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1.
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
2.
A. Noels J. Montalban A. Miglio M. Godart P. Ventura 《Astrophysics and Space Science》2010,328(1-2):227-236
Overshooting and semiconvection are two poorly known mechanisms which affect the extent and the efficiency of chemical mixing outside classical convection zones in stars. We discuss the uncertainties and the inferences of those processes in main sequence stars burning hydrogen in a convective core. We then focus on the asteroseismic signatures of partially or fully mixed zones surrounding the convective core, through the detailed shape of the induced chemical composition profile. We emphasize the potential power of asteroseismology to determine the internal structure of stars and thus to help us understand the physical processes at work inside the stars. 相似文献
3.
João P. Marques Mário J. P. F. G. Monteiro João Fernandes 《Monthly notices of the Royal Astronomical Society》2006,371(1):293-302
We discuss the effects of convective overshooting in the pre-main-sequence (PMS) evolution of intermediate-mass stars, by analysing in detail the early evolution towards the main sequence of a 2 M⊙ stellar model. These effects can be extremely important in the end of the PMS, when the abundances in CNO elements approach the equilibrium in the centre. We provide a possible physical explanation on why a moderate amount of overshooting produces, as the star approaches the zero-age main-sequence, an extra loop in the evolutionary tracks on the Hertzsprung–Russell diagram.
An interesting feature is that there is a very well defined amount of overshooting (for a given stellar mass and chemical composition) beyond which a loop is produced. For smaller amounts of overshooting such a loop does not take place and the evolutionary tracks are similar to those found in the literature. The amount of overshooting needed to produce the loop decreases with stellar mass.
We discuss the underlining physical reasons for the behaviour predicted by the evolution models and argue that it provides a crucial observational test for convective overshooting in the core of intermediate-mass stars. 相似文献
An interesting feature is that there is a very well defined amount of overshooting (for a given stellar mass and chemical composition) beyond which a loop is produced. For smaller amounts of overshooting such a loop does not take place and the evolutionary tracks are similar to those found in the literature. The amount of overshooting needed to produce the loop decreases with stellar mass.
We discuss the underlining physical reasons for the behaviour predicted by the evolution models and argue that it provides a crucial observational test for convective overshooting in the core of intermediate-mass stars. 相似文献
4.
本文利用国际上最新公布的OPAL不透明度表及相应的化学丰度，计算了三颗不同初始质量的星族互恒星的非守恒演化模型，并和过去长期采用的LAOL不透明度的结果进行详细比较，得出以下主要结论：（1）恒星HR图中的演化轨迹光度降低，主序位置明显向红方向移动；（2）恒星中心H，He燃烧寿命大大延长了；（3）小质量星氢燃烧核和对流核都变小，大、中质量星的中心对流核和燃烧核都有所增大；（4）恒星中心温度－密度关系基本不受影响． 相似文献
5.
In the evolutionary phase of central helium burning, models of horizontal-branch stars reach a stage at which an intermediate region is unstable against convection. This feature is due to the progressive increase of the core mass by overshooting during the previous evolutionary stages and to the behaviour of the radiative temperature gradient at the core boundary. By consideration of the typical time scales of propagation of convection and nuclear burning, a partial mixing (semi-convection) can be induced in a region around the convective core.
Riassunto Come conseguenza del progressivo aumento in massa per overshooting del nucleo convettivo nella fase iniziale di combustione centrale dell'elio in stelle di ramo orizzontale e per l'andamento del gradiente radiativo della temperatura al bordo del nucleo, viene raggiunto uno stadio nel quale una zona intermedia è instabile rispetto alla convezione. Dalla considerazione dei tempi caratteristici di propagazione della convezione e di combustione nucleare, si conclude che l'instabilità può indurre un mescolamento parziale (semiconvezione) in una regione esterna al nucleo.相似文献
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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
8.
In this paper we briefly discuss the effect of complete homogenization in the intermediate unstable layers of massive stars on the effective temperature of the core He-burning models. To this end, a 20M
star of Population I chemical composition (X=0.700,Z=0.020) has been allowed to evolve from the Main Sequence into the core He-exhaustion stage without taking into account semiconvective mixing. The results show that the models are systematically bluer than those computed with the same physical parameters but with the inclusion of semiconvection. 相似文献
9.
Wu-Ming Yang Shao-Lan Bi 《中国天文和天体物理学报》2008,8(6)
We investigate the rotation profile of solar-like stars with magnetic fields. A diffu-sion coefficient of magnetic angular momentum transport is deduced. Rotating stellar models with different mass incorporating the coefficient are computed to give the rotation profiles. The total angular momentum of a solar model with only hydrodynamic instabilities is about 13 times larger than that of the Sun at the age of the Sun, and this model can not reproduce quasi-solid rotation in the radiative region. However, the solar model with magnetic fields not only can reproduce an almost uniform rotation in the radiative region, but also a total angular momentum that is consistent with the helioseismic result at the 3 σ level at the age of the Sun. The rotation of solar-like stars with magnetic fields is almost uniform in the radiative region, but for models of 1.2-1.5 M⊙, there is an obvious transition region between the convective core and the radiative region, where angular velocity has a sharp radial gradient, which is different from the rotation profile of the Sun and of massive stars with magnetic fields. The change of angular velocity in the transition region increases with increasing age and mass. 相似文献
10.
E. I. Staritsin 《Astronomy Letters》2009,35(6):413-423
We consider the evolution of a rotating star with a mass of 16M ⊙ and an angular momentum of 3.25 × 1052 g cm2 s?1, along with the hydrodynamic transport of angular momentum and chemical elements in its interiors. When the partial mixing of matter of the turbulent radiative envelope and the convective core is taken into account, the efficiency of the angular momentum transport by meridional circulation in the stellar interiors and the duration of the hydrogen burning phase increase. Depending on the Schmidt number in the turbulent radiative stellar envelope, the ratio of the equatorial rotational velocity to the circular one increases with time in the process of stellar evolution and can become typical of early-type Be stars during an additional evolution time of the star on the main sequence. Partial mixing of matter is a necessary condition under which the hydrodynamic transport processes can increase the angular momentum of the outer stellar layer to an extent that the equatorial rotational velocity begins to increase during the second half of the evolutionary phase of the star on the main sequence, as shown by observations of the brightest stars in open star clusters with ages of 10–25 Myr. When the turbulent Schmidt number is 0.4, the equatorial rotational velocity of the star increases during the second half of the hydrogen burning phase in the convective core from 330 to 450 km s?1. 相似文献
11.
S. V. Chernov 《Astronomy Letters》2017,43(6):429-437
Zahn’s theory of dynamical tides is analyzed critically. We compare the results of this theory with our numerical calculations for stars with a convective core and a radiative envelope and with masses of one and a half and two solar masses. We show that for a binary system consisting of stars of one and a half or two solar masses and a point object with a mass equal to the solar mass and with an orbital period of one day under the assumption of a dense spectrum and moderately rapid dissipation, the evolution time scales of the semimajor axis will be shorter than those in Zahn’s theory by several orders of magnitude. 相似文献
12.
The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M
⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with
heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars
for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M
⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10−6 and 10−9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously.
For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and
semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations.
Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive
stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic
stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars
are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics
of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects;
also they develop large convective cores with important helium core masses which are important for explosion calculations. 相似文献
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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
15.
A. G. Massevitch E. I. Popova A. V. Tutukov L. R. Yungelson 《Astrophysics and Space Science》1979,62(2):451-463
Evolution of massive stars losing mass with the rateM H L/V
C is computed (for =1,2,7). It is shown that observed mass loss rates correspond to 0.3 and, therefore, mass loss by stellar wind cannot play any significant role in the evolution of normal massive stars. However, for several types of massive stars (WR, OH/IR, X-ray sources) enhanced mass loss explains their peculiar features. Computations of evolutionary sequences of massive stars with convective overshooting taken into account (as a formal increase of the convective core) show that a significant broadening of the hydrogen-burning band in the H-R diagram may be obtained. 相似文献
16.
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
17.
L. Hartmann 《Solar physics》1985,100(1-2):587-597
Winds are directly detected from solar-type stars only when they are very young. At ages 106 yr, these stars have mass loss rates 106 times the mass flux of the present solar wind. Although these young T Tauri stars exhibit ultraviolet transition-region and X-ray coronal emission, the large particle densities of the massive winds lead to efficient radiative cooling, and wind temperatures are only 104 K. In these circumstances thermal acceleration is unlikely to play an important role in driving the mass loss. Turbulent energy fluxes may be responsible for the observed mass loss, particularly if substantial magnetic fields are present.The presence of stellar mass loss is indirectly shown by the spindown of low-mass stars as they age. It appears that many solar-mass stars spin up as they contract toward the Main-Sequence, reaching a maximum equatorial velocity of 50 to 100 km s–1. These stars spin down rapidly upon reaching the Main Sequence. Spindown may be enhanced by a decoupling or lag between convective envelope and radiative core. Because this spindown occurs fairly early in a solar-type star's history, the internal structure of old stars like the Sun may not depend upon initial conditions. 相似文献
18.
Xiang-Jun Lai Yan Li National Astronomical Observatories /Yunnan Observatory Chinese Academy of Sciences Kunming China Key Laboratory for the Structure Evolution of Celestial Objects China Graduate University of Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2011,(11)
Based on the turbulent convection model(TCM),we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars,focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell(HR) diagram.A diffusive mixing model is adopted during the Red Giant Branch(RGB) phase.The properties of the blue loop are changed by modification of the element profiles above the H-burning shell,which results from the incomplete mixing in the bottom... 相似文献
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《New Astronomy Reviews》2004,48(9):731-739
Extragalactic eclipsing binaries open a new perspective on the study of stellar structure and evolution. Stars in different galaxies have formed and evolved in environments with chemical histories that may differ from those of the solar neighborhood. For example, the LMC and SMC contain low-metallicity, young massive stars that are no longer found in our Galaxy. Eclipsing binaries, yielding accurate determinations of masses, radii and temperatures, allow for critical tests of, e.g., convective overshooting, mass loss, and internal structure. In addition, they provide an empirical calibration of the mass-luminosity relationship. In this paper, I present some results from ongoing programs on the determination of physical properties and distances to eclipsing binaries in the LMC, the SMC, and M31. In particular, I focus on aspects relevant to stellar astrophysics, and on the contributions of these eclipsing binaries to our understanding of the structure and chemical evolution of the host galaxies. 相似文献