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We present predictions for the numbers of ultracool dwarfs in the Galactic disc population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction versus effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables such as apparent magnitudes and proper motions. Such observables are then passed through the survey selection mechanism to yield histograms in colour. This technique also produces predictions for the proper motion histograms for ultracool dwarfs and estimated numbers for the as yet undetected Y dwarfs. Finally, it is shown that these techniques could be used to constrain the ultra-low-mass mass function and birthrate of the Galactic disc population. 相似文献
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Theoretical predictions of an accretion model of star formation in galactic clusters, published quite long ago, which is found
to have some relevance with the current picture of star formation have been tested with observation of seven clusters of young
and intermediate ages. It is found that the agreement between the theory and observation is very good. 相似文献
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文章通过详细的野外地质调查和系统的岩石化学、稀土元素、微量元素及硅、氧同位素等研究,探讨了羊蹄子山-磨石山钛矿区无矿白色硅质岩和富钛硅质岩的成因及形成地质构造环境。研究结果表明,呈厚层状产出的无矿白色硅质岩具较高的SiO2、Al2O3含量及Al/(Al Fe Mn)、Al2O3/(Al2O3 Fe2O3)比值,稀土元素总量很低,其北美页岩标准化配分模式为向右倾的曲线,无明显铈异常和铕异常,表明其形成于受陆源影响的大陆边缘构造环境;赋矿岩系中薄层状富钛硅质岩的TiO2、Fe2O3、Cu、V含量较高,但Al/(Al Fe Mn)、Al2O3/(Al2O3 Fe2O3)比值较低,稀土元素总量较高,北美页岩标准化曲线为明显左倾型-平坦型,具弱的负铈异常,表明其形成于洋脊及附近环境。两种硅质岩的δ30Si值为变化较小的负值,与热水沉积和某些生物成因硅质岩的硅同位素组成相似,两者的δ18O值范围和平均值均相似。两类硅质岩的成因及形成构造环境不同,富钛硅质岩的地球化学特征表明,该矿床的形成与本区元古宙海底火山热液喷流作用有关。 相似文献
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Plagioclase ultraphyric basalts (PUBs) with up to 54% plagioclasephenocrysts were dredged in the rift valley and adjacent flanksof the ultraslow-spreading Mohns and Knipovich ridges. The PUBsshow large variations in crystal morphologies and zoning. Thelarge variations suggest that single basalt samples containa mixture of plagioclase crystals that aggregated at differentlevels in the magma conduits. Resorbed crystals and repeatedreverse zones suggest that the magma reservoirs were replenishedand heated several times. Thin concentric zones with melt inclusions,and sharp reductions in the anorthite content of 3–7%,are common between the reverse zones. These zones, and skeletalcrystals with distinctly lower anorthite contents than massivecrystals, are interpreted to be the result of rapid crystalliztionduring strong undercooling. The changes between short periodsof cooling and longer periods with reheating are explained bymultiple advances of crystal-rich magma into cool regions followedby longer periods of gradual magma inflow and temperature increase.The porphyritic basalts are characterizd by more depleted andmore fractionated compositions than the aphyric basalts, withlower (La/Sm)N, K2O and Mg-numbers. This relationship, and theobservation that PUBs are sampled only close to segment centresalong these ridges, suggests that the PUBs formed by higherdegrees of melting and evolved in more long-lived magma reservoirs.We propose that the zoning patterns of plagioclase crystalsand crystal morphologies of these PUBs reflect the developmentand flow of magma through a stacked sill complex-like conduitsystem, whereas the aphyric equivalents represent later flowof magma through the conduit. The formation of voluminous higher-degreemelts may trigger the development of the magma conduits andexplain the generally depleted compositions of PUB magmas. KEY WORDS: basalt; mineral chemistry; MORB; magma mixing; magma chamber; major element 相似文献