共查询到19条相似文献，搜索用时 125 毫秒
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Li元素是性质非常活跃且易发生核反应的轻核元素.其精确的丰度可以检验宇宙大爆炸学说.Li元素在宇宙早期化学演化、元素核合成理论和恒星的结构与演化研究中都有着非常重要的地位.近几年来,人们逐渐认识到在局部热动平衡(LTE)的假设下不能准确得到Li丰度,特别在冷的矮星中非局部热动平衡(non-LTE)效应的影响不容忽视,随着理论的发展,人们对原子参数有了更深入的了解,对non-LTE效应的研究也更趋成熟.确定元素丰度时考虑non-LTE影响已成为主要趋势.该文分析了影响Li元素non-LTE效应的各种机制,介绍了确定Li元素non-LTE效应的重要参数及Li原子模型的发展历程,最后对Li在各类恒星中的non-LTE效应进行了综合分析. 相似文献
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目前利用激光来冷却和囚禁中性原子的技术已取得激动人心的进展。通过使用廉价的二极管激光器和蒸汽室收集器，有效地降低了囚禁原子的复杂性和高成本，在零点几μＫ温度下，可得到１０１１个原子／ｃｍ３的原子密度。本文介绍了有关激光冷却与囚禁技术，讨论了其现实应用：频率标准一基本物理性质研究。 相似文献
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详细分析了铯原子喷泉频标的相对论频移机制,其中包括频标内部的二级多普勒频移和重力频移,以及与频标位置有关的重力领移。导出了频移的计算公式,估算了频移的大小并讨论了它们对铯原子喷泉频标准确度的影响。 相似文献
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“EIT waves” are a wavelike phenomenon propagating in the corona, which was initially observed in the extreme ultraviolet (EUV)
wavelength by the EUV Imaging Telescope (EIT). Their nature is still elusive, with the debate on-going between fast-mode wave
model and non-wave model. In order to distinguish between these models, we investigate the relation between the EIT wave velocity
and the local magnetic field in the corona. It is found that the two parameters show significant negative correlation in most
of the EIT wave fronts, i.e., the EIT wave propagates more slowly in the regions of stronger magnetic field. Such a result poses a big challenge to the
fast-mode wave model, which would predict a strong positive correlation between the two parameters. However, it is demonstrated
that such a result can be explained by the fieldline stretching model, i.e., that “EIT waves” are the propagation of apparent brightenings, which are generated by successive stretching of closed magnetic
field lines pushed by the erupting flux rope during coronal mass ejections (CMEs). 相似文献
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This paper investigates the tidal effect on accretion disk in CVs and sets up a simplified model in which the secondary's gravitation is substituted by a mean tidal torque. We find that a linear tidal torque will not be able to maintain an equilibrium disk. By using the result of the radius of the equilibrium disk approximately equals to the tidal radius, which was obtained by using the two dimensional numerical simulation invoking nonlinear tidal effect, we give the modified tidal dissipation function for our simplified model which could be used to interpret the outburst of the dwarf nova with tidal effect. The paper also shows that the radius of an equilibrium disk with a torus is slightly small than the Lubow-Shu radius, and the tidal effect may also cause the cycle of quiescence-superoutburst in addition to the cycle of quiescence-outbursts-superoutburst. 相似文献
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Heather M. Stoll Christine M. Klaas Ian Probert Jorge Ruiz Encinar J. Ignacio Garcia Alonso 《Global and Planetary Change》2002,34(3-4)
Five common placolith-bearing coccolithophorid algae—Gephyrocapsa oceanica, Coccolithus pelagicus, Calcidiscus leptoporus, Umbilicosphaera sibogae (var. sibogae and var. foliosa), and Emiliania huxleyi—were cultured to investigate controls on Sr partitioning in coccolith calcite. For identical temperature and media composition, Sr partitioning varies by more than 30% in exponential phase cultures of the five species and is linearly related to rates of calcite production/cell (ρ=0.91). Exponential phase culture experiments with three strains of C. leptoporus and six strains of G. oceanica at varying temperatures show variations in Sr partitioning of 20% and 30%, respectively. With C. leptoporus, Sr partitioning is equally correlated with temperature and calcification rate (ρ=0.8), which themselves are highly correlated; the slope of the relationship between DSr and calcification rate is comparable to that observed in all species at constant temperature. However, in G. oceanica, increased temperature appears to enhance Sr incorporation by up to 2% to 1.6% °C−1 in the range of 15 to 30 °C. The strong influence of calcification rate on Sr partitioning may be useful for inferring past variations in coccolithophorid productivity from Sr partitioning in coccolith sediments if the influence of temperature on Sr partitioning can be resolved. Because the relationship between calcite production and Sr partitioning is linear, a proportional change in calcification should be expressed much more strongly in the Sr/Ca ratio of large species with rapid calcite production than in smaller species, which produce calcite more slowly. Consequently, it may be possible to separate temperature and calcification influences on coccolith Sr/Ca by separately analyzing Sr/Ca in species that produce calcite rapidly and those that produce calcite slowly, if both undergo comparable relative changes in calcification rates. 相似文献
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《New Astronomy》2021
We have found an approximate solution of Dirac equation using Foldy-Wouthuysen-Tani Hamiltonian of a Dirac particle in the Kerr gravitational field. We have solved the equation approximately using time-independent perturbation theory for the positive energy states. We have found frequencies by which these states oscillate. Difference of the periods of any of these two states has an identical form of the classical gravitomagnetic clock effect where the terms are quantized. So that, we have found a quantum version of the gravitomagnetic clock effect of a Dirac fermion in the Kerr gravitational field. 相似文献
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Pan Cai-juan 《Chinese Astronomy and Astrophysics》2010,34(1):1-12
Under the assumption that the accretion disk around a Kerr (spinning) black hole is geometrically thin and optically thick, the trajectories of photons in Kerr metric are calculated by using the photon tracing method. And by numerical calculations, we have made a study on the relativistic iron line profiles and images of thin accretion disks. The result shows that viewing at large inclination angles, because of the contribution of the photons from the lower surface of the accretion disk, the line profile becomes double-peaked and the flux image is also significantly modified. 相似文献
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EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are
all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves
could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed
this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of
a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast
expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents
sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations
of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole
to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids
observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This
current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields.
It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated
over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties
of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves
are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating
in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression
inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current
shells. 相似文献
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Absolutely calibrated solar images are necessary for a variety of solar physics problems, such as the identification of solar
variability sources and the derivation of differential emission measure (DEM) maps. SOHO EIT is absolutely calibrated by using
TIMED SEE spectra to provide a method of determining physical values of irradiance for EIT images. EIT images from 1 April
2002 to 15 March 2005 in the 28.4- and 30.4-nm channels are compared to SEE daily spectra from the same time period. The resulting
fitted EIT irradiances are well correlated to SEE irradiance measurements and are within the uncertainties of both instruments.
The new cross-calibration results are compared to the currently used calibration based on the UARS SUSIM Mg ii index. 相似文献
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Shin-ya Nitta 《Monthly notices of the Royal Astronomical Society》1999,308(4):995-1005
The aim of this work is to demonstrate the properties of the magnetospheric model around Kerr black holes (BHs), the so-called fly-wheel (rotation driven) model. The fly-wheel engine of the BH–accretion disc system is applied to the statistics of QSOs/AGNs. In the model, the central BH is assumed to be formed at z ∼102 and obtains nearly maximum but finite rotation energy (∼extreme Kerr BH) at the formation stage. The inherently obtained rotation energy of the Kerr BH is released through a magnetohydrodynamic process. This model naturally leads to a finite lifetime of AGN activity.
Nitta, Takahashi & Tomimatsu clarified the individual evolution of the Kerr BH fly-wheel engine, which is parametrized by BH mass, initial Kerr parameter, magnetic field near the horizon and a dimensionless small parameter. We impose a statistical model for the initial mass function (IMF) of an ensemble of BHs using the Press–Schechter formalism. With the help of additional assumptions, we can discuss the evolution of the luminosity function and the spatial number density of QSOs/AGNs.
By comparing with observations , it is found that a somewhat flat IMF and weak dependence of the magnetic field on the BH mass are preferred. The result explains well the decrease of very bright QSOs and decrease of population after z ∼2. 相似文献
Nitta, Takahashi & Tomimatsu clarified the individual evolution of the Kerr BH fly-wheel engine, which is parametrized by BH mass, initial Kerr parameter, magnetic field near the horizon and a dimensionless small parameter. We impose a statistical model for the initial mass function (IMF) of an ensemble of BHs using the Press–Schechter formalism. With the help of additional assumptions, we can discuss the evolution of the luminosity function and the spatial number density of QSOs/AGNs.
By comparing with observations , it is found that a somewhat flat IMF and weak dependence of the magnetic field on the BH mass are preferred. The result explains well the decrease of very bright QSOs and decrease of population after z ∼2. 相似文献