2,750 research outputs found
Young stars at large distances from the galactic plane: mechanisms of formation
We have collected from the literature a list of early-type stars, situated at
large distances from the galactic plane, for which evidence of youth seems
convincing. We discuss two possible formation mechanisms for these stars:
ejection from the plane by dynamical interactions within small clusters, and
formation away from the plane, via induced shocks created by spiral density
waves. We identify the stars that could be explained by each mechanism. We
conclude that the ejection mechanism can account for about two thirds of the
stars, while a combination of star formation at z = 500-800 pc from the plane
and ejection, can account for 90 percent of these stars. Neither mechanism, nor
both together, can explain the most extreme examples.Comment: 6 pages, No figures. Sixth Pacific Rim Conference on Stellar
Astrophysics - A tribute to Helmut Abt, (Kluwer
Towards an Achievable Performance for the Loop Nests
Numerous code optimization techniques, including loop nest optimizations,
have been developed over the last four decades. Loop optimization techniques
transform loop nests to improve the performance of the code on a target
architecture, including exposing parallelism. Finding and evaluating an
optimal, semantic-preserving sequence of transformations is a complex problem.
The sequence is guided using heuristics and/or analytical models and there is
no way of knowing how close it gets to optimal performance or if there is any
headroom for improvement. This paper makes two contributions. First, it uses a
comparative analysis of loop optimizations/transformations across multiple
compilers to determine how much headroom may exist for each compiler. And
second, it presents an approach to characterize the loop nests based on their
hardware performance counter values and a Machine Learning approach that
predicts which compiler will generate the fastest code for a loop nest. The
prediction is made for both auto-vectorized, serial compilation and for
auto-parallelization. The results show that the headroom for state-of-the-art
compilers ranges from 1.10x to 1.42x for the serial code and from 1.30x to
1.71x for the auto-parallelized code. These results are based on the Machine
Learning predictions.Comment: Accepted at the 31st International Workshop on Languages and
Compilers for Parallel Computing (LCPC 2018
Assessing Predation Risk to Threatened Fauna from their Prevalence in Predator Scats: Dingoes and Rodents in Arid Australia
The prevalence of threatened species in predator scats has often been used to gauge the risks that predators pose to threatened species, with the infrequent occurrence of a given species often considered indicative of negligible predation risks. In this study, data from 4087 dingo (Canis lupus dingo and hybrids) scats were assessed alongside additional information on predator and prey distribution, dingo control effort and predation rates to evaluate whether or not the observed frequency of threatened species in dingo scats warrants more detailed investigation of dingo predation risks to them. Three small rodents (dusky hopping-mice Notomys fuscus; fawn hopping-mice Notomys cervinus; plains mice Pseudomys australis) were the only threatened species detected in <8% of dingo scats from any given site, suggesting that dingoes might not threaten them. However, consideration of dingo control effort revealed that plains mice distribution has largely retracted to the area where dingoes have been most heavily subjected to lethal control. Assessing the hypothetical predation rates of dingoes on dusky hopping-mice revealed that dingo predation alone has the potential to depopulate local hopping-mice populations within a few months. It was concluded that the occurrence of a given prey species in predator scats may be indicative of what the predator ate under the prevailing conditions, but in isolation, such data can have a poor ability to inform predation risk assessments. Some populations of threatened fauna assumed to derive a benefit from the presence of dingoes may instead be susceptible to dingo-induced declines under certain conditions
The Road to Quantum Computational Supremacy
We present an idiosyncratic view of the race for quantum computational
supremacy. Google's approach and IBM challenge are examined. An unexpected
side-effect of the race is the significant progress in designing fast classical
algorithms. Quantum supremacy, if achieved, won't make classical computing
obsolete.Comment: 15 pages, 1 figur
GLAST: Understanding the High Energy Gamma-Ray Sky
We discuss the ability of the GLAST Large Area Telescope (LAT) to identify,
resolve, and study the high energy gamma-ray sky. Compared to previous
instruments the telescope will have greatly improved sensitivity and ability to
localize gamma-ray point sources. The ability to resolve the location and
identity of EGRET unidentified sources is described. We summarize the current
knowledge of the high energy gamma-ray sky and discuss the astrophysics of
known and some prospective classes of gamma-ray emitters. In addition, we also
describe the potential of GLAST to resolve old puzzles and to discover new
classes of sources.Comment: To appear in Cosmic Gamma Ray Sources, Kluwer ASSL Series, Edited by
K.S. Cheng and G.E. Romer
The native architecture of a photosynthetic membrane
In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membrane-associated chlorophyll–protein complexes. Although the detailed structure of each complex has been determined, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found for the peripheral light-harvesting LH2 complex. In the first, groups of 10–20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)–light-harvesting 1 (RC–LH1–PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow 'energy conduits' of LH2 just two or three complexes wide
The development of path integration: combining estimations of distance and heading
Efficient daily navigation is underpinned by path integration, the mechanism by which we use self-movement information to update our position in space. This process is well-understood in adulthood, but there has been relatively little study of path integration in childhood, leading to an underrepresentation in accounts of navigational development. Previous research has shown that calculation of distance and heading both tend to be less accurate in children as they are in adults, although there have been no studies of the combined calculation of distance and heading that typifies naturalistic path integration. In the present study 5-year-olds and 7-year-olds took part in a triangle-completion task, where they were required to return to the startpoint of a multi-element path using only idiothetic information. Performance was compared to a sample of adult participants, who were found to be more accurate than children on measures of landing error, heading error, and distance error. 7-year-olds were significantly more accurate than 5-year-olds on measures of landing error and heading error, although the difference between groups was much smaller for distance error. All measures were reliably correlated with age, demonstrating a clear development of path integration abilities within the age range tested. Taken together, these data make a strong case for the inclusion of path integration within developmental models of spatial navigational processing
Comparison of outcomes following a cytological or histological diagnosis of malignant mesothelioma
Background: Survival with the epithelioid subtype of malignant mesothelioma (MM) is longer than the biphasic or sarcomatoid subtypes. There is concern that cytology-diagnosed epithelioid MM may underdiagnose the biphasic subtype. This study examines survival differences between patients with epithelioid MM diagnosed by cytology only and other subtypes diagnosed by histology. Methods: Demographics, diagnosis method, MM subtype and survival were extracted from the Western Australia (WA) Mesothelioma Registry, which records details of all MM cases occurring in WA. Results: A total of 2024 MM cases were identified over 42 years. One thousand seven hundred forty-four (86.2%) were male, median (IQR) age was 68.6 (60.4–77.0) years. A total of 1212 (59.9%) cases were identified as epithelioid subtype of which 499 (41.2%) were diagnosed using fluid cytology only. Those with a cytology-only diagnosis were older than the histology group (median 70.2 vs 67.6 years, P<0.001), but median survival was similar (cytology 10.6 (5.5–19.2) vs histology 11.1 (4.8–19.8) months, P=0.727) and Cox regression modelling adjusting for age, sex, site and time since first exposure showed no difference in survival between the different diagnostic approaches. Conclusions: Survival of cytologically and histologically diagnosed epithelioid MM cases does not differ. A diagnostic tap should be considered adequate to diagnose epithelioid MM without need for further invasive testing
A fresh look at the evolution and diversification of photochemical reaction centers
In this review, I reexamine the origin and diversification of photochemical reaction centers based on the known phylogenetic relations of the core subunits, and with the aid of sequence and structural alignments. I show, for example, that the protein folds at the C-terminus of the D1 and D2 subunits of Photosystem II, which are essential for the coordination of the water-oxidizing complex, were already in place in the most ancestral Type II reaction center subunit. I then evaluate the evolution of reaction centers in the context of the rise and expansion of the different groups of bacteria based on recent large-scale phylogenetic analyses. I find that the Heliobacteriaceae family of Firmicutes appears to be the earliest branching of the known groups of phototrophic bacteria; however, the origin of photochemical reaction centers and chlorophyll synthesis cannot be placed in this group. Moreover, it becomes evident that the Acidobacteria and the Proteobacteria shared a more recent common phototrophic ancestor, and this is also likely for the Chloroflexi and the Cyanobacteria. Finally, I argue that the discrepancies among the phylogenies of the reaction center proteins, chlorophyll synthesis enzymes, and the species tree of bacteria are best explained if both types of photochemical reaction centers evolved before the diversification of the known phyla of phototrophic bacteria. The primordial phototrophic ancestor must have had both Type I and Type II reaction centers
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201
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