Identification of new low-mass members of the Alpha Persei open cluster by ROSAT. II     

C. F. Prosser  S. Randich  T. Simon 《Astronomische Nachrichten》1998,319(4):215-234

The present investigation examines possible optical counterparts to 130 X-ray sources in the region of the α Persei open cluster (d ∽ 170pc, age ∽ 50 Myr) resulting from the analysis of three 22–25 ksec ROSAT PSPC pointings. In the same manner as for 73 X-ray sources from a raster survey in α Per (Prosser & Randich 1998), CCD photometry is employed to obtain magnitudes and colors for stars/objects close to the X-ray positions, with additional echelle and low-dispersion Hα spectra provided for some stars. For almost 60 X-ray sources, an optical counterpart with photometry acceptable for cluster membership is identified, some of which can be excluded from membership on the basis of discrepant radial velocity or X-ray characteristics. On the order of 30 new members or likely members associated with X-ray sources have been identified based on available data. A photometric rotation period has been obtained for one rapid rotator identified in X-rays.  相似文献   

The structure and sedimentology of relict talus,Trotternish, northern Skye,Scotland     

Simon Hinchliffe  Colin K. Ballantyne  John Walden 《地球表面变化过程与地形》1998,23(6):545-560

Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.  相似文献   

A method to determine warm and cool steppe biomes from pollen data; application to the Mediterranean and Kazakhstan regions     

Pavel E. Tarasov  Rachid Cheddadi  Joel Guiot  Sytze Bottema  Odile Peyron  Jordina Belmonte  Vittoria Ruiz-Sanchez  Fatima Saadi  Simon Brewer 《第四纪科学杂志》1998,13(4):335-344

An objective method for the assignment of pollen spectra to appropriate biomes has been published recently. The aim of this paper is to improve the distinction between warm and cool steppes, thus refining vegetation and climate reconstruction, particularly during the Last Glacial Maximum. A set of modern pollen spectra from the Mediterranean and Kazakhstan regions, dominated today by open vegetation types, has been analysed statistically in order to relate pollen taxa abundances to warm and cool grass/shrub plant functional types (PFTs). A statistical test using modern pollen data shows that the method is able to distinguish between cool and warm steppe biomes with a high degree of confidence. The method has been applied to two fossil pollen records. The results of this exercise showed that cool steppe dominated in central Greece between 18 000 and 13 000 yr BP, while in western Iran the vegetation was at the boundary between cool and warm steppes. These vegetation types were replaced by warm mixed forest in Greece and warm steppe in Iran after that time span. © 1998 John Wiley & Sons, Ltd.  相似文献   

Near-infrared adaptive optics imaging of nuclear spiral structure in the Seyfert galaxy NGC 3227     

Scott C. Chapman  †‡  Simon L. Morris ‡  Gordon A. H. Walker 《Monthly notices of the Royal Astronomical Society》2000,319(3):666-676

We present high spatial resolution, near-infrared images in J , H and K of the nucleus of NGC 3227, obtained with the Adaptive Optics Bonnette on the Canada–France–Hawaii Telescope. The ∼0.15 arcsec (17 pc) resolution allows structures to be probed in the core region. Dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core region. We are able to identify two main features in our maps: (i) a spiralling association of knots with a counterpart in a Hubble Space Telescope F606W image; and (ii) a smaller-scale annulus, orthogonal to the spiral of knots. These features may provide a means to transport material inwards to fuel the active nucleus.  相似文献   

Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo,Cape Verdes: Implications for rift zone and magmatic reorganisation     

Jurgen P.T. Foeken  Simon Day  Finlay M. Stuart 《Quaternary Geochronology》2009,4(1):37-49

We have used cosmogenic ³He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano. The post-collapse flows have exposure ages ranging from 62 to 11 ka. The analysis of multiple flow tops on each lava flows, often at different elevations, provides an internal check for age consistency and the exposures ages conform with stratigraphic level. The exposure ages suggest that volcanic activity along the western branch of the triple-armed rift zone was more or less continuous from before 62 ka to approximately 11 ka. The absence of magmatic activity for the last 11 kyr reflects a structural reconfiguration of the volcano and may be related to renewed flank instability. This volcanic hiatus is similar in duration to that observed in the Canary Islands. Replicate ³He exposure ages of a pre-collapse flow (123.0 ± 5.2 ka) brackets the time of the Monte Amarelo collapse between 62 ka and 123 ka. Reproducible cosmogenic ³He exposure ages of less than 123 ka from flows away from major erosion features demonstrates that the technique is a viable alternative to the radiocarbon, K/Ar and ⁴⁰Ar/³⁹Ar chronometers for dating recent volcanism in arid climate zones.  相似文献   

Large-eddy simulation of katabatic winds. Part 2: Sensitivity study and comparison with analytical models     

Simon L. Axelsen  Han van Dop 《Acta Geophysica》2009,57(4):837-856

Steady-state quasi-one-dimensional large-eddy simulations of slope winds over simple terrain are presented. The model results of up-slope flow are compared to previous simulations by Schumann (1990), and good agreement is found. Modelled downslope winds are compared to meteorological observations from two glaciers. The vertical profiles of velocity and buoyancy agree with the observations, whereas larger discrepancies are found between the modelled and the observed momentum and buoyancy flux profiles. Despite some discrepancies, the model captures the main characteristics of the observed downslope winds fairly well. The numerical model is used in a companion paper (Part II) to study how some external input parameters affect katabatic winds.  相似文献   

Response to Wilson, P. (2008): “Comment on Carr, S., and Coleman, C. (2007): An improved technique for the reconstruction of former glacier mass-balance and dynamics: Geomorphology 92, 76–90,” Geomorphology 99, 443–444     

Simon Carr  Chris Coleman 《Geomorphology》2009,106(3-4):383-384

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The effect of mantle composition on density in the extending lithosphere   总被引：1，自引：0，他引：1  

Nina S.C. Simon  Yuri Yu. Podladchikov 《Earth and Planetary Science Letters》2008,272(1-2):148-157

The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. We evaluate the influence of changes in mantle composition on lithospheric density and its evolution during horizontal stretching, using thermodynamic calculations of the density as a function of pressure, temperature and composition. We also develop a simple parameterization based on end-member mineral reactions and geometric relationships between the geotherm and the phase boundary for comparison. The garnet–spinel peridotite transition leads to a moderate decrease in density of the mantle part of the lithospheric column at the initial stages of stretching. When the crust is sufficiently thinned and temperature is relatively high, plagioclase peridotite becomes stable in the upper part of the mantle. The density reduction due to the plagioclase-in reaction is controlled by bulk Al₂O₃ in the mantle and by the depth of the plagioclase-in reaction, which is mainly governed by the Na₂O/Al₂O₃ ratio. Since Na₂O and Al₂O₃ increase with the fertility of the mantle the phase transition effect is most pronounced for relatively fertile mantle (and strong extension) and can lead to 2.3% density reduction. This is equivalent to heating the entire lithosphere by 700 °C if only the effect of thermal expansion on density is taken into account. The formation of plagioclase peridotite can explain syn-rift uplift in sedimentary basins that experienced large mantle stretching without invoking an unrealistically strong increase in temperature. It might also be responsible for the break-up unconformity observed at continental margins.  相似文献   

A model of inter-annual variability in beach levels     

Simon P. Neill  Alan J. Elliott  Mohammad R. Hashemi 《Continental Shelf Research》2008

Beach profile data, collected twice per year at 19 stations over a 25 km length of coastline in Tremadoc Bay, have been analysed to quantify the inter-annual variability in beach levels over a 7 year period and the results compared against the output of a numerical model. Using hourly wind data as forcing, the morphological development of northern Tremadoc Bay was simulated by wave, tidal, longshore transport, total transport and bed level change models. The modelling methodology was efficient and innovative, allowing realistic simulations of long duration with a time step of 1 h, hence capturing the high frequency nature of wind events. The model was run for each of the 7 autumn/winter periods (generally November–April) and the modelled net change in beach levels compared with the data from all 19 stations. The model results had reasonable agreement with the beach profile surveys. However, the observed magnitude of bed level change in the bay lagged the model output by 1 year, indicating that sediment processes acting over a larger area are important in a relatively localised study of inter-annual variability.  相似文献   

Thermal–mechanical modeling of cooling history and fracture development in inflationary basalt lava flows     

Simon A. Kattenhorn  Conrad J. Schaefer 《Journal of Volcanology and Geothermal Research》2008,170(3-4):181-197

Thermal–mechanical analyses of isotherms in low-volume basalt flows having a range of aspect ratios agree with inferred isotherm patterns deduced from cooling fracture patterns in field examples on the eastern Snake River Plain, Idaho, and highlight the caveats of analytical models of sheet flow cooling when considering low-volume flows. Our field observations show that low-volume lava flows have low aspect ratios (width divided by thickness), typically < 5. Four fracture types typically develop: column-bounding, column-normal, entablature (all of which are cooling fractures), and inflation fractures. Cooling fractures provide a proxy for isotherms during cooling and produce patterns that are strongly influenced by flow aspect ratio. Inflation fractures are induced by lava pressure-driven inflationary events and introduce a thermal perturbation to the flow interior that is clearly evidenced by fracture patterns around them. Inflation fracture growth occurs incrementally due to blunting of the lower tip within viscoelastic basalt, allowing the inflation fracture to pivot open. The final stage of growth involves propagation beyond the blunted tip towards the stress concentration at the tapered tip of a lava core, resulting in penetration of the core that causes quenching of the lava and the formation of a densely fractured entablature. We present numerical models that include the effects of inflation fractures on lava cooling and which support field-based inferences that inflation fractures depress the isotherms in the vicinity of the fracture, cause a subdivision of the lava core, control the location of the final portion of the lava flow to solidify, and cause significant changes in the local cooling fracture orientations. In addition to perturbing isotherms, inflation fractures cause a lava flow to completely solidify in a shorter amount of time than an identically shaped flow that does not contain an inflation fracture.  相似文献