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141.
Klaus Pinkau 《Experimental Astronomy》2009,25(1-3):157-171
Gamma-ray astronomy is devoted to study nuclear and elementary particle astrophysics and astronomical objects under extreme conditions of gravitational and electromagnetic forces, and temperature. Because signals from gamma rays below 1 TeV cannot be recorded on ground, observations from space are required. The photoelectric effect is dominant <100 keV, Compton scattering between 100 keV and 10 MeV, and electron–positron pair production at energies above 10 MeV. The sun and some gamma ray burst sources are the strongest gamma ray sources in the sky. For other sources, directionality is obtained by shielding / masks at low energies, by using the directional properties of the Compton effect, or of pair production at high energies. The power of angular resolution is low (fractions of a degree, depending on energy), but the gamma sky is not crowded and sometimes identification of sources is possible by time variation. The gamma ray astronomy time line lists Explorer XI in 1961, and the first discovery of gamma rays from the galactic plane with its successor OSO-3 in 1968. The first solar flare gamma ray lines were seen with OSO-7 in 1972. In the 1980’s, the Solar Maximum Mission observed a multitude of solar gamma ray phenomena for 9 years. Quite unexpectedly, gamma ray bursts were detected by the Vela-satellites in 1967. It was 30 years later, that the extragalactic nature of the gamma ray burst phenomenon was finally established by the Beppo–Sax satellite. Better telescopes were becoming available, by using spark chambers to record pair production at photon energies >30 MeV, and later by Compton telescopes for the 1–10 MeV range. In 1972, SAS-2 began to observe the Milky Way in high energy gamma rays, but, unfortunately, for a very brief observation time only due to a failure of tape recorders. COS-B from 1975 until 1982 with its wire spark chamber, and energy measurement by a total absorption counter, produced the first sky map, recording galactic continuum emission, mainly from interactions of cosmic rays with interstellar matter, and point sources (pulsars and unidentified objects). An integrated attempt at observing the gamma ray sky was launched with the Compton Observatory in 1991 which stayed in orbit for 9 years. This large shuttle-launched satellite carried a wire spark chamber “Energetic Gamma Ray Experiment Telescope” EGRET for energies >30 MeV which included a large Cesium Iodide crystal spectrometer, a “Compton Telescope” COMPTEL for the energy range 1–30 MeV, the gamma ray “Burst and Transient Source Experiment” BATSE, and the “Oriented Scintillation-Spectrometer Experiment” OSSE. The results from the “Compton Observatory” were further enlarged by the SIGMA mission, launched in 1989 with the aim to closely observe the galactic center in gamma rays, and INTEGRAL, launched in 2002. From these missions and their results, the major features of gamma ray astronomy are: Diffuse emission, i.e. interactions of cosmic rays with matter, and matter–antimatter annihilation; it is found, “...that a matter–antimatter symmetric universe is empirically excluded....” Nuclear lines, i.e. solar gamma rays, or lines from radioactive decay (nucleosynthesis), like the 1.809 MeV line of radioactive 26Al; Localized sources, i.e. pulsars, active galactic nuclei, gamma ray burst sources (compact relativistic sources), and unidentified sources. 相似文献
142.
143.
Telm Bover-Arnal Ramon Salas Josep A. Moreno-Bedmar Klaus Bitzer 《Sedimentary Geology》2009,219(1-4):280-301
The attributes of a ‘four-systems-tract’ sequence are at times difficult to identify in outcrop-scale carbonate successions. Poor exposure conditions, variable rates of sediment production, erosion and/or superposition of surfaces that are intrinsic to the nature of carbonate systems frequently conceal or remove its physical features. The late Early–Middle Aptian platform carbonates of the western Maestrat Basin (Iberian Chain, Spain) display facies heterogeneity enabling platform, platform-margin and slope geometries to be identified, and provide a case study that shows all the characteristics of a quintessential four systems tract-based sequence. Five differentiated systems tracts belonging to two distinct depositional sequences can be recognized: the Highstand Systems Tract (HST) and Forced Regressive Wedge Systems Tract (FRWST) of Depositional Sequence A; and the Lowstand Prograding Wedge Systems Tract (LPWST), Transgressive Systems Tract (TST) and subsequent return to a highstand stage of sea-level (HST) of Depositional Sequence B. An extensive carbonate platform of rudists and corals stacked in a prograding pattern marks the first HST. The FRWST is constituted by a detached, slightly cross-bedded calcarenite situated at the toe of the slope in a basinal position. The LPWST is characterized by a small carbonate platform of rudists and corals downlapping over the FRWST and onlapping landwards. The TST exhibits platform backstepping and marly sedimentation. Resumed carbonate production in shelf and slope settings characterizes the second HST. A basal surface of forced regression, a subaerial unconformity, a correlative conformity, a transgressive surface and a maximum flooding surface bound these systems tracts, and are well documented and widely mappable across the platform-to-basin transition area analyzed. Moreover, the sedimentary succession studied is made up of four types of parasequence that constitute stratigraphic units deposited within a higher-frequency sea-level cyclicity. Ten lithofacies associations form these basic accretional units. Each facies assemblage can be ascribed to an inferred depositional environment in terms of bathymetry, hydrodynamic conditions and trophic level. The architecture of the carbonate platform systems reflects a flat-topped non-rimmed depositional profile. Furthermore, these carbonate shelves are interpreted as having been formed in low hydrodynamic conditions. The long-term relative fall in sea-level occurred during the uppermost Early Aptian, which subaerially exposed the carbonate platform established during the first HST and resulted in the deposition of the FRWST, is interpreted as one of global significance. Moreover, a possible relationship between this widespread sea-level drop and glacio-eustasy seems plausible, and could be linked to the cooling event proposed in the literature for the late Early Aptian. Because of the important implications in sequence stratigraphy of this study, the sedimentary succession analyzed herein could serve as an analogue for the application of the four-systems-tract sequence stratigraphic methodology to carbonate systems. 相似文献
144.
Catherine Heymans Meghan E. Gray Chien Y. Peng Ludovic Van Waerbeke Eric F. Bell Christian Wolf David Bacon Michael Balogh Fabio D. Barazza Marco Barden Asmus Böhm John A. R. Caldwell Boris Häußler Knud Jahnke Shardha Jogee Eelco van Kampen Kyle Lane Daniel H. McIntosh Klaus Meisenheimer Yannick Mellier Sebastian F. Sánchez y N. Taylor Lutz Wisotzki Xianzhong Zheng 《Monthly notices of the Royal Astronomical Society》2008,385(3):1431-1442
We present a high-resolution dark matter reconstruction of the z = 0.165 Abell 901/902 supercluster from a weak lensing analysis of the Hubble Space Telescope STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901α. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'. 相似文献
145.
This study investigates future changes of the Arctic climate by the end of the 21st century, simulated by the regional climate model HIRHAM forced with the ECHAM5/MPI-OM general circulation model and assuming the SRES A1B emission scenario. This assessment provides the regional patterns of future circulation, temperature, and precipitation in the Arctic by the end of the 21st century. The magnitude of winter and summer temperature and precipitation is projected to increase, while their interannual variability is projected to change seasonally and is regionally dependent. The regional-scale response of the temperature and precipitation is associated with changes in storm tracks and atmospheric baroclinicity. During winter, the regions of strongest baroclinicity are shifted northward and strengthened. Changes in the seasonal temperature and precipitation are accompanied by changes in their extremes. Extreme warm and cold events are significantly projected to change, with relative changes of seasonal precipitation being larger than those of precipitation extremes. 相似文献
146.
Chelsea L. Pederson James S. Klaus Peter K. Swart Donald F. Mcneill 《Sedimentology》2019,66(6):1989-2010
Microbial carbonate mud in the modern (<4 kyr) palustrine environment of the Florida Everglades is surprisingly susceptible to early diagenetic alteration. The low‐Mg calcite crystals show an open, trellis‐like crystal structure that yields a high surface area to volume ratio. This textural complexity, likely to be a result of organic influence, leads to abundant reaction sites vulnerable to both dissolution and subsequent precipitation. Rapid degradation of organics is attributed to both aerobic and anaerobic metabolisms. Geochemical signatures suggest increased denitrification within the freshwater environment and increased sulphate reduction at the (slightly) brackish mangrove transition zone. A transition from a freshwater to brackish depositional environment is likely to follow the Holocene sea‐level transgression. The textural complexity in these microbial carbonates causes an unexpectedly low preservation potential of original textural and geochemical signatures, even in low‐Mg calcite. Given the potential for early diagenesis of palustrine and perhaps other microbial carbonates, they should be used cautiously as archives for palaeoenvironmental proxies. 相似文献
147.
Stable isotopes and trace elements in modern ostracod shells: implications for reconstructing past environments on the Tibetan Plateau,China 总被引:1,自引:0,他引:1
Nicole Börner Bart De Baere Lailah Gifty Akita Roger Francois Klaus Peter Jochum Peter Frenzel Liping Zhu Antje Schwalb 《Journal of Paleolimnology》2017,58(2):191-211
Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body \( \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) \), reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau. 相似文献
148.
149.
Abstract— We have studied a unique impact-melt rock, the Ramsdorf L chondrite, using optical and scanning microscopy and electron microprobe analysis. Ramsdorf contains not only clast-poor impact melt (Begemann and Wlotzka, 1969) but also a chondritic portion (>60 g) with what appears at low magnification to be a normal, well-defined chondritic texture. However, detailed studies at high magnification show that >90 vol% of the crystals in the chondritic portion were largely melted by the impact: the chondrules lack normal microtextures and are ghosts of the original features. The only relics from the precursor chondrules are olivine crystals, which have the highest melting temperature (~1620 °C). Pyroxene-rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine-rich melts. Fine-grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts, probably because pyroxene-rich melts are significantly more viscous. Complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of Ramsdorf formed directly from petrologic type 3–4 material by strong shock. We infer that Ramsdorf was largely melted by shock pressures of ~75–90 GPa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling. Quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes. Ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture. 相似文献
150.
Abstract— Four asteroidal bodies (the Martian satellites Phobos and Deimos and the main-belt asteroids 243 Ida and 253 Mathilde) have now been the subjects of sufficiently close encounters by spacecraft that the masses and sizes and, hence, the densities of these bodies can be estimated to ~10%. All of these asteroids are significantly less dense than most members of the classes of meteorites identified as being compositionally most nearly similar to them on the basis of spectral characteristics. We show that two processes can act, independently or in concert, during the evolutionary histories of asteroids to produce a low bulk density. One of these processes is the result of one or more impact events and can affect any asteroid type, whereas the other can occur only for certain types of small asteroids that have undergone aqueous alteration. 相似文献