对OH／IR星演化的模型研究

渐近巨星分南恒星（ＡＧＢ星）是一种晚期演化恒星,它是恒星作为以核反应释能为发光能源的天体的最后演化阶段。ＡＧＢ星阶段的恒星具有许多有趣的性质,如很大的质量损失率（因此形成很厚的拱星尘埃气体包层）,光变,热脉动（或Ｈe闪耀）,强的红外超量发射,分子脉泽发射等,弄清ＡＧＢ星的演化规律是研究恒星演化理论的重要任务。目前人们所知道的ＡＧＢ星的演化图景是,恒星经过漫长的主序演化之后,将经过红巨星（ＲＧＢ）阶     

中等质量恒星在赫罗图中由E-AGB星进入TP-AGB星的分界点

通过对3～10 M_☉恒星在赫罗图上演化轨迹的研究,分析恒星内部氦壳层燃烧峰值处能量、密度、温度、氦壳层表面光度与恒星表面光度比及恒星半径的变化,给出了中等质量恒星由早期AGB星演化至热脉冲AGB星阶段在赫罗图上的分界点,与119颗碳星的观测结果吻合得相当好.同时提出:在恒星演化至该分界点之后,其星风物质损失公式可能需要引入一个与表面光度无关的量以主导超星风的形成.在此基础上,通过对考虑湍流压效应下5 M_☉恒星的结构和演化及星风物质损失率的分析,发现湍流压在热脉冲AGB星阶段对星风物质损失影响较大,从而使得热脉冲AGB星的湍流压不可忽略,进而提出了影响热脉冲AGB星星风物质损失的可能的物理因素.     

演化晚期恒星中的结晶硅酸盐尘埃

硅酸盐是宇宙尘埃的主要成分之一。20世纪60年代,在演化晚期恒星中就曾发现硅酸盐尘埃的辐射特征,并且与实验室无定形硅酸盐的光谱特征非常吻合。到20世纪90年代,演化晚期恒星中的结晶硅酸盐特征首次在ISO卫星拍摄的光谱中探测到,其后随着更多的红外空间天文台(Spitzer、Herschel等)的发射,发现了大量具有结晶硅酸盐特征的恒星,尤其是演化晚期恒星,几乎涵盖了演化晚期的各个阶段(红巨星、渐近巨星支星、原行星状星云、行星状星云、红超巨星等)。结晶硅酸盐尘埃的特征广泛分布在10～70μm波长范围内,表现为多个丰富的谱带。阐述了演化晚期恒星中结晶硅酸盐尘埃的发现、谱特征、观测特性、结晶度和形成机制。     

8M_■恒星演化过程中湍流应力作用效果的分析

以恒星结构与演化理论中常用的混合程理论为基础,将湍流作用表现出来的宏 观应力引入恒星结构与演化模型中的流体静力学平衡方程．通过计算8M(?)恒星从主序星 到早期AGB星演化过程中湍流应力梯度与引力的比值来研究湍流作用对恒星演化与结构 的影响．结果发现:在核燃烧阶段其比值很小,湍流作用几乎可以忽略;但在RGB和早 期AGB演化阶段,发现在恒星外部存在一个湍流应力梯度为引力的几倍到几十倍的很小 区域,而该小区域以外的对流区内湍流作用力能达到引力的65％,这些对AGB星的中心 温度变化与热脉动发生的时间等恒星结构与演化规律有不同程度的影响．     

AGB星氦燃烧壳层源出现非稳定热核反应的判据

建立了热脉动AGB星氦燃烧壳层源出现非稳定热核反应的判据,新判据包含 了丰富的物理信息,它不仅与热脉动AGB星氦燃烧壳层源的几何性质有关,而且与氦燃 烧壳层源的力学、热学和化学的性质都有关． 提出了热脉动AGB星氦燃烧壳层源非稳定热核反应的发生和消失的机理,它可表述 为:热脉动AGB星氦燃烧壳层源的局部区域出现对流不稳定区会触发非稳定热核反应的 发生,非稳定热核反应会促使氦燃烧壳层源急速膨胀,氦燃烧壳层源的急速几何形变会消 除非稳定热核反应． 用改进后的Kippenhahn恒星演化程序对5M(?)恒星进行了从主序星到热脉动AGB 星的演化模型计算,结果表明新判据能很好地反映5M(?)AGB星氦燃烧壳层源的热核反应 情况．并得出5M(?)热脉动AGB星在第6次热脉动周期阶段,被挖掘到热脉动AGB星 表面的元素主要是在温度lgT2／K<8．155和密度4．0相似文献   

8M⊙恒星演化过程中湍流应力作用效果的分析

以恒星结构与演化理论中常用的混合程理论为基础，将湍流作用表现出来的宏观应力引入恒星结构与演化模型中的流体静力学平衡方程．通过计算8M⊙恒星从主序星到早期AGB星演化过程中湍流应力梯度与引力的比值来研究湍流作用对恒星演化与结构的影响．结果发现：在核燃烧阶段其比值很小，湍流作用几乎可以忽略；但在RGB和早期AGB演化阶段，发现在恒星外部存在一个湍流应力梯度为引力的几倍到几十倍的很小区域，而该小区域以外的对流区内湍流作用力能达到引力的65％，这些对AGB星的中心温度变化与热脉动发生的时间等恒星结构与演化规律有不同程度的影响．     

2.8M⊙碳AGB星的形成和演化

在混合程对流理论基础上建立了湍流压的恒星结构与演化理论,以及在对流外壳中出现动力学非稳定性的判据。在此基础上研究了初始质量为2.8M⊙的星族I恒星从主序星到碳AGB星的质量非守恒演化。结果表明,在RGB星和AGB星阶段,靠近恒星表面区域内湍流压可以达到总压强的30％。且湍流压效应可能是导致RGB星和AGB星靠近表面区域产生了动力学非稳定性,从而造成物质向外逃逸的原因,我们认为湍流压效应可能就是造成有效温度低因而辐射压也低的RGB星产生强星风,以及AGB星产生超星风的物理原因,还发现当氦燃烧层源厚度与层源质量的比值小于0.04R⊙／M⊙时,层源内会出现热核反应的非稳定现象,即出现热脉动,且2.8M⊙AGB星经过6次热脉动后,恒星表面的C／O超过1,恒星演化成碳AGB星。     

原行星盘的研究进展

原行星盘是环绕在年轻星天体(如T Tauri型星,HAe/Be星)周围的气体尘埃盘,是具有初始角动量的分子云核在塌缩形成恒星过程中的自然结果,是行星系统的起源地。原行星盘研究不仅是恒星形成理论的重要组成部分,而且是行星形成理论的基础。首先介绍了盘的形成与演化规律;然后介绍了年轻星天体的能谱分布,盘的模型和参数(质量吸积率、质量、尺度、温度、寿命);随后讨论了尘埃颗粒在盘中生长的观测证据以及行星在盘中形成的大致过程;最后对原行星盘研究的现状和未来做了总结与展望。     

恒星尘埃的实验室研究--实验天体物理学

原始球粒陨石含有来自恒星的微小固体颗粒(微米级),这些尘埃的同位素组成与太阳系物质截然不同,它们是目前唯一能直接获得的恒星固体样品．已发现的恒星尘埃有金刚石、石墨、碳化硅、刚玉、尖晶石、氮化物、和硅酸盐等,它们的母体恒星包括红巨星,AGB恒星、新星和超新星．对恒星尘埃的研究,使得更深入地了解星系的化学演化历史、恒星内部的核反应和湍流机制、恒星大气中尘埃的形成、星际介质物理现象等．恒星尘埃把天体物理领域延伸到了微观世界,它有机地结合了地球化学实验技术和天体物理理论,开辟了一门崭新的天文学分支实验天体物理学．     

Mira变星的拱星包层的速度流场

很多观测事实指出,在许多Mira变星的拱星包层中存在着气体和颗粒,存在着OH脉泽。关于Mira变星的拱星包层在膨胀过程中的速度流场问题,近来一直引起较大的注意。因为它直接关系到对Mira变星的质量损失率的估计,因此它对恒星演化过程以及星际物质的化学组成的研究有重要的价值。人们在估计晚型星质量损失率上存在的困难之一,是对拱星包层的膨胀过程的处理问题。以往的工作往往把晚型星的拱星包层的     

Evolution on the AGB and beyond: on the formation of H-deficient post-AGB stars

The evolution on the AGB and beyond is reviewed with respect to the origin of Wolf-Rayet central stars. We focus on thermal pulses due to their particular importance for the evolution of hydrogen deficient stars. It is shown that overshoot applied to all convection regions is a key ingredient to model these objects leading to intershell abundances already close to the surface abundances of Wolf-Rayetcentral stars. In contrast to standard evolutionary calculations, overshoot models do show dredge up for very low envelope masses and efficient dredge up was found even during the post-AGB stage. Three thermal pulse scenarios for Wolf-Rayet central stars can nowbe distinguished: an AGB Final Thermal Pulse (AFTP) occurring at the very end of the AGB evolution, a Late Thermal Pulse (LTP) occurring during the post-AGBevolution when hydrogen burning is still on, and a Very Late Thermal Pulse (VLTP) occurring on the cooling branch when hydrogen burning has already ceased. All scenarios lead to hydrogen-deficient post-AGB stars with carbon and oxygen abundances as observed for Wolf-Rayet stars.Hydrogen is either diluted by dredge up (AFTP, LTP) or completely burnt (VLTP).     

The Demarcation Point from E-AGB Stars to TP-AGB Stars in HR Diagram for Medium-mass Stars

Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested.     

The evolution of the Magellanic clouds

The observed maximum luminosities of AGB stars in the Magellanic Cloud clusters were compared with luminosities obtained theoretically in order to estimate the ages of the clusters. Several rates of mass loss by AGB stars are considered. The ages for 10 clusters in the SMC and 25 in the LMC are presented in the tables with the compilations of ages from the literature quoted. The discrepancies between ages derived from the AGB peak luminosities and from the Main-Sequence turn off and maximum luminosity are explained by allowing for intensive mass-loss during the AGB evolutionary phase. The method is useful only if the brightest star is on the thermally pulse AGB phase but not in the early-AGB phase.     

Stellar evolution and mass loss on the asymptotic giant branch

Mass loss dominates the stellar evolution on the Asymptotic Giant Branch. The phase of highest mass-loss occurs during the last 1–10% of the AGB and includes the so-called Miras and OH/IR stars. In this review I will discuss the characteristics and evolution of especially Miras, and discuss how they are linked to the mass loss. There are indications that high mass-loss rates are only reached for relatively young stars with massive progenitors. The mass loss rates vary both on long and short time scales: the short-term variations are likely linked to luminosity variations associated with the thermal-pulse cycle. The influence of mass loss in the post-AGB phase is also discussed.     

AGB stars in the Magellanic Clouds

The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.     

Circumstellar envelopes and Asymptotic Giant Branch stars

Summary. Red giants are sometimes surrounded by envelopes, the result of the ejection of stellar matter at a large rate (/yr) and at a low velocity (10 km/s). In this review the envelopes are discussed and the relation between stars and envelope: what stars combine with what envelopes? The envelope emits radiation by various processes and has been detected at all wavelengths between the visual and the microwave range. I review the observations of continuum radiation emitted by dust particles and of rotational transitions of molecules, where these molecules have been excited by thermal or by non–thermal (“maser”) processes. I discuss mainly the oxygen–rich stars, those of spectral type M, and only briefly the closely related carbon–rich stars. By and large the density in the envelope is well described by spherically symmetric outflow at a constant velocity; on the time scale needed to flow from stellar surface to the outermost layers, i.e. yr, the loss of mass is sometimes interrupted suddenly after which the envelope becomes “detached” from the star. The temperature decreases when moving outward; heat input is by friction between dust particles and gas and cooling occurs by line radiation by various molecules, especially by HO. The molecular composition is determined by formation in an equilibrium process deep in the atmosphere and by destruction in the outer parts of the outflow by interstellar UV radiation (H, CO, HO) or by depletion due to condensation on dust grains (SiO); dust particles of silicate material solidify where the radiation temperature is decreased to about 1000 K, and this is at a few stellar radii. The various continuum spectra produced by the dust particles in different stars are well modelled by a simple model of the density and dust temperature distribution plus the assumption that the particles consist of “dirty silicate”, i.e. silicate with Fe and Al ions added. A large range of optical depths, , is observed: from 0.01 to 10. In envelopes with large optical depth the star itself can no longer be detected directly. Model calculations also show that the momentum in the outflow, i.e. is provided by radiation pressure on the dust particles followed by the complete transfer of this momentum to the gas. The mass–loss rate itself, , is not determined by radiation pressure but by dynamic processes in the region below the dust condensation layer. When is sufficiently large its measurement, that of the stellar luminosity, and that of the outflow velocity, , permit the determination of , i.e. the total outflow rate, without making assumptions about the abundance of the dust particles or of the molecular gases. Detached envelopes have been seen in a few cases. Thermal molecular radiation is faint compared to the maser emission but has been measured in distant stars, e.g. in stars near the galactic center. Different molecules outline different “spheres” around the star. The largest sphere (a radius of 0.1 pc) is outlined by an emission line belonging to the CO() transition. Higher rotational transitions of CO give smaller diameters. A comparison of CO () and () fluxes in stars with very thick envelopes leads to the conclusion that an abrupt decrease in the mass–loss rate occurred some ten thousand years ago. Three molecules produce each several maser lines: SiO, HO and OH. Several new HO lines have recently been discovered; their exploration has hardly been started. The high intensity of the maser lines makes interferometry possible and hence detailed mapping. The SiO lines are formed deep in the envelope, below the dust condensation layer. OH maser lines are produced farthest out, HO lines in between. The excitation mechanisms for most maser lines is understood globally, but detailed models are lacking, largely because the problem is non–linear and the solution of the radiative transfer equation requires a highly anisotropic geometry. The geometrical and kinematical properties of the 1612 MHz OH maser, which in many objects is very strong, are explained by a thin shell of OH; because the angular diameter of the shell can be measured directly and the linear diameter can be determined from the difference in the time of maximum flux of blue and red maser peaks, the distance of the shell and of the star can be measured. The presence or absence of individual maser lines appears to depend on the value of and is well described by a sequence called “Lewis' chronology”. The central star is a long–period variable with a period of 300 days or longer and with a large luminosity amplitude (). Evidence is given that each star has the maximum luminosity it will reach during its evolution and that it is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB) with a main–sequence mass between 1 and 6 . Stars of the same main–sequence mass, , have different mass–loss rates, in some cases by a factor of 10. The mass–loss rate probably increases with time, and the highest mass–loss rates are reached toward the end of the evolution. Stars with higher ultimately reach higher mass–loss rates. The calibration of the main–sequence mass is reviewed. Most Mira variables with mass loss have a mass between 1.0 and 1.2 . OH/IR stars with periods over 1000 days have no counterparts among the carbon stars and thus have . Stars as discussed in this review have been found only in the thin galactic disk and in the bulge. Finally I review several recently proposed scenarios for TP–AGB evolution in which mass loss is taken into account. These scenarios represent the observations quite well; their major short–coming is the lack of an explanation why the central stars are always large–amplitude, long–period variables and why such stars are the ones with high mass–loss rates. Received: 10 January 1996     

Stellar pulsation: (I) analysis of stability of envelope

The life-time of the star on AGB is approximately 6 × 10⁴ yr. We divide it into front half and back half of AGB (including to optical Mira variable and OH/IR star) according to their evolution character. The observations show that the star has non-pulsation, but constant mass loss rate ( 5 × 10^–7 M yr^–1) on front half of AGB. Its circumstellar envelope is formed. When the star has pulsation on back half of AGB, its mass loss rate is relative with time, and increases gradually. In this time the star is on the stage of optical Mira variable. When the mass loss rate reaches the value of 3 × 10^–6 M yr^–1, the star evoluted from the stage of optical variable into the stage of radio bright OH/IR star. On the end of AGB the mass loss rate reaches 10^–4 M yr^–1. (Band and Habing 1983, Hermen and Habing 1985).     

大质量恒星演化的若干问题(英文)

大质量恒星由于其高光度、短寿命和质量损失 ,对星系的积分光谱能量分布和重元素增丰起主导作用 ,从而在研究星系的形成和演化上具有特殊的意义。特别是随着天文设备的长足进展 ,我们可以回溯宇宙演化的历史 ,得到形成初期时星系的观测性质。那时 ,大质量恒星主导星系的辐射性质 ,这更加突出了对大质量恒星进一步了解的迫切性。但是大质量恒星的演化性质相对中小质量恒星而言 ,有很多不确定性。本文通过对比现有恒星模型与实测结果 ,对现有大质量恒星演化理论中存在的几个与对流和质量损失相关的问题进行了评述 ,并对从理论上 ,特别是通过数字模拟方法对这些问题进行诊断提出了展望。     

An explosive end to intermediate-mass zero-metallicity stars and early Universe nucleosynthesis

We use the Cambridge stellar evolution code stars to model the evolution of 5 and  7 M_⊙  zero-metallicity stars. With enhanced resolution at the hydrogen- and helium-burning shell in the asymptotic giant branch (AGB) phases, we are able to model the entire thermally pulsing AGB (TP-AGB) phase. The helium luminosities of the thermal pulses are significantly lower than in higher metallicity stars so there is no third dredge-up. The envelope is enriched in nitrogen by hot-bottom burning of carbon that was previously mixed in during second dredge-up. There is no s -process enrichment owing to the lack of third dredge-up. The thermal pulses grow weaker as the core mass increases and they eventually cease. From then on the star enters a quiescent burning phase which lasts until carbon ignites at the centre of the star when the CO core mass is  1.36 M_⊙  . With such a high degeneracy and a core mass so close to the Chandrasekhar mass, we expect these stars to explode as type 1.5 supernovae, very similar to type Ia supernovae but inside a hydrogen-rich envelope.     

The structure of circumstellar envelopes

Copious mass loss on the Asymptotic Giant Branch dominates the late stages of stellar evolution. Maps of extended circumstellar envelopes provide a history of mass loss and trace out anisotropic mass loss. This review concentrates on observations of millimeter wavelength molecular line emission, on high resolution maps of maser emission and on observations of submillimeter, millimeter and radio wavelength continuum emission. Radio continuum observations show that AGB stars are larger at radio than at optical wavelengths. The extended chromospheres indicated by these observations extend to distances from the star large enough for dust to form, thereby initiating mass loss. Molecular line maps have found time-variable mass loss for some stars, including detached shells indicating interrupted mass loss and evidence for a rapid increase in the mass loss rate at the end of the AGB phase. Maps of circumstellar envelopes show evidence of flattening, bipolar outflow and angular variations in both the mass loss rate and the outflow velocity. As stars evolve away from the AGB and planetary nebula formation begins, these structures become more pronounced, and fast bipolar molecular winds are observed. The time scales derived from the dynamical times of these winds and from the expansion rates of the central planetary nebulae are very rapid in some cases, about 100 years, in agreement with the predictions of stellar evolution theory.