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21.
Jakob Wilk Christopher Hamann Agnese Fazio Robert Luther Lutz Hecht Falko Langenhorst Thomas Kenkmann 《Meteoritics & planetary science》2018,53(8):1569-1593
Shatter cones are diagnostic for the recognition of meteorite impact craters. They are unambiguously identifiable in the field and the only macroscopic shock deformation feature. However, the physical boundary conditions and exact formation mechanism(s) are still a subject of debate. Melt films found on shatter cone surfaces may allow the constraint of pressure–temperature conditions during or immediately after their formation. Within the framework of the MEMIN research group, we recovered 24 shatter cone fragments from the ejecta of hypervelocity impact experiments. Here, we focus on silicate melt films (now quenched to glass) found on shatter cone surfaces formed in experiments with 20–80 cm sized sandstone targets, impacted by aluminum and iron meteorite projectiles of 5 and 12 mm diameter at velocities of 7.0 and 4.6 km s−1, respectively. The recovered shatter cone fragments vary in size from 1.2 to 9.3 mm. They show slightly curved, striated surfaces, and conical geometries with apical angles of 36°–52°. The fragments were recovered from experiments with peak pressures ranging from 46 to 86 GPa, and emanated from a zone within 0.38 crater radii. Based on iSale modeling and petrographic investigations, the shatter coned material experienced low bulk shock pressures of 0.5–5 GPa, whereas deformation shows a steep increase toward the shatter cone surface leading to localized melting of the rock, resulting in both vesicular as well as polished melt textures visible under the SEM. Subjacent to the melt films are zones of fragmentation and brittle shear, indicating movement away from the shatter cone apex of the rock that surrounds the cone. Smearing and extension of the melt film indicates subsequent movement in opposite direction to the comminuted and brecciated shear zone. We believe the documented shear textures and the adjacent smooth melt films can be related to frictional melting, whereas the overlying highly vesiculated melt layer could indicate rapid pressure release. From the observation of melting and mixing of quartz, phyllosilicates, and rutile in this overlying texture, we infer high, but very localized postshock temperatures exceeding 2000 °C. The melted upper part of the shatter cone surface cross-cuts the fragmented lower section, and is accompanied by PDFs developed in quartz parallel to the {112} plane. Based on the overprinting textures and documented shock effects, we hypothesize shatter cones start to form during shock loading and remain an active fracture surface until pressure release during unloading and infer that shatter cone surfaces are mixed mode I/II fracture surfaces. 相似文献
22.
Vinay Udyawer Mark A. Read Mark Hamann Colin A. Simpfendorfer Michelle R. Heupel 《Marine pollution bulletin》2013
Entanglement in derelict fishing gear and other marine debris is a major threat to the survival of large marine wildlife like cetaceans, seabirds and sea turtles. However, no previous reports of entanglement or entrapment have been recorded in sea snakes (Hydrophiinae). We report here on a sea snake (Hydrophis elegans) found with a ceramic washer encircling its body captured from the north-east coast of Queensland, Australia. The ring had constricted the body and over time caused extensive damage to the underlying tissues. A post-mortem examination showed the snake was severely emaciated as the ring restricted the passage of food to the stomach and intestine. This is the first record of mortality due to marine debris entrapment in sea snakes. 相似文献
23.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
24.
D. Reckien J. Flacke R. J. Dawson O. Heidrich M. Olazabal A. Foley J. J.-P. Hamann H. Orru M. Salvia S. De Gregorio Hurtado D. Geneletti F. Pietrapertosa 《Climatic change》2014,122(1-2):331-340
Urban areas are pivotal to global adaptation and mitigation efforts. But how do cities actually perform in terms of climate change response? This study sheds light on the state of urban climate change adaptation and mitigation planning across Europe. Europe is an excellent test case given its advanced environmental policies and high urbanization. We performed a detailed analysis of 200 large and medium-sized cities across 11 European countries and analysed the cities’ climate change adaptation and mitigation plans. We investigate the regional distribution of plans, adaptation and mitigation foci and the extent to which planned greenhouse gas (GHG) reductions contribute to national and international climate objectives. To our knowledge, it is the first study of its kind as it does not rely on self-assessment (questionnaires or social surveys). Our results show that 35 % of European cities studied have no dedicated mitigation plan and 72 % have no adaptation plan. No city has an adaptation plan without a mitigation plan. One quarter of the cities have both an adaptation and a mitigation plan and set quantitative GHG reduction targets, but those vary extensively in scope and ambition. Furthermore, we show that if the planned actions within cities are nationally representative the 11 countries investigated would achieve a 37 % reduction in GHG emissions by 2050, translating into a 27 % reduction in GHG emissions for the EU as a whole. However, the actions would often be insufficient to reach national targets and fall short of the 80 % reduction in GHG emissions recommended to avoid global mean temperature rising by 2 °C above pre-industrial levels. 相似文献
25.
Wolf-Rainer Hamann 《Astrophysics and Space Science》1996,238(1):31-42
Wolf-Rayet type spectra of central stars are compared with spectra from Pop. I objects. Non-LTE models for expanding atmospheres are applied for analyzing these spectra quantitatively. 相似文献
26.
Mathur S Green PJ Arav N Brotherton M Crenshaw M deKool M Elvis M Goodrich RW Hamann F Hines DC Kashyap V Korista K Peterson BM Shields JC Shlosman I van Breugel W Voit M 《The Astrophysical journal》2000,535(2):L79-L82
We present results of new ASCA observations of the low-luminosity active galactic nucleus (LLAGN) NGC 4579 obtained on 1998 December 18 and 28, and we report on the detection of variability of an iron K emission line. The X-ray luminosities in the 2-10 keV band for the two observations are nearly identical (LX approximately 2x1041 ergs s(-1)), but they are approximately 35% larger than that measured in 1995 July by Terashima et al. An Fe K emission line is detected at 6.39+/-0.09 keV (source rest frame), which is lower than the line energy 6.73+0.13-0.12 keV in the 1995 observation. If we fit the Fe lines with a blend of two Gaussians centered at 6.39 and 6.73 keV, the intensity of the 6.7 keV line decreases, while the intensity of the 6.4 keV line increases, within an interval of 3.5 yr. This variability rules out thermal plasmas in the host galaxy as the origin of the ionized Fe line in this LLAGN. The detection and variability of the 6.4 keV line indicates that cold matter subtends a large solid angle viewed from the nucleus and that it is located within approximately 1 pc from the nucleus. It could be identified with an optically thick standard accretion disk. If this is the case, a standard accretion disk is present at the Eddington ratio of Lbol/LEdd approximately 2x10-3. A broad disk-line profile is not clearly seen, and the structure of the innermost part of accretion disk remains unclear. 相似文献
27.
Laser‐induced melting experiments: Simulation of short‐term high‐temperature impact processes 
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下载免费PDF全文 This study introduces an experimental approach using direct laser irradiation to simulate the virtually instantaneous melting of target rocks during meteorite impacts. We aim at investigating the melting and mixing processes of projectile (iron meteorite; steel) and target material (sandstone) under idealized conditions. The laser experiments (LE) were able to produce features very similar to those of impactites from meteorite craters and cratering experiments, i.e., formation of lechatelierite, partial to complete melting of sandstone, and injection of projectile droplets into target melts. The target and projectile melts have experienced significant chemical modifications during interaction of these coexisting melts. Emulsion textures, observed within projectile‐contaminated target melts, indicate phase separation of silicate melts with different chemical compositions during quenching. Reaction times of 0.6 to 1.4 s could be derived for element partitioning and phase‐separation processes by measuring time‐depended temperature profiles with a bolometric detector. Our LE allow (i) separate melting at high temperatures to constrain primary melt heterogeneities before mixing of projectile and target, (ii) quantification of element partitioning processes between coexisting projectile and target melts, (iii) determination of cooling rates, and (iv) estimation of reaction times. Moreover, we used a thermodynamic approach to calculate the entropy gain during laser melting. The entropy changes for laser‐melting of sandstone and iron meteorite correspond to shock pressures and particle velocities produced during the impact of an iron projectile striking a quartz target at a minimum impact velocity of ~6 km s?1, inducing peak shock pressures of ~100 GPa in the target. 相似文献
28.
Tracking suitable habitat for tree populations under climate change in western North America 总被引：1，自引：0，他引：1
An important criticism of bioclimate envelope models is that many wide-ranging species consist of locally adapted populations that may all lag behind their optimal climate habitat under climate change, and thus should be modeled separately. Here, we apply a bioclimate envelope model that tracks habitat of individual populations to estimate adaptational lags for 15 wide-ranging forest tree species in western North America. An ensemble classifier modeling approach (RandomForest) was used to spatially project the climate space of tree populations under observed climate trends (1970s to 2000s) and multi-model projections for the 2020s, 2050s and 2080s. We find that, on average, populations already lag behind their optimal climate niche by approximately 130 km in latitude, or 60 m in elevation. For the 2020s we expect an average lag of approximately 310 km in latitude or 140 m in elevation, with the most pronounced geographic lags in the Rocky Mountains and the boreal forest. We show that our results could in principle be applied to guide assisted migration of planting stock in reforestation programs using a general formula where 100 km north shift is equivalent to approximately 44 m upward shift in elevation. However, additional non-climatic factors should be considered when matching reforestation stock to suitable planting environments. 相似文献
29.
Boris Jeremi&#x; Gerik Scheuermann Jan Frey Zhaohui Yang Bernd Hamann Kenneth I. Joy Hans Hagen 《国际地质力学数值与分析法杂志》2002,26(10):925-944
We present a novel technique for visualizing tensors in three dimensional (3D) space. Of particular interest is the visualization of stress tensors resulting from 3D numerical simulations in computational geomechanics. To this end we present three different approaches to visualizing tensors in 3D space, namely hedgehogs, hyperstreamlines and hyperstreamsurfaces. We also present a number of examples related to stress distributions in 3D solids subjected to single and load couples. In addition, we present stress visualizations resulting from single‐pile and pile‐group computations. The main objective of this work is to investigate various techniques for visualizing general Cartesian tensors of rank 2 and it's application to geomechanics problems. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
30.
Kristen Weiss Mark Hamann Michael Kinney Helene Marsh 《Global Environmental Change》2012,22(1):178-188
Few case studies have considered the impact of network structure on the resilience of complex resource management systems that operate over large spatial scales. To help fill this knowledge gap our study examined two types of relational ties—knowledge exchange and policy influence—within a marine wildlife co-management network in Northern Australia. We conducted interviews and follow-up surveys with key-informant stakeholders in dugong and marine turtle management and used these data to perform social network analysis for the dugong and turtle co-management network. The network structure of this marine governance system supports extensive cross-scale information flow, but with a disproportionate amount of top-down policy influence compared with knowledge accumulation, an arrangement that may hinder evidence-based decision making. We developed a typological ‘map’ of stakeholder roles in the network to characterize each stakeholder's contribution of knowledge and ability to influence policy, helping to identify gaps or overlaps in network linkages. Improving communication links between knowledge producers and policy makers is important for evidence based decision making throughout the management network, while addressing overlapping management roles and functions should help decrease conflict in the system. These improvements would increase social-ecological resilience in the management network by providing better protection for marine species while meeting the needs of diverse stakeholders. 相似文献