Delta Scuti stars and Dwarf cepheids: Review and pulsation modes

Recent developments in the field of Delta Scuti stars are discussed. Considerable progress has been made in determining the multiple-period structure of individual variable stars and in identifying the excited radial and nonradial modes through a variety of methods. This allows an analysis of the change of excited modes as a function of the position in the instability strip.     

Mode discrimination in nonradially oscillating stars from light,colour and velocity observations

Expressions for the amplitudes and phases of the light, colour and radial velocity variations are derived for a star in nonradial oscillation. For stars in the cepheid instability strip the spherical harmonic mode of the oscillation can be obtained from the phase difference between the light and colour variations. For Cep stars the mode can be estimated from the amplitude ratio of the light and colour variations.     

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.     

The light and velocity curve bumps for BW Vulpeculae

Conclusions We have attempted to model bumps in the light and radial velocity curves of the Beta Cephei star BW Vulpeculae. Two mechanisms, a resonance phenomena and non-linear pulsations, were investigated. The resonance condition was clearly not fulfilled, the calculated period ratio being approximately 0.60, where a value of 0.50 ± 0.03 is required for resonance. In the non-linear calculation, the bump appears, with the correct phase, but was found at an amplitude that is too large. Further, the light curve does not show any bump-like feature. The cause of the bump is the large spurious boost given the star's velocity field by the solution methods.The calculated periods of the stellar models are shorter than those of previous calculations, enhancing the possibility that these stars pulsate in a radial fundamental mode.     

Sensitivity of Low Frequency Oscillations to Updated Solar Models

A large number of acoustic frequencies have already been detected, leading to a "seismic" model of the Sun rather close to the actual standard solar models. The core however is not yet well constrained by these observations and frequencies of low degree, low frequency modes which penetrate deeply into the solar core are needed. We present here a study on the sensitivity of low degree low frequency (50 - 900 μHz) modes to the structure of the solar interior, in order to help their detection and identification in the low frequency spectrum observed by SoHO experiments like VIRGO and GOLF. The frequencies of p and g modes have been computed for a set of solar models with updated physics (Morel et al., 1997). We analyze their sensitivity to solar parameters like age and metallicity, and to various physical processes, like convective core overshoot and mass loss during the beginning of solar evolution. This revised version was published online in June 2006 with corrections to the Cover Date.     

钛合金宽弦风扇叶片的振动特性

通过模态仿真和模态试验技术分别对某型号TC4钛合金宽弦风扇叶片的振动特性进行研究。采用ANSYS-APDL软件对风扇叶片模型进行模态仿真分析。根据风扇叶片的坎贝尔图以及不同模态的振型分布,识别风扇叶片的弯扭振型。风扇叶片不同转速不同模态下的转速裕度和频率裕度分析显示:第1阶弯曲模态在红线转速下的转速裕度和频率裕度超过20%,表明风扇叶片满足共振裕度要求。在三轴向振动台上利用扫描式激光测振仪,采用敲击法和随机带宽激励的方法测量叶片的响应,分析了风扇叶片不同模态的冲击响应特性和多轴响应特性,发现风扇叶片在周向和轴向激励下的响应以低阶弯曲模态为主,而在径向激励下的响应以扭转和复合模态为主,不同激励位置不同模态的响应特性与风扇叶片的振型息息相关。      

损伤对平纹编织陶瓷基复合材料梁振动特性影响的试验研究

为研究平纹编织陶瓷基复合材料（CMCs）梁受损伤影响的非线性振动特性,本文分别开展了拉伸试验和振动试验。拉伸试验用于获得平纹编织CMCs受损伤影响的变刚度行为,多次正弦扫频试验用于获得振动载荷下平纹编织CMCs梁受损伤影响的非线性行为。拉伸试验得到的非线性应力-应变曲线表明材料受损伤影响变刚度行为明显。梁初始固有频率的试验值与理论值相比,相对误差小于1%。振动试验结果从两方面表明了损伤对梁振动特性的影响。一是模态参数的变化,固有频率从初始值（256.44Hz）不断减小,阻尼比呈现增大趋势,而共振位移幅值受固有频率和阻尼比的耦合作用发生非线性且非单调变化。固有频率的变化造成位移幅频特性曲线的左移现象。二是位移幅频特性曲线的多峰现象,在多处载荷频率下产生了位移峰值点。本文的试验结果可用于指导平纹编织CMCs结构动力学计算理论模型的建立。     

层状介质平面交界裂纹的弹性波（P波和SV波）散射研究

 本文采用Fourier积分变换,将层状介质的弹性波传递矩阵法推广到含有平面交界裂纹层状介质的弹性渡散射问题,进而把散射问题转化为求解矩阵形式的对偶积分方程。作为特例,文中给出单一弹性层与半空间平面交界裂纹的弹性波散射远场模式,并计算几组不同材料组合情形下的远场模式幅频特性曲线,其数值结果表明幅频特性曲线有共振峰存在。     

On the Detectability and Use of Normal Modes for Determining Interior Structure of Mars

The InSight mission to Mars is well underway and will be the first mission to acquire seismic data from a planet other than Earth. In order to maximise the science return of the InSight data, a multifaceted approach will be needed that seeks to investigate the seismic data from a series of different frequency windows, including body waves, surface waves, and normal modes. Here, we present a methodology based on globally-averaged models that employs the long-period information encoded in the seismic data by looking for fundamental-mode spheroidal oscillations. From a preliminary analysis of the expected signal-to-noise ratio, we find that normal modes should be detectable during nighttime in the frequency range 5–15 mHz. For improved picking of (fundamental) normal modes, we show first that those are equally spaced between 5–15 mHz and then show how this spectral spacing, obtained through autocorrelation of the Fourier-transformed time series can be further employed to select normal mode peaks more consistently. Based on this set of normal-mode spectral frequencies, we proceed to show how this data set can be inverted for globally-averaged models of interior structure (to a depth of \(\sim 250~\mbox{km}\)), while simultaneously using the resultant synthetically-approximated normal mode peaks to verify the initial peak selection. This procedure can be applied iteratively to produce a “cleaned-up” set of spectral peaks that are ultimately inverted for a “final” interior-structure model. To investigate the effect of three-dimensional (3D) structure on normal mode spectra, we constructed a 3D model of Mars that includes variations in surface and Moho topography and lateral variations in mantle structure and employed this model to compute full 3D waveforms. The resultant time series are converted to spectra and the inter-station variation hereof is compared to the variation in spectra computed using different 1D models. The comparison shows that 3D effects are less significant than the variation incurred by the difference in radial models, which suggests that our 1D approach represents an adequate approximation of the global average structure of Mars.     

发动机谐振与箭体模态耦合稳定性及机理研究

针对发动机谐振可能与箭体弹性振动之间发生耦合共振,从而引起姿态控制系统不稳定的问题,首先将发动机谐振特性简化为一个二阶环节,建立了包含发动机谐振方程的火箭姿态动力学模型;然后分析了发动机谐振对箭体模态极点频率和阻尼比的影响,并从理论上分析了出现负阻尼比时发动机和箭体模态间的耦合共振机理,在此基础上计算了导致姿态控制系统...     

具有随机参数的周期性结构振动分析的谱方法

为了探讨整圈连接的涡轮叶片组内各个叶片的制造偏差对叶片组振动的影响,本文利用一个具有周期性随机参数的结构模型来近似地模拟叶片组结构,并提出了分析这种结构的振动的一种谱方法。假定结构参数的标准差为微小量,因而可应用摄动法。将周期性随机结构参数展开成付氏级数,从而求解结构的自由振动和受迫振动,求得结构的频率和振型,以及共振振幅及其方差估计。论证了主振型的正交性,分析了此结构共振的特殊条件。算例表明,分析结果与实验结果具有相同的量级。     

Nucleosynthesis in massive stars

We discuss three aspects of the nucleosynthesis in massive and intermediate-mass stars during their early evolutionary phases. These are related to the CNO abundances in giant or supergiant stars, to the²⁶Al yield from massive stars via stellar wind, and to the production of the s-process nuclei in massive stars.     

航空发动机风扇叶片伸根段造型设计与优化

探索了航空发动机风扇叶片伸根段造型的优化设计方法,利用它在保持叶身气动设计不变的情况下使风扇叶片振动特性得到改善。建立了基于非均匀有理B样条NURBS (non-uniform rational B-spline)桥接曲线的伸根段造型方法,实现气动叶身与榫头侧面间的光滑连接;通过改变桥接曲线控制顶点的位置实现伸根段造型的参数化设计,并在相关假设条件的基础上提取了完全定义伸根段造型所需的8个设计变量;以风扇叶片共振裕度最大化作为伸根段设计目标,在径向基函数网络(RBFN)与粒子群(PSO)算法的基础上建立了伸根段造型优化设计流程,其原理是构造RBFN近似模型来逼近和预测风扇叶片共振裕度与伸根段设计变量间的隐式目标函数,在此基础上使用PSO算法搜索使共振裕度达到最大化的伸根段设计变量组合。结果表明:某大涵道比宽弦风扇叶片采用上述流程,通过优化伸根段造型使风扇叶片轴向一弯模态的自振频率得到提高,与转速3倍频激振间的共振裕度提高约2%。      

结构动力学拓扑优化局部模态现象分析

使用拓扑优化技术进行结构自然频率最大化设计时的主要问题是结构空洞区域可能出现的局部模态现象。采用刚度矩阵和质量矩阵求解动力学特征方程,通过算例分析了结构密度差异、惩罚因子对自然频率和模态的影响以及局部模态现象产生的原因。结果表明,由于实体各向同性材料惩罚函数法(SIMP)使用了指数惩罚因子,空洞区域的密度与刚度差异远远大于实体区域,导致空洞区域局部模态的产生。分析了局部模态的产生过程,给出了结构自然频率和模态相对实体-孔洞区域密度的比值以及惩罚因子的变化趋势。同时,基于传统体胞微结构均匀化等效模型方法,提出了描述结构空洞区域的多种体胞微结构,计算证实这些微结构形式均可有效地避免局部模态的发生。     

Pseudorandom Frequency Modulation in Range-Doppler Radar

In many radar systems, efficient use of transmitter power requires the transmission of a constant-amplitude signal for a substantial fraction of time; for a monotonic transmission, however, the range resolution is restricted by the length of the transmitted pulse. Linear frequency modulation removes this constraint for targets with negligible, or known, radial velocities; it is not suitable, however, for simultaneous observations of range and radial velocity (Doppler shift). This paper describes a class of waveforms suitable for simultaneous measurement of range and Doppler shift. These waveforms are characterized by a uniform distribution in frequency and by pseudorandom frequency changes. Uniform frequency distribution is attained by a uniform spacing of frequencies with each frequency present for an identical length of time. Frequency changes are effected by sequencing the frequencies with a pseudorandom number generator. Ambiguity functions are computed for pseudorandom frequencymodulated waveforms designed for ionospheric backscatter studies. By suitable choice of parameters, the ambiguity function becomes a narrow central peak surrounded by a plateau whose height varies randomly between zero and approximately twice its average. Waveform generation by means of a digital frequency synthesizer and data reconstruction considerations are described.     

The absolute magnitudes of RR Lyrae stars

A theoretical counterpart to the Barnes-Evans relation between stellar surface brightness and V-R color has been calculated from model atmospheres for parameters appropriate to RR Lyrae stars. Such a relation can be used to derive stellar angular diameters from V,R photometry and, when applied to variable stars and combined with a radial velocity curve, to derive radii, distances, and absolute magnitudes by the method of Barnes et al. (1977, MNRAS,178, 661). This was done for RR Lyr and X Ari using the photometry of Moffett and Barnes (1980, private communication) and radial velocities from the literature. The resulting absolute magnitudes are M_v = ± 0.59 + 0.25 for X Ari and M_v = 0.61 ± 0.35 for RR Lyr. The method is shown to be a very accurate way of determining radii, distances, and absolute magnitudes for RR Lyrae stars which compares very favorably to the variations of the Baade-Wesselink technique currently in use.     

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.     

Formation of the Outer Planets

Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.     

火箭发动机泵后供应系统水力激振试验

为了研究某型发动机泵后供应系统的动力学特性,对全尺寸的氧泵后供应系统在中、高频范围内开展了水力激振试验。试验数据表明:供应系统的谐振特性被激发起来。在试验条件下,可以清晰地辨识出泵后供应系统的前4阶谐振频率和振型。第1阶谐振频率响应幅值最高,第2阶和第3阶相当,响应幅值次之,第4阶响应幅值再次之。在外界扰动下,该发动机泵后供应系统能够较容易地达到流体谐振状态,当外界扰动频率接近其谐振频率时,供应系统沿程脉动压力出现了明显的共振波形。在不同的谐振频率下,喷前腔位置均为压力振荡的波节位置,也即为流量振荡的波腹位置。