Abundance Measurements (abundance + measurement)

Distribution by Scientific Domains

Selected Abstracts

Magnetic characterization of Cretaceous-Tertiary boundary sediments

Víctor Villasante-Marcos
Studied sections' locations vary in distance to the Chicxulub structure from distal (Agost and Caravaca, Spain), through closer (ODP Hole 1049A, Blake Nose, North Atlantic), to proximal (El Mimbral and La Lajilla, Mexico). A clear magnetic signature is associated with the fireball layer in the most distal sections, consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization (SIRM), and a decrease in remanence coercivity. Magnetic properties in these sections point to a distinctive ferrimagnetic phase, probably corresponding to the reported Mg- and Ni-rich, highly oxidized spinels of meteoritic origin. At closer and proximal sections magnetic properties are different. Although there is an increase in susceptibility and SIRM associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections, and with a similar limonitic layer on top of the spherule bed that defines the boundary at Blake Nose, the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine-grained goethite. Based on previous geochemical studies at Blake Nose and new geochemical and PGE abundance measurements performed in this work at El Mimbral, this goethite-rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of Fe. There is not enough evidence to assert that this Fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material. Magnetic, geochemical, and iridium data reject it as a primary meteoritic phase. [source]

Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarf

Ayesha Begum
ABSTRACT We present an analysis of H i, H, and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disc (,8.8 times the Holmberg radius), and a dark-to-luminous (i.e. stellar) mass ratio of ,149, which makes it one of the ,darkest' dwarf irregular galaxies known. However, its ratio of baryon (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution H i images of the galaxy show evidence for a large-scale (purely gaseous) spiral arm and central bar. From our H i data, a rotation curve can be derived out to ,37,44 disc scalelengths in the J and B bands, respectively. This is just slightly short of the radius at which one would expect a Navarro,Frenk,White type rotation curve to start falling. The galaxy has an integrated star formation rate (SFR) of ,0.0034 M, yr,1, while the average SFR within the optical disc is ,0.0049 M, yr,1 kpc,2. Despite the gaseous spiral feature and the ongoing star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. This is consistent with the behaviour seen in other dwarf galaxies. We also find that the SFR is consistent with that expected from the observed correlations between H i mass and SFR and the global Kennicutt,Schmidt law, respectively. We measure the oxygen abundance to be 12 + log(O/H) = 7.66 ± 0.10, which is consistent with that expected from the metallicity,luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in the context of the closed-box model of chemical evolution. The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to the edge of the H i disc (i.e. greater than eight times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric H i maps and chemical abundance measurements, we find that the effective yield is anticorrelated with the total dynamical mass, as expected in leaky box models. [source]

A homogeneous sample of sub-damped Lyman systems , IV.

Global metallicity evolution
ABSTRACT An accurate method to measure the abundance of high-redshift galaxies involves the observation of absorbers along the line of sight towards a background quasar. Here, we present abundance measurements of 13 z, 3 sub-damped Lyman , (sub-DLA) systems (quasar absorbers with H i column density in the range 19 < log N(H i) < 20.3 cm,2) based on high-resolution observations with the VLT UVES spectrograph. These observations more than double the amount of metallicity information for sub-DLAs available at z > 3. These new data, combined with other sub-DLA measurements from the literature, confirm the stronger evolution of metallicity with redshift for sub-DLAs than for the classical damped Lyman , absorbers. In addition, these observations are used to compute for the first time, using photoionization modelling in a sample of sub-DLAs, the fraction of gas that is ionized. Based on these results, we calculate that sub-DLAs contribute no more than 6 per cent of the expected amount of metals at z, 2.5. We therefore conclude that, even if sub-DLAs are found to be more metal-rich than classical DLAs, their contribution is insufficient to solve the so-called ,missing-metals' problem. [source]

The age, metallicity and ,-element abundance of Galactic globular clusters from single stellar population models

Jon T. Mendel
ABSTRACT Establishing the reliability with which stellar population parameters can be measured is vital to extragalactic astronomy. Galactic globular clusters (GCs) provide an excellent medium in which to test the consistency of single stellar population (SSP) models as they should be our best analogue to a homogeneous (single) stellar population. Here we present age, metallicity and ,-element abundance measurements for 48 Galactic GCs as determined from integrated spectra using Lick indices and SSP models from Thomas, Maraston & Korn, Lee & Worthey and Vazdekis et al. By comparing our new measurements to independent determinations we are able to assess the ability of these SSPs to derive consistent results , a key requirement before application to heterogeneous stellar populations like galaxies. We find that metallicity determinations are extremely robust, showing good agreement for all models examined here, including a range of enhancement methods. Ages and ,-element abundances are accurate for a subset of our models, with the caveat that the range of these parameters in Galactic GCs is limited. We are able to show that the application of published Lick index response functions to models with fixed abundance ratios allows us to measure reasonable ,-element abundances from a variety of models. We also examine the age,metallicity and [,/Fe],metallicity relations predicted by SSP models, and characterize the possible effects of varied model horizontal branch morphology on our overall results. [source]

Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest

Björn D. Lindahl
Summary ,,Our understanding of how saprotrophic and mycorrhizal fungi interact to re-circulate carbon and nutrients from plant litter and soil organic matter is limited by poor understanding of their spatiotemporal dynamics. ,,In order to investigate how different functional groups of fungi contribute to carbon and nitrogen cycling at different stages of decomposition, we studied changes in fungal community composition along vertical profiles through a Pinus sylvestris forest soil. We combined molecular identification methods with 14C dating of the organic matter, analyses of carbon:nitrogen (C:N) ratios and 15N natural abundance measurements. ,,Saprotrophic fungi were primarily confined to relatively recently (< 4 yr) shed litter components on the surface of the forest floor, where organic carbon was mineralized while nitrogen was retained. Mycorrhizal fungi dominated in the underlying, more decomposed litter and humus, where they apparently mobilized N and made it available to their host plants. ,,Our observations show that the degrading and nutrient-mobilizing components of the fungal community are spatially separated. This has important implications for biogeochemical studies of boreal forest ecosystems. [source]

VLT-CRIRES: "Good Vibrations" Rotational-vibrational molecular spectroscopy in astronomy

H.U. Käufl
Abstract Near-Infrared high spectral and spatial resolution spectroscopy offers new and innovative observing opportunities for astronomy. The "traditional" benefits of IR-astronomy , strongly reduced extinction and availability of adaptive optics , more than offset for many applications the compared to CCD-based astronomy strongly reduced sensitivity. Especially in high resolution spectroscopy interferences by telluric lines can be minimized. Moreover for abundance studies many important atomic lines can be accessed in the NIR. A novel spectral feature available for quantitative spectroscopy are the molecular rotational-vibrational transitions which allow for fundamentally new studies of condensed objects and atmospheres. This is also an important complement to radio-astronomy, especially with ALMA, where molecules are generally only observed in the vibrational ground state. Rot-vib transitions also allow high precision abundance measurements , including isotopic ratios , fundamental to understand the thermo-nuclear processes in stars beyond the main sequence. Quantitative modeling of atmospheres has progressed such that the unambiguous interpretation of IR-spectra is now well established. In combination with adaptive optics spectro-astrometry is even more powerful and with VLT-CRIRES a spatial resolution of better than one milli-arcsecond has been demonstrated. Some highlights and recent results will be presented: our solar system, extrasolar planets, star- and planet formation, stellar evolution and the formation of galactic bulges (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]