β Cep stars from a spectroscopic point of view

In this review we present the current status of line-profile-variation studies of β Cep stars. Such studies have been performed for 26 bright members of this class of pulsating stars in the past 25 years. We describe all these currently available data and summarize the interpretations based on them in terms of the excited pulsation modes. We emphasize that line-profile variations offer a much more detailed picture of the pulsational behaviour of pulsating stars compared to ground-based photometric data. The latter, however, remain necessary to unravel the often complex frequency pattern and to achieve unambiguous mode identification for multiperiodic β Cep stars and also to derive the pulsational properties of the faint members of the class. We highlight the statistical properties of the sample of 26 stars for which accurate spectroscopic studies are available and point out some future prospects. This revised version was published online in June 2006 with corrections to the Cover Date.     

Pulsation modes and luminosities of the β Cephei stars

Assuming that frequencies observed in multiperiod Cephei variables correspond to normal pulsation modes leads to the following conclusions: (1) a variety of low Z oscillations are excited in these stars, (2) at most one radial mode is seen in any of them.Pulsation constants can be derived for two Cephei stars without recourse to photometric calibrations. In both cases a 0.5 mag. downward revision of the Balmer line absolute magnitudes is indicated. Implications of this result for Stel1ingwerf's opacity bump mechanism are briefly considered.The text of the paper will appear in Proceedings of the Fifth European Regional Meeting of the IAU.     

Radial and nonradial oscillations in Delta Scuti stars

Mode identifications from the phase shift between the B-V color curves and the V light curves of Delta Scuti stars are discussed. For five Delta Scuti stars the frequency of highest amplitude is due to pulsation in a radial mode with one or more frequencies due to nonradial modes lying nearby. Thus there may be a resonance between the frequencies of radial and nonradial modes. This behavior has only been found in subgiant and giant stars so far, but this could easily be a selection effect. Theoretical predictions of frequencies for different 1-modes of the same overtone for main sequence, subgiant, and giant stars are needed to compare with these observations. Theoretical predictions of the strength of resonance coupling between radial and nonradial modes as a function of frequency separation are also needed.     

有声腔推力室的声学振型及其阻尼特性研究

为确定多声学模态压力振荡条件下带有1/4波长管声腔推力室的声学振型及其阻尼特性,揭示1/4波长管声腔对推力室压力振荡的抑制作用机理,对有声腔推力室和无声腔推力室进行近圆周壁面的定容弹激励仿真,激发了多模态的声学振型,给出了推力室压力分布的时空演化,并采用半带宽法定量评价每个激发声学振型的阻尼特性.结果表明:1/4波长管...     

R Coronae Borealis Pulsations

The Cepheid-like pulsations of some of the R CrB stars should in principle make it possible to determine their masses and hence to place constraints on possible evolution scenarios. We briefly review the evidence for these pulsations and discuss the problem of how these low-mass, hydrogen-deficient carbon stars could have evolved to their present position in the H-R diagram. Linear and nonlinear pulsation calculations are reviewed. It is found that for these large luminosity to mass ratio (L/M) stars a region of pulsation instability extends considerably hotter than for normal high luminosity Cepheids. The envelopes of these models are so nonadiabatic that the identification of modes becomes very difficult since there is frequently no clearly defined nodal structure. For the most extreme L/M cases it is found that the models are unstable in the sense that they appear on the verge of ejecting the outer layers.     

β Cephei stars from a photometric point of view

This is an observational review, with an emphasis on photometric data and their interpretation. Two lists are presented, one containing β Cephei stars, and the other, β Cephei suspects. These lists then serve as a basis for discussing such topics as the location of β Cephei stars in the observational and theoretical H-R diagrams, the evolutionary state of these stars, the period-luminosity and period-luminosity-color relations, and observational identification of pulsation modes. The paper also includes references to recent work connected with the theoretical discovery that an opacity mechanism is responsible for the excitation of β Cephei-star pulsations. Finally, observational programs for verifying the consequences of this discovery are suggested.     

推力室状态对其声学振型及其阻尼特性的影响

为确定工程中冷态条件下获得的推力室声学特性能否表征真实条件下的声学特性,研究了冷态无流动、热态气相流动和湍流两相燃烧三种状态下推力室声学振型及其阻尼特性。在推力室稳态流场中的有限区域施加数值定容弹,激发其具有多模态声学振型的大幅值压力振荡,采用衰减时间和半带宽来定量评价所激发的不同声学振型压力振荡衰减快慢,进而获得其阻尼特性。在相同过载比的数值定容弹激励下,在冷态条件下能激发包含更多声学振型压力振荡,且该振荡衰减时间更长,相同振型压力振荡衰减比热态条件下慢。在冷态条件下,一阶切向振型振幅最大,为最容易被激发声学振型;一阶纵向振型半带宽最小,为最难衰减的振型。在热态条件下,一阶纵向振型为最容易激发声学振型,也为最难衰减声学振型。从所激发的主要振型及其相对衰减的快慢来看,冷态条件下获得的声学特性能够表征真实条件下的推力室的声学特征。     

ESTIMATING STRUCTURAL INERTIA PROPERTIES FROM VIBRATION MEASUREMENTS

ＥＳＴＩＭＡＴＩＮＧＳＴＲＵＣＴＵＲＡＬＩＮＥＲＴＩＡＰＲＯＰＥＲＴＩＥＳＦＲＯＭＶＩＢＲＡＴＩＯＮＭＥＡＳＵＲＥＭＥＮＴＳＥＳＴＩＭＡＴＩＮＧＳＴＲＵＣＴＵＲＡＬＩＮＥＲＴＩＡＰＲＯＰＥＲＴＩＥＳＦＲＯＭＶＩＢＲＡＴＩＯＮＭＥＡＳＵＲＥＭＥＮＴＳ￥...     

Microphysics: Equation of State

The equation of state is one of the three fundamental ingredients used to construct stellar models. The plasma of the interiors of stars such as the Sun is only slightly non-ideal. However, the extraordinary accuracy of the helioseismological data requires refined equations of state. It turned out to be necessary to include a Coulomb correction, commonly evaluated in the Debye-Hückel approximation. Higher-order non-ideal effects have implications as well, both for plasma physics and for solar physics. As a typical example, the recently studied thermodynamic consequence of excited states in compound particles is discussed. This effect is of considerable relevance in the helioseismic determination of the helium abundance in the solar convection zone. This revised version was published online in June 2006 with corrections to the Cover Date.     

A. M. Hubert and C. Jaschek (eds.), B[e] Stars

The B[e] stars are early type stars with hydrogen emission lines, forbidden [FeII] and [OI] emission lines, and with an IR excess due to circumstellar dust. These properties may occur in stars of quite different evolutionary stages. In fact, the group of B[e] stars is very inhomogeneous, and contains pre-main sequence stars, supergiants with disks, compact planetary nebulae, symbiotic stars, and a group of stars with unclear evolutionary phase. The book gives the proceedings of a workshop in Paris in 1997 in which the properties and evolutionary phases of the B[e] stars are discussed. It contains chapters on: (1) the definition of B[e] stars, (2) distances, kinematics and the distribution in our Galaxy, (3) spectroscopy, (4) infrared properties, (5) photometry, polarimetry and variability, (6) models for winds and disks, (7) evolutionary stages, (8) revised classification of B[e] stars. The book ends with an object list of all B[e] stars. The book is very useful for students and researchers of hot star winds and gives nice overviews of the observations and theories and remaining puzzles of these strange objects with winds and outflowing dust-forming disks. This revised version was published online in August 2006 with corrections to the Cover Date.     

A comparison between observed and predicted mass-loss rates and wind momentum of O stars

Empirical mass-loss rates were derived for 28 luminous O stars from radio fluxes and H equivalent widths. Comparison with theoretical values predicted by the theory of radiatively driven winds reveals a discrepancy of 0.30±0.05 dex, with the theoretical values being too low. We show that there is not only a mass-loss discrepancy but also a momentum flux discrepancy. The theoretically predicted momentum fluxes are too low by 0.17±0.04 dex. This discrepancy is independent of the adopted stellar mass. We demonstrate that the momentum discrepancy in the most luminous O stars is comparable to the one found in the least extreme Wolf-Rayet stars. We suggest that the physical reason for the break-down of the theory in Wolf-Rayet stars and the most luminous O stars may be related.     

Massive star distribution in external galaxies and starburst regions

Counts of hot and luminous stars in a number of associations in the Galaxy and Magellanic Clouds enable one to directly investigate the numbers and types of massive stars. There seems to be little, if any, dependence of the slope of the Intial Mass Function, or theM _upper on the initial composition of the stars. Indirect estimates of numbers of massive stars in various more distant environments are reviewed and discussed within a framework of acalibration of the methods using the stellar census of 30 Doradus. Very young starbursts, containing large numbers of massive stars, seem to be composed of smaller sub-units similar or somewhat larger than that object. These units might be newly born globular clusters.     

声激励柱壳响应分析中的结合受纳函数

结合受纳函数 (Joint Acceptance Function)是分析声载荷作用下结构特性时遇到的一个特殊的函数。这个函数不仅取决于结构模态 ,还取决于声场特性。本文以柱壳结构为研究对象 ,从分析结合受纳函数的定义、性质入手 ,研究如何利用结构分析中广泛采用的有限元数值结果来计算结合受纳函数 ,获得一种解决工程问题的通用方法。通过和解析解获得的结果比较说明这种结合受纳函数的数值计算方法是可行的。     

Mixing in Stars and the Evolution of the 3He Abundance

We first recall the observational and theoretical facts that constitute the so-called 3He problem. We then review the chemical anomalies that could be related to the destruction of 3He in red giants stars. We show how a simple consistent mechanism can lead to the destruction of 3He in low mass stars and simultaneously account for the low 12C/13C ratios and low lithium abundances observed in giant stars of different populations. This process should both naturally account for the recent measurements of 3He/H in galactic HII regions and allow for high values of 3He observed in some planetary nebulae. We propose a simple statistical estimation of the fraction of stars that may be affected by this process.     

Observations of the atmospheres and winds of O-stars,LBVs and Wolf-Rayet stars

This review summarises recent studies of O-stars, Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars, emphasising observations and analyses of their atmospheres and stellar winds yielding determinations of their physical and chemical properties. Studies of these stellar groups provide important tests of both stellar wind theory and stellar evolution models incorporating mass-loss effects. Quantitative analyses of O-star spectra reveal enhanced helium abundances in Of and many luminous O-supergiants, together with CNO anomalies in OBN and Ofpe/WN9 stars, indicative of evolved objects. Enhanced helium, and CNO-cycle products are observed in several LBVs, implying a highly evolved status, whilst for the WR stars there is strong evidence for the exposition of CNO-cycle products in WN stars, and helium-burning products in WC and WO stars. The observed wind properties and mass-loss rates derived for O-stars show, in general terms, good agreement with predictions from the latest radiation-driven wind models, although some discrepancies are apparent. Several LBVs show similar mass-loss rates at maximum and minimum states, contrary to previous expectations, with the mechanism responsible for the variability and outbursts remaining unclear. WR stars exhibit the most extreme levels of mass-loss and stellar wind momenta. Whilst alternative mass-loss mechanisms have been proposed, recent calculations indicate that radiation pressure alone may be sufficient, given the strong ionization stratification present in their winds.     

β Cephei stars from a photometric point of view

This is an observational review, with an emphasis on photometric data and their interpretation. Two lists are presented, one containing Cephei stars, and the other, Cephei suspects. These lists then serve as a basis for discussing such topics as the location of Cephei stars in the observational and theoretical H-R diagrams, the evolutionary state of these stars, the period-luminosity and period-luminosity-color relations, and observational identification of pulsation modes. The paper also includes references to recent work connected with the theoretical discovery that an opacity mechanism is responsible for the excitation of Cephei-star pulsations. Finally, observational programs for verifying the consequences of this discovery are suggested.Belgian Fund for Scientific Research (NFWO).     

Geometric voting algorithm for star trackers

We present an algorithm for recovering the orientation (attitude) of a satellite-based camera. The algorithm matches stars in an image taken with the camera to stars in a star catalogue. The algorithm is based on a geometric voting scheme in which a pair of stars in the catalogue votes for a pair of stars in the image if the angular distance between the stars of both pairs is similar. As angular distance is a symmetric relationship, each of the two catalogue stars votes for each of the image stars. The identity of each star in the image is set to the identity of the catalogue star that cast the most votes. Once the identity of the stars is determined, the attitude of the camera is computed using a quaternion-based method. We further present a fast tracking algorithm that estimates the attitude for subsequent images after the first algorithm has terminated successfully. Our method runs in comparable speed to state of the art algorithms but is still more robust than them. The system has been implemented and tested on simulated data and on real sky images.     

The relation between RS CVn and Algol

Late-type secondaries in Algol binaries are rapidly rotating convective stars and thus should be chromospherically active (CA). They are examined with respect to observational manifestations which characterize already known CA stars: Ca II H and K emission cores, photometric variability attributable to starspots, soft x-ray emission, non-thermal radio emission, ultraviolet and infrared excess, and alternating period changes. The conclusion is that they can be regarded as another class of CA stars. In most respects they are literally indistinguishable from other CA stars. Ca II H and K emission cores are observed in the lobe-filling component of six semi-detached binaries: U Cep, RT Lac, RV Lib, AR Mon, S Vel, HR 5110. Alternating period changes are shown to occur only in Algols containing a late-type (convective) star. It is proposed, therefore, that the Matese-Whitmire mechanism explains these changes. Specifically, the interval from one increase (or decrease) to the next can be equated with the star's magnetic cycle. Cycle lengths for 31 stars, derived in this way, range between 7 years and 109 years, with a median of 50 years.     

Stellar and solar abundances

Recent spectroscopic results on stellar and solar abundances are reviewed with special reference to (a) Standard abundance distribution (Sun, hot stars, diffuse nebulae); (b) Abundance peculiarities related to stellar evolution (red giants showing results of H-burning and s-process, peculiar and metallic-lined stars); and (c) Population effects that may be related to the evolution of the Galaxy (correlation between stellar age and metal abundance, differences in details of heavyelement mixture in atmospheric composition of normal stars that have not reached an advanced evolutionary stage).     

The relation between RS CVn and Algol

Late-type secondaries in Algol binaries are rapidly rotating convective stars and thus should be chromospherically active (CA). They are examined with respect to observational manifestations which characterize already known CA stars: Ca II H and K emission cores, photometric variability attributable to starspots, soft x-ray emission, non-thermal radio emission, ultraviolet and infrared excess, and alternating period changes. The conclusion is that they can be regarded as another class of CA stars. In most respects they are literally indistinguishable from other CA stars. Ca II H and K emission cores are observed in the lobe-filling component of six semi-detached binaries: U Cep, RT Lac, RV Lib, AR Mon, S Vel, HR 5110. Alternating period changes are shown to occur only in Algols containing a late-type (convective) star. It is proposed, therefore, that the Matese-Whitmire mechanism explains these changes. Specifically, the interval from one increase (or decrease) to the next can be equated with the star's magnetic cycle. Cycle lengths for 31 stars, derived in this way, range between 7 years and 109 years, with a median of 50 years.