
mechanism of seed particle formation for silicate dust in stellar outflows with
an oxygen rich element mixture. Previous results indicated that condensation
temperatures based on this mechanism are significant lower than what is really
observed. This negative result strongly rests on experimental data on vapour
pressure of SiO. New determinations show the older data to be seriously in
error. Here we aim to check with improved data the possibility that SiO
nucleation triggers the cosmic silicate dust formation. First we present
results of our measurements of vapour pressure of solid SiO. Second, we use the
improved vapour pressure data to re-calibrate existing experimental data on SiO
nucleation from the literature. Third, we use the re-calibrated data on SiO
nucleation in a simple model for dust-driven winds to determine the
condensation temperature of silicate in stellar outflows from AGB stars. We
show that onset of nucleation under circumstellar conditions commences at
higher temperature than was previously found. Calculated condensation
temperatures are still by about 100 K lower than observed ones, but this may be
due to the greenhouse effect of silicate dust temperatures. The assumption that
the onset of silicate dust formation in late-type M stars is triggered by
cluster formation of SiO is compatible with dust condensation temperatures
derived from IR observations.Comment: 11 pages, 11 figure
Resolving transition metal chemical space: feature selection for machine learning and structure-property relationships
Machine learning (ML) of quantum mechanical properties shows promise for
accelerating chemical discovery. For transition metal chemistry where accurate
calculations are computationally costly and available training data sets are
small, the molecular representation becomes a critical ingredient in ML model
predictive accuracy. We introduce a series of revised autocorrelation functions
(RACs) that encode relationships between the heuristic atomic properties (e.g.,
size, connectivity, and electronegativity) on a molecular graph. We alter the
starting point, scope, and nature of the quantities evaluated in standard ACs
to make these RACs amenable to inorganic chemistry. On an organic molecule set,
we first demonstrate superior standard AC performance to other
presently-available topological descriptors for ML model training, with mean
unsigned errors (MUEs) for atomization energies on set-aside test molecules as
low as 6 kcal/mol. For inorganic chemistry, our RACs yield 1 kcal/mol ML MUEs
on set-aside test molecules in spin-state splitting in comparison to 15-20x
higher errors from feature sets that encode whole-molecule structural
information. Systematic feature selection methods including univariate
filtering, recursive feature elimination, and direct optimization (e.g., random
forest and LASSO) are compared. Random-forest- or LASSO-selected subsets 4-5x
smaller than RAC-155 produce sub- to 1-kcal/mol spin-splitting MUEs, with good
transferability to metal-ligand bond length prediction (0.004-5 {\AA} MUE) and
redox potential on a smaller data set (0.2-0.3 eV MUE). Evaluation of feature
selection results across property sets reveals the relative importance of
local, electronic descriptors (e.g., electronegativity, atomic number) in
spin-splitting and distal, steric effects in redox potential and bond lengths.Comment: 43 double spaced pages, 11 figures, 4 table
High-energy astroparticle physics
In these three lectures I discuss the present status of high-energy
astroparticle physics including Ultra-High-Energy Cosmic Rays (UHECR),
high-energy gamma rays, and neutrinos. The first lecture is devoted to
ultra-high-energy cosmic rays. After a brief introduction to UHECR I discuss
the acceleration of charged particles to highest energies in the astrophysical
objects, their propagation in the intergalactic space, recent observational
results by the Auger and HiRes experiments, anisotropies of UHECR arrival
directions, and secondary gamma rays produced by UHECR. In the second lecture I
review recent results on TeV gamma rays. After a short introduction to
detection techniques, I discuss recent exciting results of the H.E.S.S., MAGIC,
and Milagro experiments on the point-like and diffuse sources of TeV gamma
rays. A special section is devoted to the detection of extragalactic magnetic
fields with TeV gamma-ray measurements. Finally, in the third lecture I discuss
Ultra-High-Energy (UHE) neutrinos. I review three different UHE neutrino
detection techniques and show the present status of searches for diffuse
neutrino flux and point sources of neutrinos.Comment: 29 pages, Lectures given at the 5th CERN-Latin-American School of
High-Energy Physics, Recinto Quirama, Colombia, 15 - 28 Mar 200
Industrial clusters and economic integration : theoretic concepts and an application to the European Metropolitan Region Nuremberg
"Economic integration typically goes along with disintegration of production through outsourcing and offshoring (Feenstra 1998). As horizontal and vertical links between firms become more and more pronounced, value chains within regions are increasingly organized by production and innovation clusters. On the basis of a literature overview, we argue that in a world of economic integration clusters can be expected to play a prominent role. Therefore clusters can also be seen as a key element in the European Metropolitan Region concept. Within such an economic space, localisation economies according to the 'Marshallian trinity' (knowledge spillovers, input sharing and labour market pooling (Rosenthal/Strange 2003)) can be realized. The paper builds on a comprehensive company survey for the core of the European Metropolitan Region Nuremberg that includes customer-supplier relationships and various forms of cooperation. As indicated by numerous empirical studies, the characteristics of clusters differ substantially. In order to overcome the fuzziness of the concept we suggest a bottom-up methodology of cluster identification using a set of qualitative and quantitative indicators. Given that many kinds of barriers to interregional and international trade are becoming less and less important and transport cost are falling, modern production clusters tend to have a higher geographical extension than traditional ones. We therefore raise the question of whether clustering is relevant for economic integration on the regional, national and supra-national level." (Author's abstract, IAB-Doku) ((en))Stadtregion, regionales Cluster, Standort, Industrieregion, Regionalökonomie, zwischenbetriebliche Kooperation, Zulieferer, Wirtschaftsstruktur, regionales Netzwerk, Nürnberg, Oberfranken, Mittelfranken, Franken, Bayern
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
Observational asteroseismology of hot subdwarf stars
Hot subdwarf stars are particularly challenging for asteroseismology due to
their rapid pulsation periods, intrinsic faintness and relative rarity both in
the field and in clusters. These features have ensured that the preferred
method of observation up to now has been white-light photometry, and all
asteroseismological solutions to date have been made by model fitting of the
frequency spectrum. Several attempts have been made to perform asteroseismology
using time-resolved spectroscopy on the brightest of these stars, but with
modest results. A few attempts at simultaneous multi-color photometry have also
been made to identify modes with the amplitude ratio method. We will review the
most recent observational results and progress in improving the observational
methods for ground-based asteroseismology of these compact pulsators.Comment: 8 pages, to appear in Astronomische Nachrichten, Vol. 331, 102
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