Short gamma-ray bursts from SGR giant flares and neutron star mergers: two populations are better than one     

Robert Chapman  Robert S. Priddey  Nial R. Tanvir 《Monthly notices of the Royal Astronomical Society》2009,395(3):1515-1522

There is increasing evidence of a local population of short duration gamma-ray bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible luminosity functions (LFs) for both neutron star binary mergers and giant flares from soft gamma repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall Burst and Transient Source Experiment (BATSE) sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertainties in the intrinsic rates, we find that at least a bimodal LF consisting of lower and higher luminosity populations is required to reproduce both the overall BATSE sGRB number counts and a local burst distribution. Furthermore, the best-fitting parameters of the lower luminosity population agree well with the known properties of SGR giant flares, and the predicted numbers are sufficient to account for previous estimates of the local sGRB population.  相似文献   

The diverse nature of optical emission lines in brightest cluster galaxies: IFU observations of the central kiloparsec     

Louise O. V. Edwards †  Carmelle Robert  Mercedes Mollá  Sean L. McGee 《Monthly notices of the Royal Astronomical Society》2009,396(4):1953-1971

We present integral field spectroscopy of the nebular line emission in a sample of nine brightest cluster galaxies (BCGs). The sample was chosen to probe both cooling flow and non-cooling flow clusters, as well as a range of cluster X-ray luminosities. The line emission morphology and velocity gradients suggest a great diversity in the properties of the line emitting gas. While some BCGs show evidence for filamentary or patchy emission (Abell 1060, Abell 1668 and MKW 3s), others have extended emission (Abell 1204, Abell 2199), while still others have centrally concentrated emission (Abell 2052). We examine diagnostic line ratios to determine the dominant ionization mechanisms in each galaxy. Most of the galaxies show regions with active galactic nucleus like spectra, however, for two BCGs, Abell 1060 and Abell 1204, the emission line diagnostics suggest regions which can be described by the emission from young stellar populations. The diversity of emission-line properties in our sample of BCGs suggests that the emission mechanism is not universal, with different ionization processes dominating different systems. Given this diversity, there is no evidence for a clear distinction of the emission-line properties between cooling flow and non-cooling flow BCGs. It is not always cooling flow BCGs which show emission (or young stellar populations), and non-cooling flow BCGs which do not.  相似文献   

Galaxies–intergalactic medium interaction calculation – I. Galaxy formation as a function of large-scale environment     

Robert A. Crain  Tom Theuns  Claudio Dalla Vecchia  Vincent R. Eke  Carlos S. Frenk  Adrian Jenkins  Scott T. Kay  John A. Peacock  Frazer R. Pearce  Joop Schaye  Volker Springel  Peter A. Thomas  Simon D. M. White  Robert P. C. Wiersma 《Monthly notices of the Royal Astronomical Society》2009,399(4):1773-1794

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The 2dF–SDSS LRG and QSO survey: the QSO luminosity function at 0.4 < z < 2.6     

Scott M. Croom  Gordon T. Richards  Tom Shanks  Brian J. Boyle  Michael A. Strauss  Adam D. Myers  Robert C. Nichol  Kevin A. Pimbblet  Nicholas P. Ross  Donald P. Schneider  Robert G. Sharp  David A. Wake 《Monthly notices of the Royal Astronomical Society》2009,399(4):1755-1772

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Magnetic zones of Mars: Deformation‐controlled origin of magnetic anomalies     

Gunther KLETETSCHKA  Robert J. LILLIS  Norman F. NESS  Mario H. ACUÑA  Jack E. P. CONNERNEY  Peter J. WASILEWSKI 《Meteoritics & planetary science》2009,44(1):131-140

Abstract— Intense magnetic anomalies over Martian surface suggest preservation of large volumes of very old crust (>3 Gyr) that formed in the presence of a global magnetic field. The global distribution of the magnetic intensities observed above the Martian crust suggests a division into three zones. Zone 1 is where the magnetic signature is negligible or of relatively low intensity at Mars Global Surveyor (MGS) satellite mapping altitude (400 km). Zone 2 is the region of intermediate crustal magnetic amplitudes and zone 3 is where the highest magnetic intensities are measured. Crater demagnetization near zone 3 reveals the presence of rocks with both high magnetic intensity and coercivity. Magnetic analyses of terrestrial rocks show that compositional banding in orogenic zones significantly enhances both magnetic coercivity and thermal remanent magnetization (TRM) efficiency. Such enhancement offers a novel explanation for the anomalously large intensities inferred of magnetic sources on Mars. We propose that both large magnetic coercivity and intensity near the South Pole is indicative of the presence of a large degree of deformation. Associated compositional zoning creates conditions for large scale magnetic anisotropy allowing magnetic minerals to acquire magnetization more efficiently, thereby causing the distinct magnetic signatures in zone 3, expressed by intense magnetic anomalies. We use a simple model to verify the magnetic enhancement. We hypothesize that magnetically enhanced zone would reside over the down welling plume at the time of magnetization acquisition.  相似文献   

Microbial activity and chemical weathering in the Middendorf aquifer,South Carolina     

Jungho Park  Robert A. Sanford  Craig M. Bethke 《Chemical Geology》2009,258(3-4):232-241

We use reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. We constructed a model accounting for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO₂ and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. We then fit the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO₂ derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO₂ dissolved in rainwater.  相似文献   

The activity of silica in kimberlites,revisited   总被引：1，自引：1，他引：0  

Robert W. Luth 《Contributions to Mineralogy and Petrology》2009,158(2):283-294

The activity of silica in a silicate liquid in equilibrium with olivine and orthopyroxene decreases with increasing pressure. In contrast, the activity of silica in an unbuffered silicate liquid changes little with pressure. Although the implications of these pressure dependencies have been considered by previous authors in terms of inferring pressures of origin of magmas, less consideration has been given to the implications of these dependencies on the evolution of the magma en route to the surface, or to the mantle through which the magma passes. In this paper, a combination of Schreinemakers’ analysis in isothermal section and calculated reactions in space is used to (a) rationalize the absence of orthopyroxene xenocrysts in kimberlites and the relative abundance of olivine “megacrysts” therein, (b) propose another reason for the paucity of xenocrystic mantle-derived carbonates in kimberlites, (c) explain why clinopyroxene is much less reactive in the kimberlite melt than is orthopyroxene, and (d) explore the implications of the relative stabilities of olivine, orthopyroxene, and clinopyroxene in kimberlitic magma for the mantle through which the magma transits.

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Robert W. LuthEmail:

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Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism   总被引：2，自引：1，他引：1  

Steven Creighton  Thomas Stachel  Sergei Matveev  Heidi Höfer  Catherine McCammon  Robert W. Luth 《Contributions to Mineralogy and Petrology》2009,157(4):491-504

The oxidation state, reflected in the oxygen fugacity (fO₂), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO₂ FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO₂ trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO₂ = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Equilibrium H/H fractionations in organic molecules: I. Experimental calibration of ab initio calculations     

Ying Wang  Alex L. Sessions  Robert J. Nielsen  William A. Goddard III 《Geochimica et cosmochimica acta》2009,73(23):7060-7075

Carbon-bound hydrogen in sedimentary organic matter can undergo exchange over geologic timescales, altering its isotopic composition. Studies investigating the natural abundance distribution of ¹H and ²H in such molecules must account for this exchange, which in turn requires quantitative knowledge regarding the endpoint of exchange, i.e., the equilibrium isotopic fractionation factor (α_eq). To date, relevant data have been lacking for molecules larger than methane. Here we describe an experimental method to measure α_eq for C-bound H positions adjacent to carbonyl group (H_α) in ketones. H at these positions equilibrates on a timescale of days as a result of keto-enol tautomerism, allowing equilibrium ²H/¹H distributions to be indirectly measured. Molecular vibrations for the same ketone molecules are then computed using Density Functional Theory at the B3LYP/6-311G** level and used to calculate α_eq values for H_α. Comparison of experimental and computational results for six different straight and branched ketones yields a temperature-dependent linear calibration curve with slope = 1.081−0.00376T and intercept = 8.404−0.387T, where T is temperature in degrees Celsius. Since the dominant systematic error in the calculation (omission of anharmonicity) is of the same size for ketones and C-bound H in most other linear compounds, we propose that this calibration can be applied to analogous calculations for a wide variety of organic molecules with linear carbon skeletons for temperatures below 100 °C. In a companion paper (Wang et al., 2009) we use this new calibration dataset to calculate the temperature-dependent equilibrium isotopic fractionation factors for a range of linear hydrocarbons, alcohols, ethers, ketones, esters and acids.  相似文献   

Are livestock a troublesome commodity?     

Benjamin Gardner 《Geoforum》2009,40(5):781-783

Rather than making a uniform case for understanding the neoliberalization of ranching and pastoralism, the articles in this volume show how everyday practices are made through historically and geographically specific struggles over the characteristics of production, exchange and rule, in which livestock play a particularly important role. Collectively, I believe the articles in this volume highlight the significant relationship between cattle and capitalism, and make a compelling argument that to understand the material implications of neoliberalism we need to pay more attention to livestock as an important and rapidly changing medium of livelihood production and accumulation strategy around the world.  相似文献