Australian National University

About: Australian National University is a education organization based out in Canberra, Australian Capital Territory, Australia. It is known for research contribution in the topics: Population & Galaxy. The organization has 34419 authors who have published 109261 publications receiving 4315448 citations. The organization is also known as: The Australian National University & ANU.

The Hamburg/ESO R-process enhanced star survey (HERES). II. Spectroscopic analysis of the survey sample

Abstract: We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars −3.8 ≤ (Fe/H) ≤− 1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically S /N ∼ 54 per pixel (ranging from 17 to 308), R ∼ 20 000, λ = 3760-4980 A. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3σ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S /N ,o r when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of (Fe/H) = −2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with (Fe/H) < −3 and 181 stars with −3 < (Fe/H) < −2. We find one star with (Fe/H) < −3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at (Fe/H) < −2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.

Reassessing the first appearance of eukaryotes and cyanobacteria

Abstract: The oldest widely accepted evidence for oxygenic photosynthesis on Earth comes from hydro-carbon biomarkers extracted from 2.7-billion-year-old shales in the Pilbara Craton of Australia, thought to be evidence of eukaryotes and photosynthetic cyanobacteria. This early date has caused controversy because of the long delay between this earliest appearance of oxygen-producing cyanobacteria and the 'great oxidation event' that caused the rise of atmospheric oxygen some 300 million years later. New work by Rasmussen et al. shows that the organic biomarkers are not of Archaean age and must have entered the rocks later, some time after about 2.2 billion years ago. The earliest unambiguous fossil evidence for eukaryotes and cyanobacteria thus reverts to 1.78–1.68 and 2.15 billion years, respectively. The oldest widely accepted evidence for oxygenic photosynthesis comes from hydrocarbon biomarkers extracted from 2.7-billion-year-old shales in the Pilbara Craton, Australia, thought to be evidence of eukaryotes and photosynthetic cyanobacteria. But evidence now shows that the organic biomarkers were not indigenous to the rocks containing them, and must have entered the rocks after ∼2.2 Gyr ago. The earliest unambiguous fossil evidence for eukaryotes and cyanobacteria thus reverts to 1.78–1.68 and 2.15 Gyr, respectively. The evolution of oxygenic photosynthesis had a profound impact on the Earth’s surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago1,2 and the onset of extreme ice ages3. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from ∼2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria4,5,6,7. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10–20‰) of 13C relative to bulk sedimentary organic matter5,8. Here we present micrometre-scale, in situ 13C/12C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in 13C, indicating that indigenous petroleum is 10–20‰ lighter than the extracted hydrocarbons5. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism ∼2.2 Gyr ago9 and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68 Gyr ago and ∼2.15 Gyr ago10,11, respectively. Our results eliminate the evidence for oxygenic photosynthesis ∼2.7 Gyr ago and exclude previous biomarker evidence for a long delay (∼300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 Gyr ago1.

Extremely Metal-Poor Stars. II. Elemental Abundances and the Early Chemical Enrichment of The Galaxy*

Abstract: We have obtained high-resolution spectra of 23 very metal-poor stars and present an abundance analysis for 19 of these for elements between Mg and Eu. The sample comprises roughly equal numbers of dwarfs and giants. All stars have [Fe/H] 0 deserve further study. CS 22897–008 has high Sr, Y, and C abundances for its [Fe/H] but normal Ba. This signature may have arisen from the weak s-process in M > 15 M stars or by r-processing. By combining an analytic description of gaseous supernova remnants with supernova yields, we show that enrichment of the interstellar medium is influenced more by supernova physics (explosive energy) than by environmental conditions (cloud density). If supernova iron-peak yields are correlated with explosion energy, we can accommodate the well-defined abundance trends with a chaotic picture for halo formation involving independently evolving clouds, as was envisaged by Searle & Zinn. We calculate that a typical enrichment in the protohalo will produce [Fe/H] = −2.7. This coincides with larger abundance variations in field stars of lower metallicity and the lower abundance limit for Galactic globular clusters.

A guide to phylogenetic metrics for conservation, community ecology and macroecology.

Abstract: The use of phylogenies in ecology is increasingly common and has broadened our understanding of biological diversity. Ecological sub-disciplines, particularly conservation, community ecology and macroecology, all recognize the value of evolutionary relationships but the resulting development of phylogenetic approaches has led to a proliferation of phylogenetic diversity metrics. The use of many metrics across the sub-disciplines hampers potential meta-analyses, syntheses, and generalizations of existing results. Further, there is no guide for selecting the appropriate metric for a given question, and different metrics are frequently used to address similar questions. To improve the choice, application, and interpretation of phylo-diversity metrics, we organize existing metrics by expanding on a unifying framework for phylogenetic information. Generally, questions about phylogenetic relationships within or between assemblages tend to ask three types of question: how much; how different; or how regular? We show that these questions reflect three dimensions of a phylogenetic tree: richness, divergence, and regularity. We classify 70 existing phylo-diversity metrics based on their mathematical form within these three dimensions and identify ‘anchor’ representatives: for α-diversity metrics these are PD (Faith's phylogenetic diversity), MPD (mean pairwise distance), and VPD (variation of pairwise distances). By analysing mathematical formulae and using simulations, we use this framework to identify metrics that mix dimensions, and we provide a guide to choosing and using the most appropriate metrics. We show that metric choice requires connecting the research question with the correct dimension of the framework and that there are logical approaches to selecting and interpreting metrics. The guide outlined herein will help researchers navigate the current jungle of indices.