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The pulsational instability develops in a time interval comparable to the dynamic timescale.


The classical theory of stability of dynamical systems is employed to demonstrate that traditional definitions of pulsational instability cannot be directly applied to stars in thermal imbalance.


If either one is unstable without the other, this constitutes a class of (probably) nonpulsational instability, not previously treated in the astrophysical literature.


It is also seen that pulsational instability is sensitive to changes in initial chemical composition and opacity parameters,n ands.


Effect of variations of opacity and heliumhydrogen ratio on pulsational instability

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Two models for gammaray bursts with WR stars as progenitors are considered: the hypernova model of Paczynski (1998) and the pulsation instability COcore collapse model proposed by Gershten (2000).


The pulsation instability of these stars is due to the effects of ionization of irongroup elements in layers with temperatures T～2×105 K.


Approximate formulas for the pulsation constant Q over the entire range of pulsation instability of the hot helium stars in terms of the mass M, radius R, effective temperature Teff, and heavyelement abundance Z are derived.


In the helium and hydrogen ionizing regions all of the Fourier harmonics drive the pulsation instability, whereas in the radiative damping region the mechanical work done by each Fourier harmonic is negative.


These are the δ Scuti type pulsations in intermediatemass PMS stars (Herbig Ae stars) and the pulsation instability of brown dwarfs and very low mass stars.

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 The theory of instability for nonlinear differential equations is applied to the rate equations of a bistable laser. It is found that there are three conditions have to be fulfilled in order to make the system stable. The first condition is automatically fulfilled for ordinary bistable lasers. The second and third conditions are related to selfpulsation and bistable characteristics of the laser respectively. It has been shown that if the third condition is not fulfilled in certain region, the linearized rate... The theory of instability for nonlinear differential equations is applied to the rate equations of a bistable laser. It is found that there are three conditions have to be fulfilled in order to make the system stable. The first condition is automatically fulfilled for ordinary bistable lasers. The second and third conditions are related to selfpulsation and bistable characteristics of the laser respectively. It has been shown that if the third condition is not fulfilled in certain region, the linearized rate equation for smallsignal will have a monotonously increasing solution in that region. This means that the light power output curve has a negative slope region, which would result in bistable character. On the other hand, if the second condition is not fulfilled, the linearized rate equation for smallsignal will have enhanced oscillating solutions. This means that the laser will selfoscillate as long as the third condition is fulfilled in that region. Besides, it is also shown that both the selfpulsation and the bistable behaviours of the laser are closely related to the nonlinearity of g(N) function, which is the necessary condition for having a linearized rate equation of third order.  本文从非线性方程稳定性理论出发,给出了双稳激光器速率方程的稳定性三条件,指出了其中两个条件分别是产生双稳和自脉动的不稳定性条件,证明了双稳与光功率曲线出现负斜率的区域相对应,这时小信号分析方程有单调上升解,自脉动则与无稳定解的区域相对应,这时小信号分析方程有振荡上升解。另外两者都属于“三次”非线性现象,但是要在g达到一定的非线性时才可能产生自脉动。  y using the perturbation method,the instabilities of thin accretion disks with viscosity and magnetic field were studied both analytically and numerically.The instability criterions were obtained for four special cases.It has been found that the pulsational instability exists both in pure viscous disks and pure magnetized disks.When viscosity and magnetic field are both considered in a disk,there are two kinds of instabilities.One is pulsational and the other one is monotonic.The former one most probably exists... y using the perturbation method,the instabilities of thin accretion disks with viscosity and magnetic field were studied both analytically and numerically.The instability criterions were obtained for four special cases.It has been found that the pulsational instability exists both in pure viscous disks and pure magnetized disks.When viscosity and magnetic field are both considered in a disk,there are two kinds of instabilities.One is pulsational and the other one is monotonic.The former one most probably exists in the inner region of disk while the latter one has physical meaning only to the short wavelength perturbations in the outer disk.Those results are very useful to the explanation of the luminosity variations of BL Lac objects,Seyfert galaxies and quasars.  本文采用微扰方法导出色散方程，并在四种情况下详细讨论了薄吸积盘的不稳定性。结果表明：在纯粘滞和纯磁场盘中都存在脉动不稳定性。而且在吸积盘内同时考虑粘滞和磁场时，存在两种不稳定性，一种是脉动不稳定性，另一种是单调不稳定性。同时数值计算还表明，脉动不稳定性更可能存在于盘的内区，而单调不稳定性则只在盘的外区，对短波扰动才有意义。这些结果为解释ＢＬＬａｃ天体、Ｓｅｙｆｅｒｔ星系及类星体等活动星系核的光变现象进一步提供了理论依据。  A model of weakly magnetized, isothermal accretion disks with viscosity isinvestigated in this paper. The influences of diffusiontype viscosity, and the vertical magnetic,field and the azimuthal magnetic field on the axisymmetric pulsational instability in this model arediscussed. It is shown that there exist four modes of axisymmetric oscillations in the disks,arising from the axisymmetric perturbations. Only when both the vertical magnetic field and theazimuthal magnetic field are present can they affect... A model of weakly magnetized, isothermal accretion disks with viscosity isinvestigated in this paper. The influences of diffusiontype viscosity, and the vertical magnetic,field and the azimuthal magnetic field on the axisymmetric pulsational instability in this model arediscussed. It is shown that there exist four modes of axisymmetric oscillations in the disks,arising from the axisymmetric perturbations. Only when both the vertical magnetic field and theazimuthal magnetic field are present can they affect the instability of these oscillation modes.Two of these modes (correspond to ) are always pulsational instable, and the other twomodes (correspond to ) are always stable. These results are of great advantage to theexplanation of the luminosity variation of the active astronomical objects.  讨论了一种含扩散型粘滞的弱磁化等温吸积盘模型，在此模型中研究了扩散型粘滞和垂向磁场ＢＺ，环向磁场Ｂ对轴对称脉动不稳定性的影响．结果表明：对于轴对称扰动，一般情况下吸积盘内存在４种轴对称振荡模式，其中２种模式（对应于）是脉动不稳定的，扩散型粘滞及磁场ＢＺ，Ｂ对它们表现为非稳因素；而另外２种模式（对应于）是稳定的．此外．仅当同时存在ＢＺ和Ｂ时，磁场才能影响这些振荡模式的不稳定性．   << 更多相关文摘 
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