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1.
自1979年以来,越来越多的观测证据表明,在许多盘状星系中普遍存在厚盘成分,它们在结构、化学组成和运动学等方面与薄盘有着明显的不同.一般而言,星系中厚盘的标高比薄盘大得多,厚盘恒星与薄盘恒星相比,金属丰度较低而年龄较老,绕星系中心的转动速度较慢而速度弥散度较大,上述观测证据表明厚盘确实是星系长期演化后的产物,对于研究河外星系的形成和演化史有着重要的天体物理意义.该文在简单回顾银河系和河外星系中厚盘发现和确认史的基础上.介绍了对星系厚盘结构参数等性质的探测途径和目前取得的一些主要认识,并与薄盘恒星的相应特征作了比较,结构参数主要包括厚盘和薄盘的标长、标高和轴比,其他观测性质有两类盘的颜色、年龄、金属度和运动学特性等.最后,结合上述观测性质,对厚盘形成的几种可能的机制及未来可以开展的工作做了简要的说明和讨论. 相似文献
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3.
恒星的Al元素丰度可以为探索星团和星系的化学演化提供重要线索.通过系统分析银河系薄盘、厚盘、核球、银晕以及M4、M5等球状星团中恒星的[Al/Fe]随恒星金属丰度[Fe/H]的变化趋势,得出银河系薄盘、厚盘和核球恒星的[Al/Fe]随着[Fe/H]的增加而缓慢下降,而球状星团M4和M5恒星的[Al/Fe]随[Fe/H]增加没有下降趋势,这暗示Ia超新星对M4和M5恒星元素丰度的贡献比较小.详细研究了银河系恒星[Al/Fe]与[Mg/Fe]、[Na/Fe]的相关性,结果表明银河系场星的[Al/Fe]与[Mg/Fe]正相关,但在球状星团M4和M5恒星中未见此相关性;银河系盘星及M4和M5等球状星团恒星的[Al/Fe]与[Na/Fe]都存在正相关. 相似文献
4.
本文是在银河系化学演化的基础上,利用银河系的三成分(threezone)(即晕、厚盘和薄盘)多相(multi phase)(气体,分子云,大、小质量恒星以及剩余物质)的化学演化的理论模型,讨论了以下观测约束:1、质量面密度、恒星形成率,各分区质量比;2、场星的年龄-金属丰度关系;3、α元素化学演化;4、太阳附近G矮星金属丰度分布;5、三成分金属丰度特征量;6、超新星爆发率;7、内落速率。结果表明,三成分多分量模型能够较好地满足观测约束,比较真实地反映星系演化过程。可以用该模型计算元素的星系化学演化。 相似文献
5.
本文是在银河系化学演化的基础上，利用银河系的三成分(threezone)(即晕、厚盘和薄盘)多相(multi-phase)(气体，分子云，大、小质量恒星以及剩余物质)的化学演化的理论模型，讨论了以下观测约束：1、质量面密度、恒星形成率，各分区质量比；2、场星的年龄-金属丰度关系；3、α元素化学演化；4、太阳附近G矮星金属丰度分布；5、三成分金属丰度特征量；6、超新星爆发率；7、内落速率。结果表明，三成分多分量模型能够较好地满足观测约束，比较真实地反映星系演化过程。可以用该模型计算元素的星系化学演化。 相似文献
6.
确定银河系的年龄是现代天体物理学的一项基本任务。其方法之一是核纪年法，即通过恒星中某一长寿命放射性元素的丰度随时间的变化来确定恒星的年龄，并以此作为银河系年龄的下限，其中目前的观测丰度来自恒星的光谱分析，恒星形成时的初始丰度来自理论模型的预言。这种方法最初是利用元素对Th／Nd来确定G矮星的年龄，近年来开始利用元素对Th／Eu和U／Th来确定晕族场星和球状星团内恒星的年龄。简要介绍了核纪年法确定银河系年龄的原理，回顾了恒星中Th和U的观测研究，其中着重介绍了极贫金属星的研究。详细讨论了用核纪年法估计银河系年龄的不确定性。作为与核纪年法的比较，简单介绍了确定银河系年龄的其他方法。提出了今后需要进一步研究的几项工作。 相似文献
7.
基于高分辨率、高信噪比光谱观测资料，确定了19颗贫金属红团簇巨星的恒星大气参数，得到样本星4种α族元素(O、Mg、Ca、Si)的化学丰度．讨论了铁丰度与恒星大气参数的相关性以及α族元素丰度随金属丰度的变化，计算了共58颗红团簇巨星在I、K波段的绝对星等，讨论了它们与恒星铁丰度之间的关系．结果表明，在分析铁丰度范围内相对于I波段，K波段的绝对星等与铁丰度的相关性更弱，并且与利用理论模型得到的红团簇巨星I、K波段绝对星等与铁丰度的关系进行了比较与验证。 相似文献
8.
对太阳邻域内银河系盘星、特别是厚盘恒星年龄-金属度关系的研究进展作了较为详细的回顾和评述；说明目前对盘星是否确实存在年龄-金属度关系还存在争议，而厚盘恒星可能存在这种关系。同时指出，要得出明晰的结论，在观测资料的基础上取得合理的恒星样本至关重要。 相似文献
9.
利用Hipparcos卫星精确天体测量观测数据,尝试研究由不同样本恒星所反映的太阳邻域银河系薄盘的标高.根据Tycho测光系统的色指数,并考虑观测样本的完备性,分别在主序段和水平支上选取了几类恒星样本,以考察银河系薄盘标高的演化特征.分析结果发现,在比较完备的样本空间上,由O-B型主序星定义的银盘标高为103.1±3.0 pc,太阳位于其平均平面以上的15.2±7.3 pc处;水平支恒星定义的银盘标高为144.0±10.0 pc,太阳位于其平均平面以上3.5±5.4pc处.A、F、G、K、M型的主序星由于观测样本完备性的限制而无法进行可信的标高研究. 相似文献
10.
利用星团谱样本的星族合成方法,研究了邻近巨椭圆星系NGC5018中的星族成分和其内部的恒星形成历史,给出了星系中不同年龄和金属丰度星族的成分占有比.星族合成结果表明,NGC5018中不仅存在大量金属丰度低的年老恒星成分,而且较年轻的星族成分(T=5×108yr)对星系光度贡献也很重要.星系吞并和相互作用过程可能是触发这些较年轻星族形成的物理原因,椭圆星系内部的恒星形成历史可能是2次爆发或者多次爆发过程.这些结果可以很好地解释NGC5018颜色偏蓝、Mg2谱指数强度偏弱等观测特征. 相似文献
11.
In this paper we first emphasize why it is important to know the successive zonal harmonics of the Sun's figure with high
accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment
why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to
emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of
Legendre's functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical
harmonic coefficients (J
2 and J
4) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties
in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical
flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper
bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce
a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J
2=−(6.13±2.52)×10−7 if all observed data are taken into account, and respectively, J
2=−(6.84±3.75)×10−7 if only sunspot data are considered, and J
2=−(3.49±1.86)×10−7 in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J
4=(2.8±2.1)×10−12. These values are compared to some others found in the literature.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005238718479 相似文献
12.
B. P. Kondratyev 《Solar System Research》2013,47(1):1-10
A two-component theoretical model of the physical libration of the Moon in longitude is constructed with account taken of the viscosity of the core. In the new version, a hydrodynamic problem of motion of a fluid filling a solid rotating shell is solved. It is found that surfaces of equal angular velocity are spherical, and a velocity field of the fluid core of the Moon is described by elementary functions. A distribution of the internal pressure in the core is found. An angular momentum exchange between the fluid core and solid mantle is described by a third-order differential equation with a right-hand side. The roots of a characteristic equation are studied and the stability of rotation is proved. A libration angle as a function of time is found using the derived solution of the differential equation. Limiting cases of infinitely large and infinitely small viscosity are considered and an effect of lag of a libration phase from a phase of action of an external moment of forces is ascertained. This makes it possible to estimate the viscosity and sizes of the lunar fluid core from data of observations. 相似文献
13.
M. V. Samokhin 《Earth, Moon, and Planets》1993,61(1):29-37
Using the well-known equation for the normal component of the current which exist near the tangential discontinuity in the plasma in the case of the frozen-in magnetic field, and supposing that the current closes in the ionosphere in the auroral oval in the region 1, one calculates and compares with the data of observations the dependence of the density of the field-aligned current at the level of the ionosphere on the local time. 相似文献
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15.
Yoshio Kubo 《Celestial Mechanics and Dynamical Astronomy》2009,105(4):261-274
We calculate the so-called convective term, which shows up in the expression for the angular velocity of the elastic Earth,
within the Andoyer formalism. The term emerges due to the fact that the elasticity-caused perturbation depends not only on
the instantaneous orientation of the Earth but also on its instantaneous angular velocity. We demonstrate that this term makes
a considerable contribution into the overall angular velocity. At the same time the convective term turns out to be automatically
included into the correction to the nutation series due to the elasticity, if the series is defined by the perturbation of
the figure axis (and not of the rotational axis) in accordance with the current IAU resolution. Hence it is not necessary
to take the effect of the convective term into consideration in the perturbation of the elastic Earth as far as the nutation
is related to the motion of the figure axis. 相似文献
16.
L. E. Cram 《Solar physics》1976,48(1):3-19
When the K-corona is formed by the scattering of photospheric radiation from free electrons, the Fraunhofer lines are greatly broadened by the thermal motions of the hot electrons. This paper discusses the possibility of measuring the coronal electron temperature from the residual depressions in the K-coronal spectrum. If the ratio of the intensities at 4100 Å and 3900 Å can be measured to an accuracy of ±1%, the coronal temperature can be inferred to an accuracy of ±0.2 MK. The temperature of a coronal inhomogeneity may also be measured by this method, provided the position angle is known.Now at Fraunhofer Institute, Freiburg, Germany. 相似文献
17.
Wu Lin-xiang 《Chinese Astronomy and Astrophysics》1992,16(4):427-433
By means of a simple relation between the velocity v of the fluid particle and the velocity vf of the photospheric footpoint of the magnetic field line vz and Bz being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of vz, and vf is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity vl of the moving curve of constant Bz is found to be equal approximately to the component of v or vf in the direction of vl. Since vl can be measured or extimated, so can the components of v and vf near the neutral line. 相似文献
18.
Range of values of the Sun's mass quadrupole moment of coefficient J2 arising both from experimental and theoretical determinations enlarge across literature on two orders of magnitude, from
around 10-7 until to 10-5. The accurate knowledge of the Moon's physical librations, for which the Lunar Laser Ranging data reach an outstanding precision
level, prove to be appropriate to reduce the interval of J2 values by giving an upper bound of J2. A solar quadrupole moment as high as 1.1 10-5 given either from the upper bounds of the error bars of the observations, or from the Roche's theory, is not compatible with
the knowledge of the lunar librations accurately modeled and observed with the LLR experiment. The suitable values of J2 have to be smaller than 3.0 10-6.
As a consequence, this upper bound of 3.0 10-6 is accepted to study the impact of the Sun's quadrupole moment of mass on the dynamics of the Earth-Moon system. Such as
effect (with J2 = 5.5±1.3 × 10-6) has been already tested in 1983 by Campbell & Moffat using analytical approximate equations, and thus for the orbits of
Mercury, Venus, the Earth and Icarus. The approximate equations are no longer sufficient compared with present observational
data and exact equations are required. As if to compute the effect on the lunar librations, we have used our BJV relativistic
model of solar system integration including the spin-orbit coupled motion of the Moon. The model is solved by numerical integration.
The BJV model stems from general relativity by using the DSX formalism for purposes of celestial mechanics when it is about
to deal with a system of n extended, weakly self-gravitating, rotating and deformable bodies in mutual interactions.
The resulting effects on the orbital elements of the Earth have been computed and plotted over 160 and 1600 years. The impact
of the quadrupole moment of the Sun on the Earth's orbital motion is mainly characterized by variations of
,
, and
. As a consequence, the Sun's quadrupole moment of mass could play a sensible role over long time periods of integration of
solar system models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
G. M. Petrov 《Astrophysics and Space Science》1991,177(1-2):269-270
The classical method of determination of the absolute azimuth (or Bessel's parameter n) can secure sufficiently precision for RA from observations of stars at high geographical latitudes during polar night only. 相似文献
20.
介绍和论述了在后牛顿引力理论(PPN形式)中在优越参考系和非优越参考系中经过参数化后引力常数变化对地球自转产生的效应,其中特别重点介绍了年周期变化的效应。此外也将理论结果同观测结果相对比。 相似文献