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231.
K. S. Krishna Swamy 《Astrophysics and Space Science》1970,8(1):86-89
Two hyades giant stars, and Tau, have been studied from an analysis of strong line profiles. We get for Tau,T
e
=4750K and logg=2.7, and for Tau,T
e
=4700K and logg=2.8. Hydrogen-to-metal ratio for the two stars is nearly the same as that of solar value. 相似文献
232.
PRABHAKAR S. NAIDU 《Geophysical Prospecting》1970,18(Z1):774-785
A point source or a dipole source of electric current, placed on a randomly layered semi-infinite medium, produces an inhomogeneous random potential field on the surface. The variance of the random potential, normalized with reference to the normal field (that is, a field due to a point source or a dipole source on a homogeneous medium), falls off inversely as the distance from the source. The conductivity of the layers is assumed to vary randomly about a mean value (unity) such that the variations can be represented by a zero mean homogeneous random function. Further the variations are assumed to be small compared to the mean so that the first order perturbation is adequate. The analysis shows that the dipole field is more sensitive than the single pole field to the conductivity variations. 相似文献
233.
The OSIRIS‐REx target asteroid (101955) Bennu: Constraints on its physical,geological, and dynamical nature from astronomical observations 下载免费PDF全文
D. S. Lauretta A. E. Bartels M. A. Barucci E. B. Bierhaus R. P. Binzel W. F. Bottke H. Campins S. R. Chesley B. C. Clark B. E. Clark E. A. Cloutis H. C. Connolly M. K. Crombie M. Delbó J. P. Dworkin J. P. Emery D. P. Glavin V. E. Hamilton C. W. Hergenrother C. L. Johnson L. P. Keller P. Michel M. C. Nolan S. A. Sandford D. J. Scheeres A. A. Simon B. M. Sutter D. Vokrouhlický K. J. Walsh 《Meteoritics & planetary science》2015,50(4):834-849
We review the results of an extensive campaign to determine the physical, geological, and dynamical properties of asteroid (101955) Bennu. This investigation provides information on the orbit, shape, mass, rotation state, radar response, photometric, spectroscopic, thermal, regolith, and environmental properties of Bennu. We combine these data with cosmochemical and dynamical models to develop a hypothetical timeline for Bennu's formation and evolution. We infer that Bennu is an ancient object that has witnessed over 4.5 Gyr of solar system history. Its chemistry and mineralogy were established within the first 10 Myr of the solar system. It likely originated as a discrete asteroid in the inner Main Belt approximately 0.7–2 Gyr ago as a fragment from the catastrophic disruption of a large (approximately 100‐km), carbonaceous asteroid. It was delivered to near‐Earth space via a combination of Yarkovsky‐induced drift and interaction with giant‐planet resonances. During its journey, YORP processes and planetary close encounters modified Bennu's spin state, potentially reshaping and resurfacing the asteroid. We also review work on Bennu's future dynamical evolution and constrain its ultimate fate. It is one of the most Potentially Hazardous Asteroids with an approximately 1‐in‐2700 chance of impacting the Earth in the late 22nd century. It will most likely end its dynamical life by falling into the Sun. The highest probability for a planetary impact is with Venus, followed by the Earth. There is a chance that Bennu will be ejected from the inner solar system after a close encounter with Jupiter. OSIRIS‐REx will return samples from the surface of this intriguing asteroid in September 2023. 相似文献
234.
Impact of Earth radiation pressure on GPS position estimates 总被引:2,自引:8,他引:2
C. J. Rodriguez-Solano U. Hugentobler P. Steigenberger S. Lutz 《Journal of Geodesy》2012,86(5):309-317
GPS satellite orbits available from the International GNSS Service (IGS) show a consistent radial bias of up to several cm
and a particular pattern in the Satellite Laser Ranging (SLR) residuals, which are suggested to be related to radiation pressure
mismodeling. In addition, orbit-related frequencies were identified in geodetic time series such as apparent geocenter motion
and station displacements derived from GPS tracking data. A potential solution to these discrepancies is the inclusion of
Earth radiation pressure (visible and infrared) modeling in the orbit determination process. This is currently not yet considered
by all analysis centers contributing to the IGS final orbits. The acceleration, accounting for Earth radiation and satellite
models, is introduced in this paper in the computation of a global GPS network (around 200 IGS sites) adopting the analysis
strategies from the Center for Orbit Determination in Europe (CODE). Two solutions covering 9 years (2000–2008) with and without
Earth radiation pressure were computed and form the basis for this study. In previous studies, it has been shown that Earth
radiation pressure has a non-negligible effect on the GPS orbits, mainly in the radial component. In this paper, the effect
on the along-track and cross-track components is studied in more detail. Also in this paper, it is shown that Earth radiation
pressure leads to a change in the estimates of GPS ground station positions, which is systematic over large regions of the
Earth. This observed “deformation” of the Earth is towards North–South and with large scale patterns that repeat six times
per GPS draconitic year (350 days), reaching a magnitude of up to 1 mm. The impact of Earth radiation pressure on the geocenter
and length of day estimates was also investigated, but the effect is found to be less significant as compared to the orbits
and position estimates. 相似文献
235.
Tidal Computations for Morecambe Bay 总被引:9,自引:0,他引:9
236.
Emma?S.?Msaky Daniel?Livingstone Owen?K.?DavisEmail author 《Journal of Paleolimnology》2005,34(1):73-83
Pollen spectra from seven short cores taken from deltaic sites in the central and northern parts of Lake Tanganyika provide information about vegetation changes around the lake during the last 5000 years. Pollen analysis was undertaken to understand the history and timing of catchment deforestation and its causal linkage to excess sedimentation and ecosystem change in Lake Tanganyika. The spectra are dominated by grass pollen at all levels in every core. Grass pollen percentage values range between 40 and 80%. Low values of arboreal pollen taxa (1–20%) were documented from most cores except core LT-98-2M. Core LT-98-2M represents the longest duration vegetation record of this study (close to 5000 years BP), and records the onset of increasingly arid conditions in the Late Holocene, especially after 500 A.D., with the probable replacement of forest by open grassland in the Mahale Mountains region. The pollen/spore content for other cores showed a consistent trend of a decrease in grass pollen and an increase in pteridophyte and forest indicator pollen taxa during the last few centuries, contemporaneous with other indications of increased watershed disturbance from forest clearing (especially isotopes and lake faunal change). The timing or strength of this trend is not tied to specific levels of watershed disturbance. However, increasing fern spore abundance does occur progressively later towards the south, where modern human population densities are lower. Although increasing fern spore abundance is consistent with a land-clearing hypothesis, the rising arboreal pollen percentages are seemingly counterintuitive. One possible explanation is that increasing arboreal pollen proportions reflects the recycling of abundant pollen of this type from rapidly eroding soils. Another likely explanation for this finding is that land clearing involved the replacement of miombo woodland, with its mixture of trees producing little pollen and understory grasses producing large amounts of pollen, by the present day cassava, banana, and legume agricultural systems, all of which are poor pollen producers. This shift in catchment vegetation would increase the relative contribution of Afromontane pollen transported long distances from the surrounding highland regions. This hypothesis is consistent with both the lack of correlation of palynological history with specific watershed deforestation attributes, as well as the broader historical record of human habitation in the Lake Tanganyika region. This study also highlights the need for both modern pollen transect data from the region and comparative cores from low elevation swamps or ponds (wetlands) in the region with smaller catchment areas. 相似文献
237.
238.
R.D. Oudmaijer M.E. van den Ancker D. Baines P. Caselli J.E. Drew M.G. Hoare S.L. Lumsden B. Montesinos S. Sim J.S. Vink H.E. Wheelwright W.J. de Wit 《Astronomische Nachrichten》2011,332(3):238-241
The Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
239.
Summary. Lg -wave observations at NORSAR from a sequence of explosions straddling the Central Graben in the North Sea, show that the phase is almost extinguished by passing through this structure. For models of the graben structure, based on seismic refraction experiments, with crustal thinning beneath a zone of thickened sediments, numerical modelling of Lg propagation shows very poor transmission. The inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity. The theoretical effect of a crustal pinch is to redistribute S energy over a wide range of groups velocities, a pattern that fits well with the observations.
Three components recordings at NORSAR of explosive charges fired in water covering a wide range of azimuths, show significant transverse components to the seismograms. The transverse energy builds up with travel time and for Lg is of the same order as the vertical component. These results are consistent with the theoretical prediction of progressive net transfer of energy from Rayleigh to Love modes as the wavetrain propagates through a three-dimensionally heterogeneous medium. 相似文献
Three components recordings at NORSAR of explosive charges fired in water covering a wide range of azimuths, show significant transverse components to the seismograms. The transverse energy builds up with travel time and for Lg is of the same order as the vertical component. These results are consistent with the theoretical prediction of progressive net transfer of energy from Rayleigh to Love modes as the wavetrain propagates through a three-dimensionally heterogeneous medium. 相似文献
240.
Europa's surface is chemically altered by radiolysis from energetic charged particle bombardment. It has been suggested that hydrated sulfuric acid (H2SO4·nH2O) is a major surface species and is part of a radiolytic sulfur cycle, where a dynamic equilibrium exists between continuous production and destruction of sulfur polymers Sx, sulfur dioxide SO2, hydrogen sulfide H2S, and H2SO4·nH2O. We measured the rate of sulfate anion production for cyclo-octal sulfur grains in frozen water at temperatures, energies, and dose rates appropriate for Europa using energetic electrons. The measured rate is GMixture(SO42−)=fSulfur (r0/r)βG1 molecules (100 eV)−1, where fSulfur is the sulfur weight fraction, r is the grain radius, r0=50 μm, β≈1.9, and G1=0.4±0.1. Equilibrium column densities N are derived for Europa's surface and follow the ordering N(H2SO4) » N(S)>N(SO2)>N(H2S). The lifetime of a sulfur atom on Europa's surface for radiolysis to H2SO4 is τ(−S)=120(r/r0)β years. Rapid radiolytic processing hides the identity of the original source of the sulfurous material, but Iogenic plasma ion implantation and an acidic or salty ocean are candidate sources. Sulfate salts, if present, would be decomposed in <3800 years and be rapidly assimilated into the sulfur cycle. 相似文献