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1.
《天文和天体物理学研究(英文版)》2014,(8)
In massive stars,convection in the interior is different from that of intermediate and small mass stars. In the main-sequence phase of small mass stars,there is a convective core and a radiative envelope,between which are the radiative intermediate layers with uneven chemical abundances. Semiconvection would occur in the intermediate layers between the convective core and the homogeneous envelope in massive stars. We treat core convective overshooting and semiconvection together as a process. We found that when decreasing overshooting,the semiconvection is more pronounced. In these two processes,we introduce one diffusive parameter D,which is different from other authors who have introduced different parameters for these two zones. The influences of the turbulent diffusion process on chemical evolution and other quantities of the stellar structure are shown in the present paper. 相似文献
2.
A simple idealized nonlinear model applicable to long period variable stars has been formulated that assumes the convective envelope ofM giants is composed of giant convection cells, which are comparable in size to the stellar radius. The simplicity of this model essentially constitutes a physical analog to the strong dynamic coupling that occurs if the convective envelope of the star undergoes both modes of motion. As shown implicitly in the time scales associated with these motions, the coupling produces asymmetrical fluctuations of the entire star, the mean velocity of which is comparable to the escape velocity of the star at particular values of the ratio of the pulsation and convection time scales. It is suggested that this can account for the mass loss from late type stars, and the circumstellar dust shells that are associated extensively with long period variables.For critical values of the pulsation and convection time scales, the solutions correspond to the rapid expansion of the entire convective envelope, and is the basis of a new mechanism that simulates the manner in which pulsating stars ballistically accelerate their convective shells to form planetary nebulae. 相似文献
3.
Klaus G. Strassmeier 《Astrophysics and Space Science》2006,304(1-4):333-336
With the advent of 8–12m-class telescopes and powerful new spectrographs, we can now extend the Doppler-imaging technique to the cool (and faint) end of the main sequence. At a spectral type of approximately M2, stars are thought to become fully convective and cannot possess an overshoot layer between a radiative core and a convective envelope which, as in the case of the Sun and similar stars, likely harbors the dynamo. Therefore, one could expect a fundamentally different magnetic-field topology than on the Sun and thus a qualitatively different surface temperature distribution with new, hitherto unknown, magnetic activity phenomena. Unfortunately, most single M stars do not rotate sufficiently fast for Doppler imaging and one has to “use” binaries or pre-main-sequence stars in which M stars appear spun up or, in binaries, synchronized to the orbital motion. 相似文献
4.
Mário J. P. F. G. Monteiro Jørgen Christensen-Dalsgaard Michael J. Thompson 《Monthly notices of the Royal Astronomical Society》2000,316(1):165-172
The possibility of observing solar-type oscillations on other stars is of great relevance to investigating the uncertain aspects of the internal structure of stars. One of these aspects is the convective overshoot that takes place at the borders of the envelopes of stars of mass similar to, or lower than, the Sun. It affects the temperature stratification, mixing, rotation and magnetic-field generation. Asteroseismology can provide an observational test for the studies of the structure of such overshoot regions.
The seismic study of the transition in the Sun, located at the base of the convection zone, has been successful in determining the characteristics of this layer in the Sun. In this work we consider the extension of the analysis to other solar-type stars (of mass between 0.85 and 1.2 M⊙ ) in order to establish a method for determining the characteristics of their convective envelopes. In particular, we hope to be able to establish seismologically that a star does indeed possess a convective envelope, to measure the size of the convective region and also to constrain the properties of an overshoot layer at the bottom of the envelope. The limitations in terms of observational uncertainties and stellar characteristics, and the detectability of an overshoot layer, are discussed. 相似文献
The seismic study of the transition in the Sun, located at the base of the convection zone, has been successful in determining the characteristics of this layer in the Sun. In this work we consider the extension of the analysis to other solar-type stars (of mass between 0.85 and 1.2 M
5.
L. Tao M. R. E. Proctor & N. O. Weiss 《Monthly notices of the Royal Astronomical Society》1998,300(3):907-914
Flux expulsion is an important consequence of the interaction of magnetic fields with fluid convection and has been well studied for particular cases of steady, single-cell flows. Here we examine a related phenomenon in inhomogeneous turbulence using direct numerical simulations. To understand our numerical results, we analyse average properties of our model, and obtain mean transport coefficients which can be used to describe the approach of the system to its final state. For the kinematic problem these transport coefficients give an excellent prediction of the expulsion process; however, the enhanced transport is suppressed by dynamical back-reaction of the Lorentz force. Finally, we discuss the astrophysical implications for magnetic fields in stellar convection zones. Segregation of magnetic fields from turbulent motion not only allows strong toroidal fields to accumulate in regions of convective overshoot but also permits significant poloidal fields to be maintained by dynamo action in stars like the Sun. 相似文献
6.
Understanding transport processes inside stars is one of the main goals of asteroseismology. Chemical turbulent mixing can affect the internal distribution of μ near the energy generating core, having an effect on the evolutionary tracks similar to that of overshooting. This mixing leads to a smoother chemical composition profile near the edge of the convective core, which is reflected in the behavior of the buoyancy frequency and, therefore, in the frequencies of gravity modes. We describe the effects of convective overshooting and turbulent mixing on the frequencies of gravity modes in B‐type main sequence stars. In particular, the cases of p‐g mixed modes in β Cep stars and high‐order modes in SPBs are considered. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
Eric S. Saunders Tim Naylor Nathan Mayne S. P. Littlefair 《Monthly notices of the Royal Astronomical Society》2009,397(1):405-410
We use I -band imaging to perform a variability survey of the 13-Myr-old cluster h Per. We find a significant fraction of the cluster members to be variable. Most importantly, we find that variable members lie almost entirely on the convective side of the gap in the cluster sequence between fully convective stars and those which have a radiative core. This result is consistent with a scenario in which the magnetic field changes topology when the star changes from being fully convective to one containing a radiative core. When the star is convective, the magnetic field appears dominated by large-scale structures, resulting in global-size spots that drive the observed variability. For those stars with radiative cores, we observe a marked absence of variability due to spots, which suggests a switch to a magnetic field dominated by smaller-scale structures, resulting in many smaller spots and thus less apparent variability. This implies that wide field variability surveys may only be sensitive to fully convective stars. On the one hand, this reduces the chances of picking out young groups (since the convective stars are the lower mass and therefore fainter objects), but conversely the absolute magnitude of the head of the convective sequence provides a straightforward measure of age for those groups which are discovered. 相似文献
8.
A topological approach in the HR diagram is given for the occurrence and the characteristics of convective envelopes in population II stars. The location of the Hayashi track and of the Red Giants branch are both investigated for various assumed masses of the stars and chemical compositions of the convective layers. An analysis is performed on the influence of the assumed mixing length. The observational consequences of a possible mass loss is discussed for both the Red Giants and the Asymptotic branches. Possible causes for separation between these two branches are briefly examined. The case of the globular cluster ω Cen is investigated, in order to give some suggestions for a large dispersion in colours observed among the giants belonging to this cluster. 相似文献
9.
M. Godart A. Noels M.-A. Dupret Y. Lebreton 《Monthly notices of the Royal Astronomical Society》2009,396(4):1833-1841
Thanks to their past history on the main-sequence phase, supergiant massive stars develop a convective shell around the helium core. This intermediate convective zone (ICZ) plays an essential role in governing which g-modes are excited. Indeed, a strong radiative damping occurs in the high-density radiative core but the ICZ acts as a barrier preventing the propagation of some g-modes into the core. These g-modes can thus be excited in supergiant stars by the κ-mechanism in the superficial layers due to the opacity bump of iron, at log T = 5.2 . However, massive stars are submitted to various complex phenomena such as rotation, magnetic fields, semiconvection, mass loss, overshooting. Each of these phenomena exerts a significant effect on the evolution and some of them could prevent the onset of the convective zone. We develop a numerical method which allows us to select the reflected, thus the potentially excited, modes only. We study different cases in order to show that mass loss and overshooting, in a large enough amount, reduce the extent of the ICZ and are unfavourable to the excitation of g-modes. 相似文献
10.
We suggest a model based on the representation of the stellar magnetic field as a superposition of a finite number of poloidal and toroidal free decay modes to describe the dynamo action in fully convective stars. For the adopted law of stellar differential rotation, we determined the dynamo number in exceeding which the generation of a cyclically varying magnetic field is possible in stars without a radiative core and derived an expression for the period of the cycle. The dynamo cycles in fully convective stars and in stars with thin convective envelopes are shown to differ qualitatively: first, the distributions of spots in latitude during the cycle are different for these two types of stars and, second, the model predicts a great weakening of the spot formation in fully convective stars at certain phases of the cycle. To compare the theory with observations, we have analyzed the historical light curve for the weak-line T Tauri star V410 Tau and found that its long-term activity is not a well-defined cycle with a definite period—its activity is more likely quasi-cyclic with a characteristic time of ～4 yr and with a chaotic component superimposed. we have also concluded that a redistribution of spots in longitude is responsible for the secular brightness variations in the star. This does not allow the results of photometric observations to be directly compared with predictions of ourmodel, in which, for simplicity, we assumed a symmetry in longitude and investigated the temporal evolution of the spot distribution in latitude. Therefore, we discuss the questions of what and how observations can be compared with predictions of the dynamo theory. 相似文献
11.
12.
Hideyuki Saio 《Astrophysics and Space Science》1974,29(1):41-49
The validity of current ideas to set up the semiconvective zone of the stellar models in corehelium-burning phase has been considered mainly from the view point of times-cales related to the mechanism. The time-scale of outward motion of convective shell has been estimated as being longer than that of evolution. From this point of view, it might be said that the models constructed with semiconvective zone are not appropriate for the core-helium-burning stars. 相似文献
13.
E. Schatzman 《Astrophysics and Space Science》1999,265(1-4):97-97
The aim of this work is to present a transport process which is likely to have a great importance for the internal constitution
of the stars. In order to set the problem, we first give a short presentation of the physical properties of the Sun and stars,
described usually under the names of `Standard Solar Model' or `Standard Stellar Models' (SSM). Next we show that an important question about SSM is that they do not explain the age dependance of lithium deficiency
of stars of known age: stars of galactic clusters and the Sun. It has been suggested a long time ago to assume the presence
of a macrosocpic diffusion process in the radiative zone, below the surface convective zone of solar like stars. It is then
possible for the lithium present in the convective zone to be carried to the thermonuclear burning level below the convective
zone. The first assumption was that differential rotation generates turbulence and therefore that a turbulent diffusion process
takes place. However, this model predicts a lithium abundance which is strongly rotation dependant, contrary to the observations.
Furthermore, the diffusion coefficient being large all over the radiative zone, it prevents the possibility of gravitational
separation by diffusion and consequently leads to an impossibility of explaining the difference of helium abundance between
the surface and the center of the Sun. The consequence is obviously that we need to take into account another physical process.
Stars having a mass M < 1.3 M
⊙ have a convective zone which begins close to the stellar surface and extends down to a depth which is an appreciable fraction
of stellar radius. In the convective zone, strong stochastic motions take care, at least partially, of heat transfer. These
motions do not vanish at the lower boundary and generate internal waves into the radiative zone. These random internal waves
are at the origin of a diffusion process which can be considered as responsible of the diffusive transport of lithium down
to the lithium burning level. This is certainly not the only physical process responsible of lithium deficiency in main sequence
stars, but its properties open the way to a completely consistent analysis of lithium deficiency. The model of generation
of gravity waves is based on a model of heat transport in the convective zone by diving plumes. The horizontal component of
the turbulent motion at the boundary of the convective zone is supposed to generate the horizontal motion of internal waves.
The result is a large horizontal component of the diffusion coefficient, which produces in a short time an horizontal uniform
chemical composition. It is known that gravity waves, in the absence of any dissipative process, cannot generate vertical
mixing. Therefore, the vertical component of the diffusion coefficient is entirely dependant of radiative damping. It decreases
quickly in the radiative zone, but is large enough to be responsible of lithium burning. Due to the radial dependance of velocity
amplitude, the diffusion coeficient increases when approaching the stellar center. However, very close to the center, non-linear
dissipative and radiative damping of internal waves become large and the diffusion coefficient vanishes at the very center.
The development of this abstract can be found in E. Schatzman (1996, J. Fluid Mech.
322, 355).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
We model stellar differential rotation based on the mean-field theory of fluid dynamics. DR is mainly driven by Reynolds stress, which is anisotropic and has a non-diffusive component because the Coriolis force affects the convection pattern. Likewise, the convective heat transport is not strictly radial but slightly tilted towards the rotation axis, causing the polar caps to be slightly warmer than the equator. This drives a flow opposite to that caused by differential rotation and so allows the system to avoid the Taylor-Proudman state. Our model reproduces the rotation pattern in the solar convection zone and allows predictions for other stars with outer convection zones. The surface shear turns out to depend mainly on the spectral type and only weakly on the rotation rate. We present results for stars of spectral type F which show signs of very strong differential rotation in some cases. Stars just below the mass limit for outer convection zones have shallow convection zones with short convective turnover times. We find solar-type rotation and meridional flow patterns at much shorter rotation periods and horizontal shear much larger than on the solar surface, in agreement with recent observations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
K. H. Böhm 《Astrophysics and Space Science》1975,37(2):L7-11
Very different amounts of convective mixing are predicted for DA and for non-DA white dwarfs if we accept the recently determined mass differences between these two classes of stars. Using convection zone models calculated by Fontaine (1973) we show that this effect contributes strongly to the atmospheric abundance differences provided DA and non-DA stars have originally comparable total hydrogen abundances. 相似文献
16.
Some recent solar observations show that a bipolar magnetic flux in an active region tends to disappear in situ, in less than one solar rotation, without evidence of spreading. This feature is difficult to explain if it is assumed that magnetic buoyancy is the dominant force in controlling dynamics of a magnetic flux tube, since the assumption implies no other force to submerge the tube. These observations may be explained by assuming that a convective motion is the major cause for the formation of a -shaped geometry of the magnetic flux tube, but that the flux tube thus arisen is submerged by the counteracting Lorentz force as the convective motion decays. A two-dimensional MHD simulation method is used to demonstrate this possibility. 相似文献
17.
极端质量比旋进系统是空间引力波探测器最重要的波源之一。对引力波的探测需要高精度波形模版。当前主流的极端质量比旋进系统引力波计算模型中,人们一般将小质量天体当作试验粒子进行计算,而忽略了其结构及自身引力对背景引力场的影响。利用Mathisson-Papapetrou-Dixon方程研究延展体在弯曲时空中的运动,以及小天体自旋和质量多极矩对引力波信号识别产生的影响。结果表明,质量比在10?6-10?4范围的旋进系统,其自旋达到很大时,自旋对延展体的轨道运动有不可忽略的影响;在质量比10?4-10?2区间内,需要考虑中心黑洞潮汐作用导致的白矮星形变;在质量比大于10?4,且白矮星自旋很大时,其自旋产生的形变会对小天体轨道运动产生不可忽略的影响。大质量黑洞潮汐作用导致的恒星级黑洞或中子星产生的形变可以忽略,中子星和黑洞的自旋会对轨道运动产生不可忽略的影响,而自旋产生的四极矩对轨道运动不产生影响。 相似文献
18.
1 INTRODUCTIONThe mixing length theory (MLT) for stellar convection originally developed by Vitense(1953, 1958) has been the most popularly used local convection theory in the studies of stellarstructure and evolution. The theory was later modified and revised by many investigators,who suggested some different expressions. In fact, MLT is not a real hydrodynamic theory,rather, it is a simple "ballistic" theory which traces the motion of imaginary convective elements. In reality j stell… 相似文献
19.
The radiative acceleration on iron inside stars may lead to an accumulation of this element in stellar internal layers. As discussed by several authors, this iron accumulation has many important consequences. It may lead to an extra convective zone, and in some cases it may help triggering stellar pulsations. However, the computations which have been done up to now ignore an important effect: the double-diffusive, or “thermohaline” convection induced by the inverse μ gradient. Detailed computations of all these processes have been introduced in the TGEC stellar evolution code. We show how thermohaline convection modifies the profiles of iron inside stars, with important consequences 相似文献
20.
Effect of Coriolis force on the equilibrium structures of rotating stars and stars in binary systems
Kopal (Adv. Astron. Astrophys. 9:1–65, 1972) introduced the concept of Roche equipotentials to incorporate the effects of rotation and tidal distortions on the equilibrium structure and periods of small oscillations of rotating stars and stars in binary systems. However his expression for the Roche equipotential accounts for only the effects of centrifugal and gravitational forces and does not take into account the effect of Coriolis force. In this paper we have suitably modified Kopal’s expression for Roche equipotentials to incorporate into it the effect of Coriolis force as well. The modified expression for the Roche equipotential has then been used to compute the equilibrium structures and shapes of polytropic models of rotating stars and stars in binary systems. 相似文献