共查询到20条相似文献，搜索用时 500 毫秒
1.
金属丰度是表征星系特征的重要参量,对于理解星系的形成与演化具有重要意义。随着观测设备的不断完善与发展,已经获得越来越多的星系(包括近邻星系、中等红移及高红移星系)金属丰度的观测资料。在光致电离模型方面也取得了很大进展．这些对于理解星系中金属及恒星成分的累积历史有非常重要的作用。有多种方法可以用来估计星系的金属丰度(主要是星际介质中气相的O丰度),介绍和评述了估计星系的O丰度的多种方法,包括电子温度的方法、R23方法以及其他一些“强线”比值的方法,并给出了相应的解析公式,同时介绍了电离星云的物理本质及由光致电离模型估计星系金属丰度的方法,还给出了估计星系的C和N元素丰度的方法。 相似文献
2.
In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared. 相似文献
3.
The application of biomarker climate proxies to lacustrine settings requires a thorough understanding of those processes that give rise to and mediate in the burial and preservation of organic matter in the sediments. This information is to date missing for Lake Baikal in Central Asia. The biogeochemistry of the lake cannot be considered analogous to that in other lacustrine environments given its enormous size, depth, remote location and unique biology. The aim of this paper is to report on the main compound classes in the water column and sediments. As part of an ongoing evaluation of the inputs and fluxes of organic matter in Lake Baikal, we have found that there are significant differences in the inputs and preservation of organic matter between the North and South Basins. Both basins have dominant algal input and a contribution from the terrestrial plants of the lake edges and surrounding environments. However, the concentration of organic material in the sediment traps and sediments of the South Basin is much higher than that of the North Basin. In addition, the South Basin contains much higher proportions of the more labile material than does the North Basin. This is likely due to the South Basin being free of surface ice for a much longer time than the North, and in consequence, primary producers having an overall longer productive season. There is some evidence of microbial activity in the sediment traps and sediments from both the North and South Basins, but it does not appear to be more predominant in one basin than the other. It is probable that the differences in the composition and concentration of biomarkers in sediments between basins is due to the length of and the intensity of the productive season, which is in turn influenced by the climate. 相似文献
4.
The evolution of the galaxy mass function MF (and luminosity function LF) depending on merging of galaxies is discussed. The
richness and masses of most massive (and most luminous) galaxies of a cluster are chosen as a characteristic of the bright
edge of LF. Mergers are simulated by a Monte-Carlo method. The probability of merging depends on the masses of galaxies. The
ratio of the current number of galaxies to the initial one plays the role of the time scale. Transformation to real time and
redshift is realized with help of the Smoluchowsky kinetic equation (SE) solution, describing the merging process and making
possible to find the dependence of the galaxy number on time. The dependencies of richness, masses and magnitude differences
of most massive and brightest galaxies of a cluster on redshift have been obtained. Creation of cD-galaxy has been retraced
on small redshifts. The masses of the second and less massive cluster galaxies grow, but after the creation of the cD-galaxy
they begin to decrease. Comparison of obtained results with data of the catalogues is provided. Correspondence in mentioned
dependencies is observed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
A pulsar has the very stable rotation and can be used as the time standard. The astrometric parameters and astrophysical parameters of many pulsars, such as the spatial position, proper motion, distance, rotation period and its derivative, etc., can be all accurately determined. Since the pulsar can provide the time signal and the coordinates of its spatial position simultaneously, the pulsar navigation system installed on a spacecraft enables the autonomous navigation of the spacecraft to be realized. Firstly, the position of the spacecraft is predicted based on the equation of orbit dynamics of the spacecraft and then the Kalman filtering is applied to calculating the estimation error of the spacecraft position through the difference between the pulse arrival time observed on the spacecraft and the predicted pulse arrival time, thereby modifying the position of the spacecraft. Finally, the effects of the initial error, measuring accuracy of the pulse arrival time and number of pulsars on the navigation accuracy are analyzed. 相似文献
6.
白矮星演化与宇宙纪年学 总被引：2，自引：0，他引：2
近几年白矮星研究在理论与观测方面都取得了很大进展，这推动了白矮星宇宙纪年学的应用和发展。白矮星宇宙纪年学，即利用白矮星的演化结果计算白矮星理论光度函数，并通过与现测光度函数的对比来确定天体年龄。简要描述了白矮星的基本性质，回顾了白矮星演化研究的历史和基本方法；介绍了白矮星宇宙纪年学研究的基本原理及其在银盘、球状星团、疏散星团以及银晕年龄确定方面的应用现状。白矮星宇宙纪年学是一个很新的研究领域，但它已经在确定天体年龄方面显示出很大的潜力。最后讨论了目前研究中存在的问题，并提出今后需要进一步研究的几项工作。 相似文献
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The surface area of Venus (∼460×106 km2) is ∼90% of that of the Earth. Using Magellan radar image and altimetry data, supplemented by Venera-15/16 radar images, we compiled a global geologic map of Venus at a scale of 1:10 M. We outline the history of geological mapping of the Earth and planets to illustrate the importance of utilizing the dual stratigraphic classification approach to geological mapping. Using this established approach, we identify 13 distinctive units on the surface of Venus and a series of structures and related features. We present the history and evolution of the definition and characterization of these units, explore and assess alternate methods and approaches that have been suggested, and trace the sequence of mapping from small areas to regional and global scales. We outline the specific defining nature and characteristics of these units, map their distribution, and assess their stratigraphic relationships. On the basis of these data, we then compare local and regional stratigraphic columns and compile a global stratigraphic column, defining rock-stratigraphic units, time-stratigraphic units, and geological time units. We use superposed craters, stratigraphic relationships and impact crater parabola degradation to assess the geologic time represented by the global stratigraphic column. Using the characteristics of these units, we interpret the geological processes that were responsible for their formation. On the basis of unit superposition and stratigraphic relationships, we interpret the sequence of events and processes recorded in the global stratigraphic column. The earliest part of the history of Venus (Pre-Fortunian) predates the observed surface geological features and units, although remnants may exist in the form of deformed rocks and minerals. We find that the observable geological history of Venus can be subdivided into three distinctive phases. The earlier phase (Fortunian Period, its lower stratigraphic boundary cannot be determined with the available data sets) involved intense deformation and building of regions of thicker crust (tessera). This was followed by the Guineverian Period. Distributed deformed plains, mountain belts, and regional interconnected groove belts characterize the first part and the vast majority of coronae began to form during this time. The second part of the Guineverian Period involved global emplacement of vast and mildly deformed plains of volcanic origin. A period of global wrinkle ridge formation largely followed the emplacement of these plains. The third phase (Atlian Period) involved the formation of prominent rift zones and fields of lava flows unmodified by wrinkle ridges that are often associated with large shield volcanoes and, in places, with earlier-formed coronae. Atlian volcanism may continue to the present. About 70% of the exposed surface of Venus was resurfaced during the Guineverian Period and only about 16% during the Atlian Period. Estimates of model absolute ages suggest that the Atlian Period was about twice as long as the Guineverian and, thus, characterized by significantly reduced rates of volcanism and tectonism. The three major phases of activity documented in the global stratigraphy and geological map, and their interpreted temporal relations, provide a basis for assessing the geodynamical processes operating earlier in Venus history that led to the preserved record. 相似文献
10.
Yehouda Enzel Rivka Amit Uri Dayan Onn Crouvi Ron Kahana Baruch Ziv David Sharon 《Global and Planetary Change》2008,60(3-4):165-192
Modern-day synoptic-scale eastern Mediterranean climatology provides a useful context to synthesize the diverse late Pleistocene (60–12 ka) paleohydrologic and paleoenvironmental indicators of past climatic conditions in the Levant and the deserts to its south and east. We first critically evaluate, extract, and summarize paleoenvironmental and paleohydrologic records. Then, we propose a framework of eastern Mediterranean atmospheric circulation features interacting with the morphology and location of the southeast Mediterranean coast. Together they strongly control the spatial distribution of rainfall and wind pattern. This cyclone–physiography interaction enforces the observed rainfall patterns by hampering rainfall generation south and southeast of the latitude of the north Sinai coast, currently at 31°15′.The proposed framework explains the much-increased rains in Lebanon and northern Israel and Jordan as deduced from pollen, rise and maintenance of Lake Lisan, and speleothem formation in areas currently arid and semiarid. The proposed framework also accounts for the southward and eastward transition into semiarid, arid, and hyperarid deserts as expressed in thick loess accumulation at the deserts' margins, dune migration from west to east in the Sinai and the western Negev, and the formation of hyperarid (< 80 mm yr− 1) gypsic–salic soils in the southern Negev and Sinai. Our climatic synthesis explains the hyperarid condition in the southern Negev, located only 200–250 km south of the much-increased rains in the north, probably reflecting a steeper rainfall gradient than the present-day gradient from the wetter Levant into its bordering southern and eastern deserts.At present, the rainiest winter seasons in Lebanon and northern and central Israel are associated with more frequent (+ 20%), deeper Cyprus Lows traversing the eastern Mediterranean at approximately the latitude of southern Turkey. Even these wettest years in northern Israel do not yield above average annual rainfall amounts in the hyperarid southern Negev. This region is mainly influenced by the Active Red Sea Troughs that produce only localized rains. The eastern Mediterranean Cyprus Lows also produce more dust storms and transport higher amounts of suspended dust to the loess area than any other atmospheric pattern. Concurrent rainfall and dust are essential to the late Pleistocene formation of the elongated thick loess zone along the desert northern margin. Even with existing dust storms, the lack of rain and very sparse vegetation account for the absence of late Pleistocene loess sequences from the southern Negev and the formation of hyperarid soils.When the north Sinai coast shifted 30–70 km northwest due to last glacial global sea level lowering, the newly exposed coastal areas supplied the sand and dust to these active eastern Mediterranean cyclones. This enforced the latitude of the northern boundary of the loess zone to be directly due east of the LGM shoreline. This shift of coast to the northwest inhibited rainfall in the southern Levant deserts and maintained their hyperaridity. Concurrently, frequent deep eastern Mediterranean Cyprus Lows were funneled along the northern Mediterranean increasing (probably doubling) the rains in central and northern Israel, Lebanon, southwestern Syria and northern Jordan. These storms and rains formed lakes, forests, and speleothems only a short distance north of the deserts in the southern Levant. 相似文献
11.
20世纪人类在探索宇宙奥秘的道路上取得了辉煌的成就 .其主要标志是恒星的内部结构与演化理论和宇宙大爆炸标准模型的建立。这一理论框架为宇宙中一切物质 ,包括基本粒子 ,原子 ,分子 ,行星 ,恒星 ,生命的产生和发展提供统一的科学图象 ,受到越来越丰富的观测的支持。然而在大爆炸宇宙模型和恒星演化理论之间存在一个人们了解甚少的领域 :就是由大量恒星组成而又作为宇宙基本单元的星系是如何形成和演化的 ?回答这一重要问题无疑是对 2 1世纪天文学的重大挑战。世界发达国家正在投巨资建造新一代巨型天文设备 ,如各个波段的地面的和空间的望远镜 ,把人类探测能力延伸到宇宙演化的早期。中国也己确定星系形成与演化作为我国天文学科发展的国家目标之一 ,在国家重大科学工程中安排大天区面积多目标光纤光谱天文望远镜———LAMOST以增加我国在这一重大领域的竞争力。“星系形成和演化”项目的目标 ,就是要在LAMOST建成之前 ,充分利用国内中小设备和国外大型设备资料 ,开展大样本统计 ,数值模拟和理论研究 ,提高我国在这一领域的开拓创新的能力 ,形成在国际上有竞争力的队伍 ,在若干优选的课题方向上先期做出有高显示度的成果 ,以期在LAMOST建成之后 ,我国在宇宙大尺度结构 ,星系形成与演化方面跻身国际先 相似文献
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时间频率量的特征及其对时频系统建设的影响 总被引：2，自引：1，他引：1
分析了物理量测量中时间频率量的特点，主要有：时间的流逝性；其基准是自然基准；时间和频率既密切相关又有区别；时间频率具有最高的测量精细度(分辨率)与准确度；其计量标准可通过电磁波发播；其测量精确度与测量时间有关。另外，从基准、守时、授时、时间频率设备的研制、生产和队伍建设等方面阐发了这些特点对时间频率系统建设的影响。 相似文献
14.
As shown by a series of previous researches, on the solar photospheric surface, together with the intensi?cation of longitudinal magnetic ?eld, the ob- served energy of seismic waves decreases. In order to further investigate the relation between seismic waves and magnetic ?elds, using the data of Helioseis- mic and Magnetic Imager (HMI) and via the extrapolation of the potential ?eld from the line-of-sight magnetic ?eld, we can get the total intensity of magnetic ?eld and correlate it with the energy of seismic waves. By ?tting the energies of p-mode seismic waves within a certain region of total magnetic ?eld, the relation between the energy of seismic waves and the total intensity of magnetic ?eld can be obtained. Our study has con?rmed that between the energy of seismic waves and the total intensity of magnetic ?eld there is a stronger correlation. As shown by the results of various active regions, the tendencies of decrease of the energy of p-mode seismic waves with the total intensity of magnetic ?eld below 500 GS are very close to each other, but in the range from 500 GS to 1000 GS there are small differences. Besides, between the logarithm of the energy of p-mode seis- mic waves and the total intensity of magnetic ?eld there is a rather strong linear correlation. As revealed by comparing the results of Michelson Doppler Imager (MDI) and HMI, although in totality the observational results of magnetic ?elds of MDI are larger than those of HMI, but the results of comparison and analysis on the relation between the normalized energy of p-mode seismic waves and the intensity of magnetic ?eld do not differ much in these two cases. 相似文献
15.
Recent studies have pointed out that persistence of the atmospheric circulation over Europe, as measured by residence times of circulation types, has increased since the mid-1980s in all seasons and for most groups of the types. The greater persistence may affect surface climatic anomalies, particularly the frequency and severity of heat and cold waves associated with severe impacts on society and environment. In this paper, relationships between the persistence of circulation types over Europe and extreme surface air temperature anomalies are studied over the 20th century using the Hess–Brezowsky catalogue of large-scale circulation patterns and long-term temperature series at stations covering most of the European continent. Types significantly conducive to heat and cold waves are identified, and temperature anomalies are linked to their persistence. It is shown that more persistent circulation enhances the severity of temperature extremes over the whole area, which is slightly more important for warm than cold temperature anomalies. The changes in both frequencies and residence times of circulation patterns have been supporting sharply rising trends in warm temperature extremes observed over Europe in recent decades, and the circulation changes may also contributed to the fact that trends in cold temperature extremes have been less pronounced or absent in the same period. The findings also emphasize the need for taking into account the persistence of circulation types together with their frequencies when evaluating links between the atmospheric circulation and the surface climate. In global warming context, the effects of the future climate change on the occurrence and severity of temperature extremes may be exacerbated by a more persistent circulation related to a decreased cyclone activity over mid-latitudes and a northward shift of storm tracks. 相似文献
16.
An Improved Grey Model and Its Application Research on the Prediction of Real-time GPS Satellite Clock Errors 总被引：2，自引：0，他引：2
In the work on the real-time GPS precise point positioning, the realtime and reliable prediction of the satellite clock error is one of the keys to the realization of the GPS real-time high accuracy point positioning. The satelliteborne GPS atomic clock has high frequency, is very sensitive and extremely easy to be influenced by the outside world and its own factors. Therefore, it is very difficult for one to know well its complicated and detailed law of change, with these attributes being in accordance with the characteristics of the theory of grey system. Thus, it is considered that the process of variation of the clock error is regarded as a grey system. On the basis of the exploration of the limitations of the quadratic polynomial and grey model satellite clock error predictions, the research on the real-time prediction of the GPS satellite clock error by taking advantage of the improved grey model is proposed. Finally, the materials of the GPS satellite clock error of 3 different time intervals are used to make the accuracy analysis of the clock error prediction of different sampling intervals, to study the relation between the grey model exponential coefficient and the prediction accuracy and to make the analysis of the comparison of the prediction accuracy with that of the quadratic polynomial method. The general relation between the different types of satellite clock errors and the model exponential coefficients is summarized and compared with the IGS final clock error ephemeris product to test and verify the feasibility and availability of the improved prediction model proposed in the present article so as to provide the higher-accuracy satellite clock error products for the real-time GPS dynamic precise point positioning. 相似文献
17.
Several long sediment cores (max. 12 m) from various parts (up to 150 km apart) of Lake Onega, Russian Karelia, have been studied for lithology, varve chronology and palaeomagnetism. The two longest varve records from the central basin contain 1300 varves. These indicate the length of the deglaciation period from these localities to the north of Lake Onega, where the drainage of glacial meltwaters was directed towards the White Sea and the deposition of varves in the Onega basin terminated. An estimate of the duration of deglaciation of the whole Onega basin is 1500 years. Natural remanent magnetization (NRM) is strong and stable in these sediments and accurately records changes in the Earth's magnetic field. A distinct change in the magnetic field, when the declination shifted from east to west by at least 60° in 350 varve years, is clearly identifiable in all cores. This palaeomagnetic feature was used for core to core correlation together with other variations in magnetic parameters and widely distributed lithological marker horizons. On the basis of the correlations between the cores and calibration of AMS radiocarbon dates from varves obtained from the northern archipelago of Lake Onega, the age of the westerly declination peak is dated to 13 090 cal. BP and accordingly the deglaciation of the Onega basin took place between 14 250 and 12 750±100 cal. BP. The westerly declination peak was also recognized earlier by Bakhmutov and Zagniy in the Helylä varved clay sequence near Sortavala on the northern shore of Lake Ladoga. Helylä is situated outside the Salpausselkä end moraines and the accumulation of varved clays continued there 1500 years after the declination peak, up until the drainage of the Baltic Ice Lake, which more or less coincides with the ice margin retreat from Salpausselkä II end moraine and the termination of the Younger Dryas event. The date thus arrived at for this event is 11 590±100 cal. BP, close to the recent results from Greenland ice cores and from varved lake sediments and tree rings from Central Europe. It is further suggested that the formation of the major Younger Dryas end moraines, the Finnish Salpausselkä I and Salpausselkä II and their correlatives in Russian Karelia, took place between 12 250 and 11 590 calendar years ago, clearly earlier than earlier estimated through correlation with the Swedish varve chronology. 相似文献
18.
David M.H. Baker James W. Head Samuel C. Schon Carolyn M. Ernst Louise M. Prockter Scott L. Murchie Brett W. Denevi Sean C. Solomon Robert G. Strom 《Planetary and Space Science》2011,59(15):1932-1948
The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet's surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury's peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011. 相似文献
19.
L.A. Pustil'nik 《Astrophysics and Space Science》1997,252(1-2):353-362
A critical analysis of standard accretion models is presented. We consider the stability of models in the theories of disc accretion onto black holes and spherical/disc accretion onto a magnetosphere. We take into account realistic physics processes and geometry (inner magnetic field in the accreted plasma, finite conductivity, finite length of the field lines, finite rotation of the accreted object, and magnetic shear on the boundary between the magnetosphere and accreted plasma). The influence of these factors leads to radical changes of both the accretion as whole and the energy release in the accreting system. Strong current-sheet and Z-pinch-like structures should arise over the polar region of the accreting object. Particle acceleration in the electric fields of current discharges in these regions may be a source of efficient conversion of energy into nonthermal particles and of the emission observed from many accreting objects. 相似文献
20.
随着天文观测数据的爆炸式增长以及数据网格技术等新兴IT技术的日趋成熟,虚拟天文台应运而生。虚拟天文台成为基于数据网格技术的解决复杂海量天文数据的访问和管理的一种有效手段。我们对现有中国虚拟天文台体系结构底层进行设计与实现,并完成了其核心部分一天文数据结点。天文数据结点可发布和管理异构的数据资源,为用户提供统一的数据访问接口,进而为天文学家实现对各种天文数据的无缝透明访问和处理提供了方便。本文重点介绍了我们设计的天文数据结点的体系结构,以及结点各部分的功能,并讨论了其关键技术的实现。 相似文献