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651.
We present new ultraviolet spectra of the hottest known, peculiar white dwarf H1504+65, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. H1504+65 is the hottest known white dwarf (T
eff=200 000 K) and has an atmosphere mainly composed by carbon and oxygen, augmented with high amounts of neon and magnesium.
This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either
a C–O white dwarf or even a O–Ne–Mg white dwarf. In the latter case, this would be the first proof that such white dwarfs
can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious
object. 相似文献
652.
Boris Shustov Mikhail Sachkov Ana I. Gómez?de?Castro Klaus Werner Norbert Kappelmann Alexander Moisheev 《Astrophysics and Space Science》2011,335(1):273-282
Continuous access to the UV domain has been considered of importance to astrophysicists and planetary scientists since the
mid-sixties. However, the future of UV missions for the post-HST era is believed by a significant part of astronomical community
to be less encouraging. We argue that key science problems of the coming years will require further development of UV observational
technologies. Among these hot astrophysical issues are: the search for missing baryons, revealing the nature of astronomical
engines, properties of atmospheres of exoplanets as well as of the planets of the Solar System etc. We give a brief review
of UV-missions both in the past and in the future. We conclude that UV astronomy has a great future but the epoch of very
large and efficient space UV facilities seems to be a prospect for the next decades. As to the current state of the UV instrumentation
we think that this decade will be dominated by the HST and coming World Space Observatory-Ultraviolet (WSO-UV) with a 1.7 m
UV-telescope onboard. The international WSO-UV mission is briefly described. It will allow high resolution/high sensitivity
imaging and high/low resolution spectroscopy from the middle of the decade. 相似文献
653.
654.
A review of seawater intrusion and its management in Australia 总被引:5,自引:3,他引:2
Adrian D. Werner 《Hydrogeology Journal》2010,18(1):281-285
Extended periods of below-average rainfall combined with a rising population density in the Australian coastal margin have led to higher stresses on coastal water resources, and the risk of seawater intrusion has increased. Despite reports of seawater intrusion in the majority of states and evidence that some Australian coastal aquifers are seriously depleted, comprehensive seawater intrusion investigations have only been completed for coastal systems in Queensland and to a lesser degree in Western Australia and South Australia. The degree of assessment appears to be linked to the perceived economic value of the groundwater resource. The most detailed studies include those of the Pioneer Valley and Burnett basins in Queensland, for which conceptual and mathematical models have been developed at the regional scale, and have been used to underpin trigger-level management approaches to protect against further seawater intrusion. Historical responses to seawater intrusion include the establishment of artificial recharge schemes; the most prominent being that of the Lower Burdekin aquifers in Queensland. Recommendations for future solutions include enhanced fit-for-purpose seawater intrusion monitoring, continuing research into investigation methods, and improved knowledge-sharing through education programs and the development of national guidelines for seawater intrusion assessment and management. 相似文献
655.
The emergence of the Metazoa can be dated back to the Neoproterozoic Era which comprises the Cryogenian Period during which two major glaciations occurred, the Sturtian and the Varanger-Marinoan. At that time, the phylum Porifera (sponges) evolved as the first animals and developed a hard skeleton. The two classes of siliceous sponges, the Hexactinellida and the Demospongiae, are already provided with the major genetic repertoire and gene regulatory networks that also exist in modern multicellular animals. Besides these metazoan innovations, the siliceous sponges display one autapomorphic character, silicatein, an enzyme which mediates bio-silica formation. Well preserved siliceous sponge fossils have been excavated from the Cambrian Burgess Shale- and Chengjiang deposits. It is concluded that it was the hard skeleton of the siliceous sponges that contributed to the successful evolution and survival of the Porifera during the last 500 Ma. 相似文献
656.
1IntroductionThe Tibet plateau consists of terranes which were accreted in a southward-younging succession from Late Permian to Mid-Eocene (Dewey etal., 1988; Mattern etal., 1998; Mattern and Schneider, 2000). The Songpan-Ganzi terrane was accreted to the Kunlun during Late Permian along Kunlun-Qin-ling suture. The Qiangtang block was added onto the Songpan-Ganzi terrane at the Jinsha suture dur-ing Late Triassic-Early Jurassic. The Lhasa block became attached to the Qia… 相似文献
657.