恒星类太阳和类长周期变星脉动激发机制的理论探讨

利用一种非局部和非定常的恒星对流理论,计算了0.6～3.0 M☉恒星演化模型的线性非绝热脉动.结果表明,赫罗图上脉动不稳定带右方的脉动不稳定的低温恒星可以分成2大类(群).一类由主序矮星、亚巨星和中低光度红巨星组成的类太阳振荡恒星,它们的中高阶(径向阶nr≥12)p模是脉动不稳定的,而低阶(nr≤ 5)模是脉动稳定的;另一类是由亮的红巨星和渐近巨星支星(AGB)组成的类长周期变星,它们的低阶模(nr≤5)是脉动不稳定的,而高阶模(nr≥12)却是脉动稳定的.能利用对流与脉动的耦合统一解释造父变星脉动不稳定带红边界、类太阳和类长周期变星脉动.对赫罗图右方的低温恒星的中低阶p模振荡,对流与脉动的耦合是主要的脉动激发和阻尼机制,而湍流的随机激发机制仅对类太阳振荡高阶p模重要.     

关于恒星大气超声对流的注释

1引言尽管有诸多的不满之处,由于其物理上的直观性和应用上的简单性,至今混合长理论仍几乎是唯一一个广泛用于恒星结构、演化和脉动计算的恒星对流理论.混合长理论预言,在红、黄巨星和超巨星大气中,对流是超声速的.我们曾指出,混合长理论隐含了一个假定,对流是亚声速的.对于超声速对流,无论从物理的真实性,还是从混合长公式的数学表述来看,混合长理论都是不正确的.因此超声对流的真实性是存在问题     

球状星团中低光度红巨星的脉动稳定性

利用一种非定常的恒星非局部对流理论,对球状星团中低光度的红巨星进行了线性非绝热脉动理论计算．结果表明,对所有温度高于约5400 K模型的基音到4阶泛音都是脉动稳定的．随着恒星光度的增大,低阶泛音也变得脉动不稳定．对中低光度的红巨星,脉动稳定性非常低,接近中性稳定．因此他们将是不变星或非常小振幅的短周期变星(P<2天)．     

太阳存在脉动不稳定的低球谐阶p1模吗?

利用一种非局部和非定常的恒星对流理论,计算了太阳中低球谐阶(l＜25)模的非绝热脉动.结果表明,低阶(0＜l＜6)非径向p1模是脉动不稳定的,而与之相邻的g模、f模和p2-p5模都是脉动稳定的.根据累积功图的分析,发现振荡的激发来自对流区底部区域.太阳是否存在不稳定的低球谐阶p1模对判断太阳5 min振荡的激发机制有重要意义.     

恒星内部的超声速对流区

依传统的局部混合长理论进行了对流包层模型计算,在黄红巨星和超巨星对流区的顶部出现超声速对流的情况,这与局部对流理论本身是不自恰的．文章详细分析了恒星大气超声速对流的成因,并按照星族Ⅰ恒星的演化轨迹,在赫罗图上给出了出现超声速对流的区域．其结果是。在演化过程中, (1)低质量星基本不出现超声速对流; (2)中等质量星的超声速对流主要出现在3．6相似文献   

恒星的非径向脉动和模式证认

介绍了恒星非径向脉动的一般特性和可观测特征．着重评述早型恒星非径向脉动研究的最新进展以及非径向脉动模式证认的各种方法．     

脉动稳定性对恒星对流包层氦丰度的依赖

捅姜利用一种非局部和非定常的恒星对流理论,计算了3种不同氦丰度(Y=0.28,0.13,0.00)、太阳金属丰度(Z=0.02)、质量1.4~3.0 M_☉恒星演化模型的径向和低球谐阶F-p8模的线性非绝热脉动.数值计算结果表明,理论的δScuti变星脉动不稳定区红边界几乎不随氦丰度变化而改变.随着氦丰度降低,δScuti变星脉动不稳定区蓝边界向低温方向推移,脉动不稳定区中热方的高温星变得脉动更稳定;而冷方的低温星则变得脉动更不稳定.这似乎表明不大可能用氦弥散去解释δScuti脉动不稳定带中的不变星.然而脉动不稳定带热方和冷方中的不变星与变星的比例可能作为是否存在氦弥散的一种观测证据.     

恒星内部的贯穿对流(英文)

贯穿对流是恒星演化理论长期未获很好解决的理论问题。尽管它已有近五十年的研究历史 ,但至今人们对贯穿对流仍存在很多的错误的理解。究其原因是因为人们对贯穿对流的理解依然停留在唯象的混合长理论图象上。根据一种非局部对流理论 ,我们揭示了贯穿对流区的结构。在太阳下部的贯穿对流区 ,温度梯度是亚绝热的 ,但是是超辐射的。本文还对恒星对流理论给出了一个简短的评述。     

湍动对流与变星脉动稳定性

对低温变星的脉动激发机制中的一些争议问题进行了评述:(1)大多数人相信γDoradus型变星是由所谓的阻塞对流机制所激发.研究表明,γDoradus和δScuti变星的激发机制不存在任何实质性的差别,他们是更大一类δScuti-γDoradus变星的2个次群:δScuti变星是p模脉动子群;而γDoradus是g模脉动子群.(2)大多数人相信太阳5分钟振荡和恒星的类太阳振荡被对流所阻尼,他们是由所谓湍流随机激发机制驱动的.研究表明,对流并非单纯只是脉动的阻尼机制,否则无法解释Mira变星和类Mira变星.利用非局部和非定常的恒星对流理论,不仅可以重现δScuti和γDoradus脉动不稳定区,也可重现低光度红巨星的类太阳振荡和高光度红巨星的类Mira振荡特性.     

关于太阳底部贯穿对流区的延伸程度

众所周知 ,贯穿对流对恒星演化有至关重要的影响 .然而至今对此问题尚无完善的理论处理方法 .尽管该问题已经取得了很大的进展[1-4 ] ,但在恒星演化计算中处理贯穿对流最广泛的仍是某种唯象的混合长理论[5,6] .日震学提供了一个探测太阳内部结构的强有力手段 ,它对恒星对流理论给出了一个严格的限制 .　　在过去几年中 ,利用日震方法探测太阳对流区之下贯穿对流区的结构引起了人们的特别关注 .所有的理论研究都是根据这样一种简单的唯象贯穿对流模型[5-9] :在稍许高于绝热温度梯度的对流区的下方 ,紧贴着的是一个稍许低于绝热温度梯度的贯穿…     

Unstable Non-radial Modes of Upper Main-sequence Stars†

Using a non-local time-dependent theory of convection, the linear stability is analyzed for low-degree (l = 1 − 4) g39-p4 modes of the evolutionary sequences of stars with masses from 3M_ to 30M_. The influence of convection on the pulsation stability of the non-radial modes is studied. The results show that this influence is not negligible. However, if the details of the single star and the single mode are not concerned, the location and scope of instability strip are not affected significantly by convection.     

太阳5分钟振荡的激发机制

1 引言 太阳5分钟振荡是上世纪1个重要的发现[1],它使得人们可以通过观测太阳表面的振动来探测其内部的结构,日震学已取得了巨大的进展,然而我们至今仍不了解其脉动的激发机制,它依然是1个存在争议的问题.太阳位于造父变星脉动不稳定区之外,所以大多数人都相信,由于对流的阻尼,太阳是脉动稳定的,太阳和太阳型恒星的振荡都是由所谓的湍流随机激发机制所激发[2-8].     

On the effect of turbulent anisotropy on pulsation stability of stars

Within the framework of a non-local time-dependent stellar convection theory, we study in detail the effect of turbulent anisotropy on stellar pulsation stability. The results show that anisotropy has no substantial influence on pulsation stability of g modes and low-order(radial order n_r 5) p modes.The effect of turbulent anisotropy increases as the radial order increases. When turbulent anisotropy is neglected, most high-order(n r 5) p modes of all low-temperature stars become unstable. Fortunately,within a wide range of the anisotropic parameter c_3, stellar pulsation stability is not sensitive to the specific value of c_3. Therefore it is safe to say that calibration errors of the convective parameter c_3 do not cause any uncertainty in the calculation of stellar pulsation stability.     

Numerical simulations of the κ-mechanism with convection

A strong coupling between convection and pulsations is known to play a major role in the disappearance of unstable modes close to the red edge of the classical Cepheid instability strip. As mean-field models of time-dependent convection rely on weakly-constrained parameters (see, e.g., Baker in Physical Processes in Comets, Stars and Active Galaxies, p. 105, 1987), we tackle this problem by the means of 2-D Direct Numerical Simulations (DNS) of the κ-mechanism with convection. Using a linear stability analysis, we first determine the physical conditions favourable for the κ-mechanism to occur inside a purely-radiative layer. Both the instability strips and the nonlinear saturation of unstable modes are then confirmed by the corresponding DNS. We next present the new simulations with convection, where a convective zone and the driving region overlap. The coupling between the convective motions and acoustic modes is then addressed by using projections onto an acoustic subspace.     

On the effects of higher convection modes on the thermal evolution of small planetary bodies

The effects of higher modes of convection on the thermal evolution of a small planetary body is investigated. Three sets of models are designed to specify an initially cold and differentiated, an initially hot and differentiated, and an initially cold and undifferentiated Moon-type body. The strong temperature dependence of viscosity enhances the thickening of lithosphere so that a lithosphere of about 400 km thickness is developed within the first billion years of the evolution of a Moon-type body. The thermally isolating effect of such a lithosphere hampers the heat flux out of the body and increases the temperature of the interior, causing the solid-state convection to occur with high velocity so that even the lower modes of convection can maintain an adiabatic temperature gradient there. It is demonstrated that the effect of solid-state convection on the thermal evolution of the models may be adequately determined by a combination of convection modes up to the third or the fourth order harmonic. The inclusion of higher modes does not affect the results significantly.     

The effect of finite electrical conductivity on compressible magnetoconvection

The stability of a polytropic fluid layer in the presence of a uniform vertical magnetic field is studied under the combined influence of thermal and magnetic diffusion. The main objective of the present investigation is to examine the effect of finite electrical conductivity of the medium on the stability of hydromagnetic modes which are believed to be important in the sunspot phenomena. It is shown that the inclusion of finite electrical conductivity has destabilizing effect on convective modes and small-scale convection can occur in the presence of strong magnetic field, provided the magnetic diffusivity is larger than the thermal diffusivity. The magnetic diffusivity, however, has a tendency to stabilize the fast, slow, and Alfvén-modes.     

Solar p and g modes in a model with a convection layer above a sub-adiabatic interior

We study high-order acoustic modes which reside in the outer layers of the solar interior. Magnetic field effects are not taken into account in this paper as we wish first to filter out how the modal frequencies depend on physical characteristics of a particular model structure of the Sun. In particular, we are interested in how the modal frequencies of solar global oscillations depend on the thickness of the convection layer and on the temperature gradient of the solar interior below. The model we use consists of three planar layers: an isothermal atmosphere, while the convection layer and the interior have temperature gradients that are adiabatic and sub-adiabatic, respectively. The presence of a convection layer with a finite thickness brings in additional modes while the variations in temperature gradient of the interior cause shifts in eigenfrequencies that are more pronounced for the p modes than for the g modes. These shifts can easily be of the order of several hundreds of Hz, which is much larger than the observational accuracy.     

Hydrodynamic instability of convectively unstable atmospheres in shear flow

Results are presented concerning the interaction between regions of convectively unstable fluid, bounded above and below by stable fluid, with a basic horizontal flow field, sheared in a vertical direction. The analysis is conveniently based on the definition of the mechanical energy flux associated with wave motion in a stratified compressible fluid, and enables bounds to be placed on the real and complex phase velocities of overstable modes, in addition to some general results on the net upward wave energy flux. It is shown that purely exponentially growing modes (with horizontal wavevectors spanwise to the shear) do not exist. A known sufficient condition for the stability of stable atmospheres is reproduced here with an interesting modification, and details of energy-flux discontinuities at certain singular points of the equations are given. The work is relevant to any astrophysical and geophysical situations in which convectively unstable regions and shear flows are likely to be together present, but the special motivation here is that of describing some aspects of the interaction between supergranular flow and granular convection.     

Trapped gravity waves and the five-minute oscillations of the solar atmosphere

The various modes of hydrodynamic waves are considered for a model of the solar atmosphere which is based on the Bilderberg model and includes the effects of ionization. The atmosphere forms a potential well for internal gravity waves, since the stability is low at the base (near the convection) and low again in the region of partial ionization in the chromosphere. Calculations show that there are two resonant (trapped) modes of internal gravity waves for horizontal wavelengths based on the scale of the granulation. The properties of these modes are in close agreement with the two modes of oscillation observed by Frazier (1968). Trapped acoustic modes are found to have periods too short to account for the observations.Presently Visiting Fellow, Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, Colo.