The infrared [WC] stars

A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for10⁴ yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M_⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars.     

天体运动的轨道偏心率研究概述---从恒星系统到行星系统

偏心率是描述天体运动轨道的重要参数之一, 能够为揭示天体的动力学演化提供重要线索, 进而帮助理解天体形成与演化的过程及背后的物理机制. 随着天文观测技术的不断发展, 人们对于天体运动轨道的研究已经走出太阳系, 包含的系统也从大质量端的恒星系统延伸到了低质量端的行星系统. 聚焦天体轨道偏心率研究, 回顾了目前在恒星系统(包括主序恒星、褐矮星以及致密星)和行星系统(包括太阳系外巨行星以及``超级地球''、``亚海王星''等小质量系外行星)方面取得的进展, 总结了不同尺度结构下偏心率研究的一些共同之处和待解决的问题. 并结合当下和未来的相关天文观测设备和项目, 对未来天体轨道偏心率方面的研究工作进行了展望.     

蓝离散星研究现状(Ⅱ):观测特性

不同恒星系统(银河系晕、疏散星团、球状星团、矮星系)中蓝离散星所表现出的观测特性各不相同,这与恒星系统动力学环境及相应的蓝离散星主导形成机制直接相关。因此,分析研究蓝离散星的不同观测特性,也就成为研究蓝离散星形成机制、恒星及双星系统演化以及恒星系统动力学演化的有效方法。     

Kinematical properties of early-type galaxies

Summary In the last decade, our understanding of early-type galaxies has greatly changed: from rather uninteresting oblate spheroids flattened by rotation to multicomponent stellar systems whose structure, formation mechanisms, and evolution, are far from being understood. This new scenario is mainly the consequence of the huge growth, in quantity and in quality, of kinematical data obtained from high signal to noise spectral data. Rotation curves and velocity dispersion profiles extending out to almost 2 effective radii are now available, together with line asymmetry measurements, for the stellar components of a fairly large sample of galaxies. For a few galaxies, outer halo tracers such as globular clusters and/or planetary nebulae allow to explore the kinematics out at 4 6r _e. In this article we focus on these data giving particular emphasis on the most recent results. Reference is given to other review articles complementing the approach presented here.     

Star formation and molecular clouds

The hypothesis advanced by V. A. Ambartsumyan according to which stars are formed from prestellar superdense objects-- protostars-- was an alternative to the hypothesis of the 1950's (and even now, not much changed) according to which stars are formed by accretion with subsequent collapse (in various modifications). Ambartsumyan's basic inferences were based on an analysis of the observational data available at that time. This paper presents both Ambartsumyan's pioneering ideas and some modern hypotheses of star formation. Some results from studies of molecular clouds and star formation regions are also discussed. One of the distinctive features of young stellar objects (YSO) is the outflow of matter from these objects (molecular, in the form of jets, etc.), a phenomenon whose importance for the evolution of stars was noted by Ambartsumyan as long ago as 1937. Radial systems of dark globules are examined, as well as H-H objects associated with star formation regions, cometary nebulae, and close Trapeziumtype systems (consisting of YSO). Translated from Astrofizika, Vol. 52, No. 2, pp. 185–202 (May 2009).     

A multiphysics and multiscale software environment for modeling astrophysical systems

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computational efficiency. This approach allows scientists to use combinations of codes to solve highly coupled problems without the need to write new codes for other domains or significantly alter their existing codes. MUSE currently incorporates the domains of stellar dynamics, stellar evolution and stellar hydrodynamics for studying generalized stellar systems. We have now reached a “Noah’s Ark” milestone, with (at least) two available numerical solvers for each domain. MUSE can treat multiscale and multiphysics systems in which the time- and size-scales are well separated, like simulating the evolution of planetary systems, small stellar associations, dense stellar clusters, galaxies and galactic nuclei. In this paper we describe three examples calculated using MUSE: the merger of two galaxies, the merger of two evolving stars, and a hybrid N-body simulation. In addition, we demonstrate an implementation of MUSE on a distributed computer which may also include special-purpose hardware, such as GRAPEs or GPUs, to accelerate computations. The current MUSE code base is publicly available as open source at http://muse.li.     

On synchronism between axial rotation and orbital motion in close binary systems

In this paper we examine the possible outcome of the tidal evolution of a close binary system using a method from which the outline has already been given by Counselman (1973). If the value of the total angular momentum of the system is sufficiently large, two equilibrium states corresponding to synchronism between stellar rotation and orbital motion are possible. In one of these states the total energy attains no extreme value. The considered evolution can be visualized geometrically by the motion of a point along a hyperbolic cylinder in three-dimensional space. A comparison with some observational data reveals that most of the synchronously rotating detached systems have attained a stable equilibrium state of minimum total energy for the given value of total angular momentum.     

Observational approach to star formation

The observational approach to the early stages of stellar evolution has been applied to some problems relating to the formation and dissipation of stellar associations, the origin of OB field stars, and low-mass star formation in OB associations. The OB field stars ejected from parent associations are older on the average than the OB stars in the associations. The average duration of active OB-star formation in associations is evaluated. It is suggested that, under the conditions in OB associations, low-mass stars may be formed from dense protostellar objects.Translated fromAstrofizika, Vol. 39, No. 3, pp. 393–406, July–September, 1996.     

Determining the character of motion in quiet and active galaxies with a satellite companion

A galaxy model with a satellite companion is used to study the character of motion for stars moving in the x ‐y plane. It is observed that a large part of the phase plane is covered by chaotic orbits. The percentage of chaotic orbits increases when the galaxy has a dense nucleus of massM_n. The presence of the dense nucleus also increases the stellar velocities near the center of the galaxy. For small values of the distance R between the two bodies, low energy stars display a chaotic region near the centre of the galaxy, when the dense nucleus is present, while for larger values of R the motion in active galaxies is regular for low energy stars. Our results suggest that in galaxies with a satellite companion, the chaotic character of motion is not only a result of galactic interaction but also a result caused by the dense nucleus. Theoretical arguments are used to support the numerical outcomes. We follow the evolution of the galaxy, as mass is transported adiabatically from the disk to the nucleus. Our numerical results are in satisfactory agreement with observational data from M51‐type binary galaxies (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

蓝离散星研究现状(Ⅰ):形成机制

大量研究表明,蓝离散星的形成可能有多种机制,目前比较流行的形成机制可以概括为以下几类:密近双星系统的质量传输及双星并合、密集星场的恒星碰撞,以及包含双星系统的恒星间 (双星-单星、双星-双星)相互作用导致的恒星并合。与此同时,蓝离散星在各类恒星系统中的普遍存在,也使得研究这类恒星的形成及演化成为追踪恒星系统动力学演化、化学演化及积分光谱性质变化的有效指针。     

Formation and evolution of planets

Hydrodynamical calculations are becoming increasingly successful at understanding the shapes and kinematics of planetary nebulae (PNs). The most successful models are two-dimensional interacting stellar wind models for which the PN nucleus is assumed to originally expel much or most of its mass in an equatorial waistband. The physics of the ensuing evolution seems to be explained nicely by a combination of hydrodynamics coupled with time-dependent stellar ionization and energy loss through nebular radiation. Recent radiation gas dynamic calculations are shown to yield excellent agreement with data.     

Bubbles in planetary nebulae and clusters of galaxies: jet bending

We study the bending of jets in binary stellar systems. A compact companion accretes mass from the slow wind of the mass-losing primary star, forms an accretion disc and blows two opposite jets. These fast jets are bent by the slow wind. Disregarding the orbital motion, we find the dependence of the bending angle on the properties of the slow wind and the jets. Bending of jets is observed in planetary nebulae which are thought to be the descendants of interacting binary stars. For example, in some of these planetary nebulae, the two bubbles (lobes) which are inflated by the two opposite jets are displaced to the same side of the symmetry axis of the nebula. Similar displacements are observed in bubble pairs in the centre of some clusters and groups of galaxies. We compare the bending of jets in binary stellar systems with that in clusters of galaxies.     

Effect of the rotation and tidal dissipation history of stars on the evolution of close-in planets

Since 20 years, a large population of close-in planets orbiting various classes of low-mass stars (from M-type to A-type stars) has been discovered. In such systems, the dissipation of the kinetic energy of tidal flows in the host star may modify its rotational evolution and shape the orbital architecture of the surrounding planetary system. In this context, recent observational and theoretical works demonstrated that the amplitude of this dissipation can vary over several orders of magnitude as a function of stellar mass, age and rotation. In addition, stellar spin-up occurring during the Pre-Main-Sequence (PMS) phase because of the contraction of stars and their spin-down because of the torque applied by magnetized stellar winds strongly impact angular momentum exchanges within star–planet systems. Therefore, it is now necessary to take into account the structural and rotational evolution of stars when studying the orbital evolution of close-in planets. At the same time, the presence of planets may modify the rotational dynamics of the host stars and as a consequence their evolution, magnetic activity and mixing. In this work, we present the first study of the dynamics of close-in planets of various masses orbiting low-mass stars (from \(0.6~M_\odot \) to \(1.2~M_\odot \)) where we compute the simultaneous evolution of the star’s structure, rotation and tidal dissipation in its external convective envelope. We demonstrate that tidal friction due to the stellar dynamical tide, i.e. tidal inertial waves excited in the convection zone, can be larger by several orders of magnitude than the one of the equilibrium tide currently used in Celestial Mechanics, especially during the PMS phase. Moreover, because of this stronger tidal friction in the star, the orbital migration of the planet is now more pronounced and depends more on the stellar mass, rotation and age. This would very weakly affect the planets in the habitable zone because they are located at orbital distances such that stellar tide-induced migration happens on very long timescales. We also demonstrate that the rotational evolution of host stars is only weakly affected by the presence of planets except for massive companions.     

On the evolution of central stars of planetary nebulae

The evolution of nuclei of planetary nebulae has been calculated from the end of the ejection stage that produces the nebulae to the white, dwarf stage. The structure of the central star is in agreement with the general picture of Finzi (1973) about the mass ejection from the progenitors of planetary nebulae. It has been found that in order to obtain evolutionary track consistent with the Harman-Seaton track (O'Dell, 1968) one has to assume that the masses of the nuclei stars are less than 0.7M . The calculated evolutionary time scale of the central stars of planetary nebulae is 2×10⁴ yr. This time scale is negatively correlated with the stellar mass: the heavier the stellar mass, the shorter the evolutionary time scale.     

Bipolar outflows and the evolution of stars

Hypersonic bipolar outflows are a ubiquitous phenomena associated with both young and highly evolved stars. Observations of Planetary Nebulae, the nebulae surrounding Luminous Blue Variables such as η Carinae, Wolf Rayet bubbles, the circumstellar environment of SN 1987A and Young Stellar Objects all reveal high velocity outflows with a wide range of shapes. In this paper the current state of our theoretical understanding of these outflows is reviewed. Beginning with Planetary Nebulae considerable progress has been made in understanding bipolar outflows as the result of stellar winds interacting with the circumstellar environment. In what has been called the “Generalized Wind Blown Bubble” (GWBB) scenario, a fast tenuous wind from the central star expands into a ambient medium with an aspherical (toroidal) density distribution. Inertial gradients due to the gaseous torus quickly lead to an expanding prolate or bipolar shell of swept-up gas bounded by strong shock waves. Numerical simulations of the GWBB scenario show a surprisingly rich variety of gasdynamical behavior, allowing models to recover many of the observed properties of stellar bipolar outflows including the development of collimated supersonic jets. In this paper the physics behind the GWBB scenario is reviewed in detail and its strengths and weakness are considered. Alternative models involving MHD processes are also examined. Applications of these models to each of the principle classes of stellar bipolar outflow (YSO, PNe, LBV, SN87A) are then reviewed. Outstanding issues in the study of bipolar outflows are considered as are those questions which arise when the outflows are viewed as a single class of phenomena occurring across the HR diagram.     

Ring nebulae associated with of stars: Statistics,classification, origin

The ring nebulae associated with galactic Of stars is considered on the grounds of the list of Of nebulae proposed by lozinskaya and Lomovsky (1982). Taking into account the selection effects, about 80% of Of stars are shown to be associated withHii regions and about 30–50% of these regions have shell structures. Four types of nebulae associated with Of stars are resolved: amorphousHii regions, ring-likeHii regions, wind-blown bubbles, and stellar ejectas. These types appear to be identical to the morphology of nebulae around WR stars proposed by Chu (1981). Observational data are presented and the nature of a number of Of ring nebulae of different types is discussed.     

Infrared helium–hydrogen line ratios as a measure of stellar effective temperature

We have observed a large sample of compact planetary nebulae in the near-infrared to determine how the 2¹P–2¹S He  i line at 2.058 μm varies as a function of stellar effective temperature, T _eff. The ratio of this line with H  i Br γ at 2.166 μm has often been used as a measure of the highest T _eff present in a stellar cluster, and hence of whether there is a cut-off in the stellar initial mass function at high masses. However, recent photoionization modelling has revealed that the behaviour of this line is more complex than previously anticipated. Our work shows that in most aspects the photoionization models are correct. In particular, we confirm the weakening of the 2¹P–2¹S line as T _eff increases beyond 40 000 K. However, in many cases the model underpredicts the observed ratio when we consider the detailed physical conditions in the individual planetary nebulae. Furthermore, there is evidence that there is still significant 2¹P–2¹S He  i line emission even in the planetary nebulae with very hot     central stars. It is clear from our work that this ratio cannot be considered as a reliable measure of effective temperature on its own.     

A new, efficient stellar evolution code for calculating complete evolutionary tracks

We present a new stellar evolution code and a set of results, demonstrating its capability at calculating full evolutionary tracks for a wide range of masses and metallicities. The code is fast and efficient, and is capable of following through all evolutionary phases, without interruption or human intervention. It is meant to be used also in the context of modelling the evolution of dense stellar systems, for performing live calculations for both normal star models and merger products.
The code is based on a fully implicit, adaptive-grid numerical scheme that solves simultaneously for structure, mesh and chemical composition. Full details are given for the treatment of convection, equation of state, opacity, nuclear reactions and mass loss.
Results of evolutionary calculations are shown for a solar model that matches the characteristics of the present sun to an accuracy of better than 1 per cent; a  1 M_⊙  model for a wide range of metallicities; a series of models of stellar Populations I and II, for the mass range 0.25 to  64 M_⊙  , followed from pre-main-sequence to a cool white dwarf or core collapse. An initial–final mass relationship is derived and compared with previous studies. Finally, we briefly address the evolution of non-canonical configurations, merger products of low-mass main-sequence parents.     

The (sub)Structure of the Halo

The fossil record of the Milky Way indicates an evolution including periodic accretions of smaller galaxies and clusters, consistent with hierarchical models of galaxy formation. I discuss three observational programs that demonstrate that the phase space distribution of stars, clusters and dwarf galaxies in the Galactic halo contains degrees of substructure left by the débris of tidally disrupted stellar systems. This revised version was published online in July 2006 with corrections to the Cover Date.