共查询到20条相似文献，搜索用时 15 毫秒
1.
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
2.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
3.
Garrelt Mellema 《Astrophysics and Space Science》2001,275(1-2):147-157
This review discusses the physics of the formation of planetarynebulae around low mass WR stars, or [WR] stars. It especially focuseson the differences which can be expected due to the differentcharacter of the fast winds from these [WR] stars. Their fast windsare more massive and are highly H deficient and metal enrichedcompared to the winds of normal central stars of planetarynebulae. This is expected to lead to faster expansion velocities forthe nebulae and a longer momentum-driven phase in the evolution of thewind-driven bubble, leading to more turbulent nebulae. Theobservational evidence also shows that the process which produces the[WR] stars is unlikely to influence the onset of aspherical mass loss,something which can be used as a test for models for aspherical massloss from AGB and post-AGB stars. Finally it is shown that thenebular characteristics rule out a very late He shell flash as theorigin of most [WR] stars. 相似文献
4.
D. C. V. Mallik 《Journal of Astrophysics and Astronomy》1981,2(2):171-185
The current rate of nucleosynthesis in the solar neighbourhood is re-evaluated on the basis of Arnett’s (1978) stellar yields,
the mass loss models of Chiosi, Nasi and Sreenivasan (1978) and the initial mass function determined by Lequeux (1978). If
massive stars are held responsible for most of the metals we observe, a higher birthrate of these stars in the past is indicated
in view of the low current rate of nucleosynthesis. The intermediate mass stars may not supply the bulk of the metals unless
total disruption of their carbon core takes place.
While a declining birthrate is in conflict with the result obtained from the age-metallicity relation of stars, it is supported
by some galactic evolution models which interpret successfully the white dwarf mass distribution data. If the constraint of
a nearly time-invariant birthrate were strictly accepted, then models of the prompt initial enrichment type are required to
explain the observed abundances in terms of nucleosynthesis in massive stars. 相似文献
5.
M. V. Mekkaden 《Astrophysics and Space Science》1985,110(2):413-416
A study of the relationship between soft X-ray luminosity and Hei 10830 equivalent width in RS CVn binaries and very active late-type Main-Sequence stars is made. Long-period RS CVns and very active F and G Main-Sequence stars show strong dependence on their X-ray luminosities. This is attributed to the dominance of coronal excitation of the line in these stars. Short-period RS CVn binaries have lower Hei 10830 absorption compared to their X-ray emission. This phenomenon is explained by the presence of Hei emission region also in their chromospheres.On leave from Indian Institute of Astrophysics, Bangalore, India. 相似文献
6.
7.
We present quantitative analyses of 7 galactic O stars of spectral types from O3 to O6. By means of classical hydrostatic
and planeparallel models we find the so-called He and Mass Discrepancies for a number of stars of our sample. We also prove
the need of line-blocking for the correct reproduction of these spectra. For stars with effective temperature around 50 000
K and above, there is no possibility of reproducing their spectra with these kind of models. Further analyses with unified
models, including sphericity and mass loss, allow us to reproduce even the earliest spectra. They also show that the Mass
Discrepancy is not only due to the neglect of sphericity and mass loss in the planeparallel analysis, and give us the mass
loss rate of these stars. We find that HD15 570, with a present mass of 100 M⊙, can be the most massive star known in the solar neighbourhood.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
大质量双星系统的非守恒演化 总被引：1，自引：0，他引：1
由于大质量双星系统有强大的星风物质损失,因而在研究其结构和演化时必须考虑星风物质损失,动量损失,物质交换以及由以上原因引起的轨道参量的变化,此外,天文观测又证实,一些大质量双星系统中存在星风冲击波,有Ｘ射线辐射以及有致密天体（白矮星,中子星）的存在,因此在研究大质量双星的演化时,又会遇到在星风冲击波理论及其对演化的影响,双星系统何时会演化成为公共外壳的系统,以及双星系统中如果发生超新星爆发,是否会 相似文献
9.
The mass-radius relation is determined for Main-Sequence stars from observational data of well-detached binary systems. The results are compared to previous empirical relations and we discuss their application to the study of the light curves of eclipsing binaries. A comparison with theoretical Main-Sequence models has been carried out. 相似文献
10.
恒星形成于分子云环境中。近30多年的观测研究使得天文学家对小质量恒星的形成有了相对明确的认识：小质量恒星通过坍缩、吸积和外向流的路标而形成。至于大质量恒星，其形成过程还存在着许多不确定因素，现有的观测证据表明：大质量恒星也可能通过坍缩、吸积和外向流的路标来形成，但也不排除在星团中通过中小质量恒星聚合而成的因素。大质量恒星形成与致密电离氢区(UCHII)成协较好，而与大质量恒星形成区成协的分子云环境中，既有大质量恒星也有小质量恒星形成。综述了恒星形成各个阶段的观测结果和研究现状以及成协的天体物理环境情况。未来的观测和研究重点在于，大质量恒星形成以及星团环境中的恒星形成。 相似文献
11.
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
12.
Ju. L. Frantsman 《Astrophysics and Space Science》1988,145(2):287-292
The observed maximum luminosities of AGB stars in the Magellanic Cloud clusters were compared with luminosities obtained theoretically in order to estimate the ages of the clusters. Several rates of mass loss by AGB stars are considered. The ages for 10 clusters in the SMC and 25 in the LMC are presented in the tables with the compilations of ages from the literature quoted. The discrepancies between ages derived from the AGB peak luminosities and from the Main-Sequence turn off and maximum luminosity are explained by allowing for intensive mass-loss during the AGB evolutionary phase. The method is useful only if the brightest star is on the thermally pulse AGB phase but not in the early-AGB phase. 相似文献
13.
The initial condition of the formation of massive stars is still unclear at present. In particular, it is still debatable whether or not massive stars are formed in the cluster center. Some people considered from the viewpoint of time scale and thought that the mass segregation phenomena in embedded clusters means that the massive stars can only be born in the cluster center. In this paper we used the Monte Carlo method to make numerical simulation of the dynamical evolution of embedded clusters and the result is compared with the observations. It is assumed that at the initial time massive stars are randomly distributed. It was found that, due to the random motions of massive stars, temporary mass segregation may exist at certain times in the course of evolution of a given embedded cluster, and this phenomenon may be very prominent in some of them. It is pointed out that massive star formation in the center is not the only explanation for mass segregation in embedded clusters. In addition, dynamical friction from the gas can effectively reduce the time scale of the dynamical mass segregation. In consequence, the probability of temporary mass segregation is increased. 相似文献
14.
Nilgün Kiziloĝlu 《Astrophysics and Space Science》1992,192(1):83-89
The evolutionary behaviour of rotating solar models with different initial angular-momentum distributions has been investigated through the pre-Main-Sequence and Main-Sequence phases. The angular momentum was removed from the convective evelope of the solar models according to the Kawaler's model of magnetic stellar wind (Kawaler, 1988). The models show that (i) the surface rotational velocities of the solar mass stars are independent of initial angular momentum for ages greater than 108 years and (ii) it is not possible to explain the neutrino problem and the sufficient depletion of lithium in the Sun. 相似文献
15.
We have investigated the evolutionary behaviour of intermediate mass (2, 3, 4, 5, and 7M
) Population I stars, assuming two different rates of rotation at the threshold of stability.In the first part of the study, stars are assumed to start with a critical rotation (fast rotation model) and to progress to the point of rotational instability. The stars evolve by losing mass and become rotationally unstable before they reach the zero-age Main Sequence. It is argued that multiple star systems might be formed through the evolution of rapidly rotating stars. An expression for the rotational mass loss rate is derived as a function of the physical parameters of stars.In the second part of the study, stars are assumed to rotate at a rate below the critical value (slow rotation model). The evolution of slowly rotating stars is followed as far as zero-age Main Sequence on the theoretical Hertzsprung-Russell diagram and compared with that of normal stars. The evolutionary paths are found to be more or less similar to those of normal stars; but their positions on the Main Sequence are characterized by effective temperatures and luminosities lower than those of normal stars. The zero-age Main-Sequence times of these stars are longer than those of normal stars. The rotational rates obtained for the zero-age Main Sequence are in good agreement with observed values. 相似文献
16.
邓李才 《紫金山天文台台刊》2003,(2)
大质量恒星由于其高光度、短寿命和质量损失 ,对星系的积分光谱能量分布和重元素增丰起主导作用 ,从而在研究星系的形成和演化上具有特殊的意义。特别是随着天文设备的长足进展 ,我们可以回溯宇宙演化的历史 ,得到形成初期时星系的观测性质。那时 ,大质量恒星主导星系的辐射性质 ,这更加突出了对大质量恒星进一步了解的迫切性。但是大质量恒星的演化性质相对中小质量恒星而言 ,有很多不确定性。本文通过对比现有恒星模型与实测结果 ,对现有大质量恒星演化理论中存在的几个与对流和质量损失相关的问题进行了评述 ,并对从理论上 ,特别是通过数字模拟方法对这些问题进行诊断提出了展望。 相似文献
17.
U. Hopp 《Astrophysics and Space Science》1980,72(1):233-236
By star counts on four plates with different limiting magnitudes, it was found that the relative frequency of Main-Sequence stars increases from the centre of the cluster to the boundary. It is shown that the stars of NGC 7789 are separated according to their masses. 相似文献
18.
Allan J. Willis 《Astrophysics and Space Science》1996,237(1-2):145-168
This paper reviews the current status of knowledge regarding the basic physical and chemical properties of Wolf-Rayet stars;
their overall mass loss and stellar wind characteristics and current ideas about their evolutionary status. WR stars are believed
to be the evolved descendents of massive O-type stars, in which extensive mass loss reveals successive stages of nuclear processed
material: WN stars the products of interior CNO-cycle hydrogen burning, and WC and WO stars the products of interior helium
burning. Recent stellar evolution models, particularly those incorporating internal mixing, predict results which are in good
accord with the different chemical compositions observationally inferred for WN, WC and WO stars. WR stars exhibit the highest
levels of mass loss amongst earlytype stars: mass loss rates, typically, lie in the range [1–10]×10−5
M
⊙yr−1. Radiation pressure-driven winds incorporating multi-scattering in high ionisation-stratified winds may cause these levels,
but additional mechanisms may also be needed. 相似文献
19.
Massimo Marengo &#;eljko Ivezi&#; Gillian R. Knapp 《Monthly notices of the Royal Astronomical Society》2001,324(4):1117-1130
We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically significant. In particular, we find that these stars segregate in the K–[12] versus [12]–[25] colour–colour diagram, and have distinct properties of the IRAS LRS spectra, including the peak position of the silicate emission feature. We show that the infrared emission from the majority of non-Mira stars cannot be explained within the context of standard steady-state outflow models.
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch. 相似文献
The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical silicate grain condensation temperatures (800–1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (>500 K) in their envelopes.
The absence of hot dust can be interpreted as a recent (∼100 yr) decrease in the mass-loss rate. The distribution of best-fitting model parameters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be supported by a number of spatially resolved observations (e.g. recent Hubble Space Telescope images of the multiple shells in the Egg Nebula) and is consistent with new dynamical models for mass loss on the asymptotic giant branch. 相似文献
20.
Dany Vanbeveren Houria Belkus Joris Van Bever Nicki Mennekens 《Astrophysics and Space Science》2009,324(2-4):271-276
In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics.
- The formation and the evolution of very massive stars (with masses >120 M⊙) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collisions of the 10–20 most massive stars in the cluster (this process is known as ‘runaway merging’). The further evolution is governed by stellar wind mass loss during core hydrogen and core helium burning (the WR phase of very massive stars). Our simulations reveal that, as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed, but with a maximum mass ≈70 M⊙. In low-metallicity clusters, however, it cannot be excluded that the runaway-merging process is responsible for pair-instability supernovae or for the formation of intermediate-mass black holes with a mass of several 100 M⊙.
- Massive runaways can be formed via the supernova explosion of one of the components in a binary system (the Blaauw scenario), or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g. ζ Pup, λ Cep, BD+43°3654) are the product of the collision and merger of two or three massive stars.