Bücher- und Zeitschriftenschau

Ohne Zusammenfassung     

Tidal tails of star clusters

Based on recent findings of a formation mechanism of substructure in tidal tails by Küpper et al., we investigate a more comprehensive set of N -body models of star clusters on orbits about a Milky Way like potential. We find that the predicted epicyclic overdensities arise in any tidal tail no matter which orbit the cluster follows as long as the cluster lives long enough for the overdensities to build up.
The distance of the overdensities along the tidal tail from the cluster centre depends for circular orbits only on the mass of the cluster and the strength of the tidal field, and therefore decreases monotonically with time, while for eccentric orbits the orbital motion influences the distance, causing a periodic compression and stretching of the tails and making the distance oscillate with time. We provide an approximation for estimating the distance of the overdensities in this case.
We describe an additional type of overdensity which arises in extended tidal tails of clusters on eccentric orbits, when the acceleration of the tidal field on the stellar stream is no longer homogeneous. Moreover, we conclude that a pericentre passage or a disc shock is not the direct origin of an overdensity within a tidal tail. Escape due to such tidal perturbations does not take place immediately after the perturbation but is rather delayed and spread over the orbit of the cluster. All observable overdensities are therefore of the mentioned two types. In particular, we note that substructured tidal tails do not imply the existence of dark matter substructures in the haloes of galaxies.     

<Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr anomalies in Late Cretaceous-Early Tertiary strata of the Cauvery basin,south India: Constraints on nature and rate of environmental changes across K-T boundary

The Ariyalur-Pondicherry sub-basin of the Cauvery basin comprises a near complete stratigraphic record of Upper Cretaceous-Lower Tertiary periods. Earlier studies have documented variations of clay mineral assemblages, change in microtexture of siliciclasts and many geochemical and stable isotopic anomalies far below the Cretaceous-Tertiary boundary (KTB) in these strata. This paper documents the occurrences of two positive ⁸⁷Sr/⁸⁶Sr anomalies preceding K-T boundary in this basin and discusses plausible causes. Analysis of trace elemental and stable isotopic profiles, sedimentation history, petrography and mineralogy of the rocks reveal that while both the anomalies may be due to increased detrital influx caused by sea level and climatic changes, the second anomaly might have been influenced by Deccan volcanism which in turn predated KTB. Record of such anomalies preceding K-T boundary supports the view of multi-causal step-wise extinction of biota across KTB.     

Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications

Lattice diffusion coefficients have been determined for 19 elements (Li, Be, Na, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Eu, Gd, Lu and Hf) in a single crystal of San Carlos olivine as a function of crystallographic orientation, at 1,300°C, 1 bar and fO₂ = 10^−8.3 bars, by equilibration with a synthetic silicate melt. Results for Li, Na, V, Cr, Fe and Zn are from diffusion of these elements out of the olivine, starting from their indigenous concentrations; those for all other elements are from diffusion into the olivine, from the silicate melt reservoir. Our 25-day experiment produced diffusion profiles 50 to > 700 μm in length, which are sufficiently long that precise analyses could be achieved by scanning laser ablation inductively coupled plasma mass spectrometry, even at concentration levels well below 1 μg g⁻¹. For the divalent cations Ca, Mn, Fe and Ni, profiles were also obtained by electron microprobe analysis. The results of the two methods agree well with each other, and are consistent with divalent cation diffusion coefficients previously determined using different experimental methodologies. Olivine/melt partition coefficients retrieved from the data are also consistent with other published partitioning data, indicating that element incorporation and transport in olivine in our experiment occurred via mechanisms appropriate to natural conditions. Most of the examined trace elements diffuse through olivine at similar rates to the major octahedral cations Fe and Mg, showing that cation charge and radius have little direct influence on diffusion rates. Aluminium and P remain low and constant in the olivine, implying negligible transport at our analytical scale, hence Al and P diffusion rates that are at least two orders of magnitude slower than the other cations studied here. All determined element diffusivities are anisotropic, with diffusion fastest along the [001] axis, except Y and the REEs, which diffuse isotropically. The results suggest that element diffusivity in olivine is largely controlled by cation site preference, charge balance mechanisms and point-defect concentrations. Elements that are present on multiple cation sites in olivine (e.g. Be and Ti) and trivalent elements that are charge-balanced by octahedral site vacancies tend to diffuse at relatively fast rates.     

Near-Sun Flux-Rope Structure of CMEs

We have used the Krall flux-rope model (Krall and St. Cyr, Astrophys. J. 2006, 657, 1740) (KFR) to fit 23 magnetic cloud (MC)-CMEs and 30 non-cloud ejecta (EJ)-CMEs in the Living With a Star (LWS) Coordinated Data Analysis Workshop (CDAW) 2011 list. The KFR-fit results shows that the CMEs associated with MCs (EJs) have been deflected closer to (away from) the solar disk center (DC), likely by both the intrinsic magnetic structures inside an active region (AR) and ambient magnetic structures (e.g. nearby ARs, coronal holes, and streamers, etc.). The mean absolute propagation latitudes and longitudes of the EJ-CMEs (18^°, 11^°) were larger than those of the MC-CMEs (11^°, 6^°) by 7^° and 5^°, respectively. Furthermore, the KFR-fit widths showed that the MC-CMEs are wider than the EJ-CMEs. The mean fitting face-on width and edge-on width of the MC-CMEs (EJ-CMEs) were 87 (85)^° and 70 (63)^°, respectively. The deflection away from DC and narrower angular widths of the EJ-CMEs have caused the observing spacecraft to pass over only their flanks and miss the central flux-rope structures. The results of this work support the idea that all CMEs have a flux-rope structure.     

The Corossol structure: A possible impact crater on the seafloor of the northwestern Gulf of St. Lawrence,Eastern Canada

We report on a 4.1 (±0.2) km diameter and 185 m deep circular submarine structure exposed on the seabed in >40 m water depths in the northwestern Gulf of St. Lawrence (Eastern Canada) from the analysis of high‐resolution multibeam bathymetric and seismic data. The presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures. Also, other origins, such as kimberlites, intrusions, karsts, or diapirs, can be eliminated on geological criteria. A single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition, assumed to be impact melts, and a single quartz grain with planar intersection features thought to be shock‐induced planar deformation features (PDFs). The absolute age of this possible impact structure is unknown, but its geological setting indicates that it was formed long after the Mid‐Ordovician and before regional pre‐Quaternary sea‐level lowstands. Present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure.     

The pseudometeorite Angara

Several pallasites are known to have formed strewn fields with multiple fragments. Therefore, it seems possible that the famous Krasnojarsk pallasite—the Pallas Iron of 687 kg—could have been accompanied by one or more additional fragments that had not been recovered due to incomplete observations from the overgrown and remote place of fall. During a survey for literature accounts of distant fragments in such a hypothetical strewn field, a report of native iron was found, dating to 1847. The fragments of nickel‐free iron—amounting to at least a few kg—had been recovered 1836–1843 during placer gold mining north of the Angara River. The position of these finds is coincident with Burovaya, Murozhna, and Uderei, the three known fragments of the pseudometeorite Angara, all collected in 1885.     

Circulation dynamics and its influence on European and Mediterranean January–April climate over the past half millennium: results and insights from instrumental data, documentary evidence and coupled climate models

We use long instrumental temperature series together with available field reconstructions of sea-level pressure (SLP) and three-dimensional climate model simulations to analyze relations between temperature anomalies and atmospheric circulation patterns over much of Europe and the Mediterranean for the late winter/early spring (January–April, JFMA) season. A Canonical Correlation Analysis (CCA) investigates interannual to interdecadal covariability between a new gridded SLP field reconstruction and seven long instrumental temperature series covering the past 250 years. We then present and discuss prominent atmospheric circulation patterns related to anomalous warm and cold JFMA conditions within different European areas spanning the period 1760–2007. Next, using a data assimilation technique, we link gridded SLP data with a climate model (EC-Bilt-Clio) for a better dynamical understanding of the relationship between large scale circulation and European climate. We thus present an alternative approach to reconstruct climate for the pre-instrumental period based on the assimilated model simulations. Furthermore, we present an independent method to extend the dynamic circulation analysis for anomalously cold European JFMA conditions back to the sixteenth century. To this end, we use documentary records that are spatially representative for the long instrumental records and derive, through modern analogs, large-scale SLP, surface temperature and precipitation fields. The skill of the analog method is tested in the virtual world of two three-dimensional climate simulations (ECHO-G and HadCM3). This endeavor offers new possibilities to both constrain climate model into a reconstruction mode (through the assimilation approach) and to better asses documentary data in a quantitative way.     

Aircraft measurements and model simulations of stratospheric ozone and N2O: implications for chemistry and transport processes in the models

Airborne measurements of stratospheric ozone and N₂O from the SCIAMACHY (Scanning Imaging Absorption Spectrometer) Validation and Utilization Experiment (SCIA-VALUE) are presented. The campaign was conducted in September 2002 and February–March 2003. The Airborne Submillimeter Radiometer (ASUR) observed stratospheric constituents like O₃ and N₂O, among others, spanning a latitude from 5°S to 80°N during the survey. The tropical ozone source regions show high ozone volume mixing ratios (VMRs) of around 11 ppmv at 33 km altitude, and the altitude of the maximum VMR increases from the tropics to the Arctic. The N₂O VMRs show the largest value of 325 ppbv in the lower stratosphere, indicating their tropospheric origin, and they decrease with increasing altitude and latitude due to photolysis. The sub-tropical and polar mixing barriers are well represented in the N₂O measurements. The most striking seasonal difference found in the measurements is the large polar descent in February–March. The observed features are interpreted with the help of SLIMCAT and Bremen Chemical Transport Model (CTMB) simulations. The SLIMCAT simulations are in good agreement with the measured O₃ and N₂O values, where the differences are within 1 ppmv for O₃ and 15 ppbv for N₂O. However, the CTMB simulations underestimate the tropical middle stratospheric O₃ (1–1.5 ppmv) and the tropical lower stratospheric N₂O (15–30 ppbv) measurements. A detailed analysis with various measurements and model simulations suggests that the biases in the CTMB simulations are related to its parameterised chemistry schemes.     

Sensitivity of an ecosystem model to hydrology and temperature

We tested the sensitivity of a dynamic ecosystem model (LPJ-GUESS) to the representation of soil moisture and soil temperature and to uncertainties in the prediction of precipitation and air temperature. We linked the ecosystem model with an advanced hydrological model (JULES) and used its soil moisture and soil temperature as input into the ecosystem model. We analysed these sensitivities along a latitudinal gradient in northern Russia. Differences in soil temperature and soil moisture had only little influence on the vegetation carbon fluxes, whereas the soil carbon fluxes were very sensitive to the JULES soil estimations. The sensitivity changed with latitude, showing stronger influence in the more northern grid cell. The sensitivity of modelled responses of both soil carbon fluxes and vegetation carbon fluxes to uncertainties in soil temperature were high, as both soil and vegetation carbon fluxes were strongly impacted. In contrast, uncertainties in the estimation of the amount of precipitation had little influence on the soil or vegetation carbon fluxes. The high sensitivity of soil respiration to soil temperature and moisture suggests that we should strive for a better understanding and representation of soil processes in ecosystem models to improve the reliability of predictions of future ecosystem changes.