A study on the long term periodicities among the X‐ray binaries: Two new superorbital periods     

S. zdemir 《Astronomische Nachrichten》2010,331(3):300-303

Long term behaviours of a few selected X‐ray binaries are studied on the basis of RXTE/ASM archive covering a period of ∼13 years of continuous X‐ray data. Two new superorbital periods seem to be revealed for the GX354‐0 and X Persei systems as 8.7 years and 345 days, respectively. Although the X Persei system has two strongest peaks in its power density spectrum at 15.7 years and at 345 days, the latter is much more plausible. The GX354‐0 system also has few additional periodicities appeared in different time intervals (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

A uvbyβ photometric analysis of the Monoceros star‐forming field     

N. Kaltcheva  A. Kuchera  C. Hathaway 《Astronomische Nachrichten》2010,331(4):384-396

This investigation presents a new analysis of the spatial distribution of the bright early‐type stars in the field of Northern Monoceros. A database of all O–B9 stars with available uvbyβ photometry is collated and a homogeneous distance scale is established for the clusters and layers of field stars. We provide revised distances for NGC 2264 and NGC 2244 of 833±38 (s.e.) pc and 1585±60 (s.e.) pc, respectively. We present arguments that there might be substructures in the clusters projected along the line of sight. According to the present sample the classical Mon OB2 association at 1.6 kpc is represented by a relatively compact group at 1.26 kpc in the vicinity of NGC 2244 and a layer of massive stars located between 1.5 and 3 kpc (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Metallicity and kinematical clues to the formation of the Local Group     

R.F.G. Wyse 《Astronomische Nachrichten》2010,331(5):526-534

The kinematics and elemental abundances of resolved stars in the nearby Universe can be used to infer conditions at high redshift, trace how galaxies evolve and constrain the nature of dark matter. This approach is complementary to direct study of systems at high redshift, but I will show that analysis of individual stars allows one to break degeneracies, such as between star formation rate and stellar Initial Mass Function, that complicate the analysis of unresolved, distant galaxies (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Comparisons and connections between mean field dynamo theory and accretion disc theory     

E.G. Blackman 《Astronomische Nachrichten》2010,331(1):101-109

The origin of large scale magnetic fields in astrophysical rotators, and the conversion of gravitational energy into radiation near stars and compact objects via accretion have been subjects of active research for a half century. Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both subjects have benefitted from numerical simulations.However, understanding the key principles and practical modeling of observations warrants testable semi‐analytic mean field theories that distill the essential physics. Mean field dynamo (MFD) theory and alpha‐viscosity accretion disc theory exemplify this pursuit. That the latter is a mean field theory is not always made explicit but the combination of turbulence and global symmetry imply such. The more commonly explicit presentation of assumptions in 20th century textbook MFDT has exposed it to arguably more widespread criticism than incurred by 20th century alpha‐accretion theory despite complementary weaknesses. In the 21st century however, MFDT has experienced a breakthrough with a dynamical saturation theory that consistently agrees with simulations. Such has not yet occurred in accretion disc theory, though progress is emerging. Ironically however, for accretion engines, MFDT and accretion theory are presently two artificially uncoupled pieces of what should be a single coupled theory. Large scale fields and accretion flows are dynamically intertwined because large scale fields likely play a key role in angular momentum transport. I discuss and synthesize aspects of recent progress in MFDT and accretion disc theory to suggest why the two likely conspire in a unified theory (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Supersoft X‐ray sources     

F. Haberl 《Astronomische Nachrichten》2010,331(2):239-240

The workshop “Supersoft X‐ray Sources – New Developments” brought together observers and theoretician to discuss the present status and unsolved problems of supersoft source research. A large part of the workshop was devoted to optical novae and their supersoft state. Large samples of supersoft X‐ray sources were presented from nearby galaxies, as well as extensive monitoring campaigns ofbright individual sources. The theoretical modelling oflight curves and high‐resolution X‐ray spectra are well underway, but details are often not yet understood (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

基于CCD星图的空间目标精确光学定位方法研究     

谌钊  罗成  华卫红 《天文研究与技术》2010,7(3)

通过光学定位方法来确定空间目标的轨道是实现空间目标监测的重要手段之一。介绍一套小型的空间目标光学定位系统,针对该系统获取的CCD星图,提出Top-hat变换和中值滤波相结合的算法去除星图噪声,并提出基于梯度Hough变换的算法完成对CCD星图星点和轨迹的同时精确定位,最后利用星角距的旋转不变性和球面几何原理,实现对空间目标的精确定位。对分辨率为752×580的CCD实拍星图的应用研究结果表明,星图去噪算法和星点、轨迹同时精确定位算法是有效的,且空间目标的定位精度也可达到角秒量级。  相似文献   

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field     

Valérie Malavergne  Michael J. Toplis  John Jones 《Icarus》2010,206(1):199-1852

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3-6 log units below the iron-wüstite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO₂ being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core-mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe-S-Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe-S liquid, poor in Si; (ii) a middle layer of Fe-Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.  相似文献   

Barchan dune asymmetry: Observations from Mars and Earth     

Mary C. Bourke 《Icarus》2010,205(1):183-197

Barchan dune asymmetry refers to the extension of one barchan limb downwind. It is a common dune form on Earth and also occurs on Mars and Titan. A new classification of barchan limbs is presented where three types of limb morphology are identified: linear, kinked and beaded. These, along with other dune-scale morphological signatures, are used to identify three of the causes of barchan asymmetry on Mars: bi-directional winds, dune collision and the influence of inclined topography.The potential for specific dune asymmetric morphologies to indicate aspects of the formative wind regime on planetary surfaces is shown. For example, the placement of dune limbs can indicate the general direction and relative strength of formative oblique winds; an extreme barchan limb length may indicate a long duration oblique wind; a kinked limb may be evidence of the passage of a storm; beaded limbs may represent surface-wave instabilities caused by an increase in wind energy parallel to the dune. A preliminary application of these signatures finds evidence for bi-modal winds on Mars. However, these and other morphological signatures of wind direction and relative strength should be applied to planetary landforms with caution as more than one process (e.g., bi-modal winds and collision) may be operating together or sequentially on the dunefield. In addition, analysis should be undertaken at the dunefield scale and not on individual dunes. Finally, morphological data should be acquired from similar-scale dunes within a dunefield.In addition to bi-modal wind regimes on Mars, the frequent parallel alignment of the extended barchan limb to the dune suggests that dune collision is also an important cause of asymmetry on Mars. Some of the more complex dunefield patterns result from a combination of dune collision, limb extension and merging with downwind dunes.Dune asymmetric form does not inhibit dune migration in the Namib Desert or on Mars. Data from the Namib suggest that dune migration rates are similar for symmetric and asymmetric dunes. Further modeling and field studies are needed to refine our understanding of the potential range of limb and dune morphologies that can result from specific asymmetry causes.  相似文献   

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies     

Joseph S. Levy  David R. Marchant 《Icarus》2010,206(1):229-216

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.  相似文献   

Refractive index measurements of ammonia and hydrocarbon ices at 632.8 nm     

Constantin Romanescu  Ajeeta Khatiwada 《Icarus》2010,205(2):695-701

Optical constants in a broad temperature and wavelength range are important input parameters in radiative transfer models used in studies of planetary atmospheres. In the laboratory, the refractive index values of ices at the HeNe laser wavelength (632.8 nm) are often used to monitor the growth rate and thickness of ice films. In this report we present laboratory measurements determining the refractive index at 632.8 nm of ammonia and hydrocarbon ices in the temperature range 80-100 K. Thin ice films are vapor-deposited on a cryogenically cooled mirror located inside a high-vacuum apparatus. The real component of the refractive index of these ice films is determined by a two-angle interferometric technique. Optical modeling calculations of the transmittance and reflectance through the thin ice films assist in the interpretation of the experimental results. We discuss our results and compare them with other measurements available in the literature. The results reported here are relevant to the spectroscopy of icy objects in the solar system; they are needed to perform laboratory characterization of ices, derive optical constants, and model spectra.  相似文献