星流的搜寻

在冷暗物质的宇宙学模型框架里,银河系(尤其是银晕)至少部分起源于对周围卫星星系的吸积。随着新技术和新设备的不断应用,大规模巡天项目的成功实施使天文学家获得了越来越多、越来越全面的关于银河系的数据。通过对这些数据进行分析,天文学家发现了很多星流的候选体,为银河系的等级并合理论提供了观测上的证据。随着对星流研究的扩大和深入,人们对银河系及其结构和起源的了解也越来越详细。介绍了搜寻星流的方法,以及基于这些方法获得的星流研究结果。     

银河系恒星盘翘曲的秘密

Richard de Grijs     

银河系中球状星团的空间分布和运动轨道

选用银河系中29个累积光谱型为F型的球状星团样本。根据它们的视向速度，绝对自行等参数，归算处理后得出了各样本星团的空间分布和运动速度。并以此作为初始条件，在给定的3种银河系引力势模型中，采用数值积分方法计算出各样本星团的运动轨道。计算结果表明：（1）大部分样本星团都位于银心距5kpc-10kpc的范围内，相对于银心呈球对称分布，它们的速度也呈椭球分布；（2）29个样本星团按其金属度大小和基本性发类，可分属HB和MP两个次系，且样本星团数随金属度[Fe/H]而变化，在[Fe/H]＝－1．6处出现一个峰值；（3）所有样本星团的轨道运动都呈周期性，大都在一个有界而不封闭的周期轨道上运动，其最大银心距大都在40kpc以内。不同的引力势模型对球状星团轨道的具体形态影响不大，在给定的引力势模型下，当某些星团的运动轨道穿越距银心1kpc附近的区域时会出现“混沌”行为。而样本星团的金属度与其轨道形态之间的相关性并不明显；（4）29个样本星团的轨道半长轴、远银心距和方位周期随金属度的变化规律基本相似。轨道偏心率与金属度有关，对于所选的晕族样本星团而言，大约有24％的样本星团的轨道偏心率低于0．4，不同的引力势模型对近银心距、偏心率和参数的不确定度等量影响较小，但是对远银心距、径向周期和方位周期等参数影响较为明显。     

球状星团致密度及其演化

根据球状星团动力学演化理论,本文探讨了球状星团致密度的演化与球状星用质量和位置的关系。结果表明,银心距和球状星团质量都与致密度的演化紧密相关。一般来说,银心距很大时致密度演化极少,球状星团质量愈大致密度演化愈缓慢。对现有球状星团致密度的分布作了统计研究,结果表明,在银心距较小的区域,致密度的分布与理论分析结果一致。在大银心距处发现,质量不同的球状星团其致密度分布有明显不同,它可能反映了球状星团形成阶段其致密度与质量紧密相关。     

银河系恒星形成区的OH,H2O脉泽

本文对银河系恒星形成区的OH和H_2O脉泽的观测、研究的各个方面:形态特征、生成区域、运动状况、时变性质、物理环境和脉泽产生机制,作了简要的综述。     

GD-1星流金属丰度与运动特性分析

星流在星系形成与演化过程中扮演了重要的角色,对银河系中星流的研究将有助于进一步探究银河系的合并历史.将LAMOST(Large Sky Area Multi-Object Fiber Spectroscopic Telescope)DR6光谱数据以及SDSS(Sloan Digital Sky Survey)DR12光谱数据分别与Gaia(Global Astrometric Interferometer for Astrophysics)DR2天体测量数据交叉匹配,获得恒星自行等数据.对GD-1星流在速度空间、几何空间和金属丰度上进行限制,从LAMOST DR6和SDSS DR12数据中共获得了157颗星流成员星.GD-1星流的平均金属丰度为[Fe/H]=-2.16±0.10 dex,延伸长度超过80°.收集前人给出的GD-1星流高概率成员星,组成较大的成员星样本进行对比分析,发现GD-1星流的金属丰度分布呈现内低外高的特点,沿着星流方向径向速度分布特点是两端大、中间小,?₁=-20°(?₁为GD-1星流坐标系横坐标)和?₁=-60°附近的间隙是因为成员星运动差异形成的.根据成员星分布及其速度分布特性,推测GD-1星流起源位置是在?₁=-40°附近.     

河外星系恒星潮汐流的探测

不少河外星系的周边发现有不同形态的恒星潮汐流,它们大多源自主星系潮汐力场对伴星系的作用,少数来自球状星团,而对它们进行探测有着重要的天体物理意义。对几个代表性河外星系的潮汐流、星系团环境中的星流结构、潮汐流探测的统计性质和球状星团潮汐尾的探测现状及若干相关问题作了简要介绍。     

球状星团的金属丰度特征与其恒星形成史

本文根据球状星团所特有的金属丰度特征,利用星族综合方法,探讨了球状星团诸恒星的形成史。研究表明,这些恒星不可能通过恒星形成率和初始质量函数均不随时间变化的单一恒星形成模式产生。原初云通过恒星演化而得到金属丰度污染的过程和多数恒星的形成过程必须分为两个不同的阶段。 球状星团得到金属丰度污染的过程中,若恒星形成具有通常的初始质量函数,则其恒星形成率必须较低,且初始质量函数不能太陡,从而使污染过程中只形成数量较少的低质量恒星,以保证单个球状星团内金属丰度的均匀性。另一种可能性是污染阶段有非常特殊的初始质量函数,只形成大质量的恒星,从而除提供适量污染外不留下任何痕迹。 多数恒星应是在原初云不同部位得到适当污染后通过局域的短暂的爆发性恒星形成(星暴过程)产生。本文进一步探讨了在Fan和Rees球状星团形成模型的框架下,两相介质中温云块相互碰撞造成星暴过程的可能性。     

球状星团M3绝对自行的测定对其轨道计算结果的影响

采用球状星团M3的两组不同的绝对自行值，其中有一组为基于Hpaarcos星表参考是生的新绝对自行值，推导出M3对于银心坐标系的空间位置和速度，并利用Dauphole和Colin的银河系质量分布模型，计算了M3的轨道，考察了绝对自行的取值不同对其轨道计算结果的影响。     

银河系分子云总体性质的研究及恒星形成区成协天体的探测

综述了作者通过＾１３ＣＯ观测示研究银河系分子云总体特征和展示大尺度结构，以及探测恒星形成区新成协天体。评述了研究结果及其应用和有关研究课题的深入与发展，并与同类工作作了比较。     

Possible Streams of the Globular Clusters in the Galaxy

1 INTRODUCTION The globular cluster (GC hereafter), as the oldest star group in the universe, has been a target that astro- physics has paid close attention to all the time. The near-field (Galaxy) cosmology makes contacts with the far-field cosmology by …     

Tidal effects of a massive spheroidal halo on an inner spheroidal contracting visible body: Application to an evolutionary disk-galaxy model

The effects of the tidal interactions between two coaxial, homogeneous spheroids, one (the “Brigt Component”: B) completely embedded in the other (the “Dark Halo”: D), along a quasi-static contraction, are considered. The aim is to look how the dynamical properties and the final morphology of the B subsystem may be affected by the presence of the D component. Three initial configurations are considered: the quasi-spherical “Dark Halo” D coincides with the “Visible Component” B (case C); D is flatter than B but the two spheroids have the same semiminor axis (case N); no D component is assumed; the “visible” spheroid is single (case S). The application to an evolutionary disk-galaxy model is considered under some simple assumptions: i) in cases C and N the spheroidal halo is massive (mass ratio “dark”/“bright” about ten) and dissipationless. For a mass ratio like this, the tidal interaction of the B component over D turns to be negligible in the course of the contraction; adding to that the lack of dissipation, it appears plausible to take the D component as frozen along the evolution; ii) the degree of anisotropy and the angular momentum of B, J_B, are conserved. The conservation of J_B provides us the time-independent relationship among the key physical quantities and gives the possibility to draw the evolutionary tracks on the plane (axis ratio σ_B, semimajor axis a_B) without any explicit time-scale; iii) the global “star formation rate” is parametrized according to a simple “Schmidt power law” proportional to the square of gas density. At every step of the quasi-static contraction, the structure is determined by the tensor virial theorem extended to a double configuration. The model is very idealized, particularly because there are no available tidal gravitational terms other than for the case of two homogeneous. Nevertheless, the method based on the tensor virial appears powerful to gain insight into the correlations among the physical quantities involved and into their trend along the evolution. One of the main result is a clear indication of a leading role played by the axis ratio of the “Dark-Halo” component which might be, to the extent that this simple picture can be compared with a real galactic system, a possible new physical parameter to be added to mass and angular momentum for separating spirals from S0 galaxies.     

The stellar halo of the Galaxy

Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia.     

The orbits of open clusters in the Galaxy

We present and analyse the kinematics and orbits for a sample of 488 open clusters (OCs) in the Galaxy. The velocity ellipsoid for our present sample is derived as  (σ _U , σ _V , σ _W ) = (28.7, 15.8, 11.0) km s⁻¹  which represents a young thin-disc population. We also confirm that the velocity dispersions increase with the age of a cluster subsample. The orbits of OCs are calculated with three Galactic gravitational potential models. The errors of orbital parameters are also calculated considering the intrinsic variation of the orbital parameters and the effects of observational uncertainties. The observational uncertainties dominate the errors of derived orbital parameters. The vertical motions of clusters calculated using different Galactic disc models are rather different. The observed radial metallicity gradient of clusters is derived with a slope of   b =−0.070 ± 0.011   dex kpc⁻¹. The radial metallicity gradient of clusters based on their apogalactic distances is also derived with a slope of   b =−0.082 ± 0.014   dex kpc⁻¹. The distribution of derived orbital eccentricities for OCs is very similar to that derived for the field population of dwarfs and giants in the thin disc.     

On the Sun’s distance from the center and the shape of the inner halo in the Galaxy: Gaia EDR3, HST and literature globular clusters

A statistical method is used to derive both the Sun’s distance $r_0$ from the Galactic Center (GC) and the 3D geometry of the inner ($<$ 25 kpc) halo. The spatial distribution of the 138 Gaia EDR3 globular clusters (GCs) with distances established on a combination of HST and literature data of Baumgardt and Vasiliev (2021) is explored. An estimate by using these ancient objects of the pressure-supported subsystem of the Galaxy with newly derived distances leads to the mean $r_0=7.81\pm 0.14$ kpc. The distribution of GCs within 25 kpc is almost spherically symmetric, and has the shape of an ellipsoid with a major axis of its symmetry slightly elongated toward the Sun and two minor axes of almost the same length. The obtained scale-length ratio of the major axis to the minor axis in the plane and to the vertical axis of the ellipsoid is $\approx$ 1:0.8:0.7. Based on the papers of a series, for practical use we argue to employ the following Sun’s distances from the GC and the plane: $r_0=8.15\pm 0.15$ kpc and $z_0=15\pm 5$ pc.     

Open star clusters in the spiral arms of our Galaxy

We consider the age distributions of open star clusters attributed to three segments of Galactic spiral arms. The smoothed distributions of clusters on the age-Galactocentric angle plane show a great nonuniformity. The time dependence of the formation rate of Galactic disk clusters recovered by taking into account selection effects and dynamical evolution of clusters shows that, on average, the formation rate of open star clusters decreases with time. This is in agreement with the increase in star formation rate into the past, as follows from the study of this process by the method of stellar population synthesis. The present time is the epoch of a current maximum of the cluster formation rate. In addition to the current maximum, there have been at least three more maxima with a period of 300–400 Myr and a duration of no more than 300 Myr. The age distributions are consistent with the pattern of star formation governed by the successive passages of density waves through each examined volume of the Galactic disk. The spiral structure becomes more complex when passing from the inner regions of the Galaxy to its outer regions.