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1.
Via a study of the evolutionary tracks of 3∼10 M stars on the Hertzsprung-Russell diagram, the variations of the energy, density, temperature at the peak of helium-shell burning, ratio of surface luminosity of helium shell to stellar surface luminosity as well as the stellar radius are analyzed. Then the demarcation point of medium-mass stars in the evolution from early AGB stars to thermally pulsing AGB stars on the HR diagram is determined, and for 119 carbon stars our analysis agrees rather well with observation. At the same time the following is suggested. After arriving at this demarcation point in stellar evolution, in the formula of the loss of stellar wind material it is probably needed to introduce a quantity which is not concerned with the surface luminosity, but it dominates the formation of super stellar wind. On this basis and via the analysis of the structure and evolution of 5 M stars as well as the rate of mass loss of stellar wind, it is found that the effect of turbulent pressure on the mass loss of stellar wind in the stage of thermally pulsing AGB stars is rather great, hence the turbulent pressure of thermally pulsing AGB stars cannot be overlooked. Furthermore, the physical factors which possibly affect the matter loss of the stellar winds of thermally pulsing AGB stars are suggested. 相似文献
2.
渐近巨星分支恒星 (AGB星 )是一种晚期演化恒星 ,它是恒星作为以核反应释能为发光能源的天体的最后演化阶段。AGB星阶段的恒星具有许多有趣的性质 ,如很大的质量损失率 (因此形成很厚的拱星尘埃气体包层 ) ,光变 ,热脉动 (或He闪耀 ) ,强的红外超量发射 ,分子脉泽发射等 ,弄清AGB星的演化规律是研究恒星演化理论的重要任务。目前人们所知道的AGB星的演化图景是 ,恒星经过漫长的主序演化之后 ,将经过红巨星 (RGB)阶段 ,然后才进入AGB阶段 ,在其演化过程中AGB星的光度和质量损失率要逐渐增大 ,它的光变周期也逐渐变长 ,在其中心星经历了一系列的由He核反应不稳定性引起的热脉动之后 ,它的质量损失很快停止 ,恒星开始向行星状星云 (PN)演化 ,最后行星状星云将会变成一个白矮星 ,这将是许多初始质量不很大的恒星的最终结局。OH/IR星阶段是AGB星演化的一个阶段 ,OH/IR星是那些质量稍大的恒星在AGB阶段后期演化而成的天体。现阶段人们对OH/IR星的具体演化过程还知道得很少。我们利用了球对称包层中的尘埃辐射转移模型来研究OH/IR星的演化性质 ,并且收集了尽量多的具有可靠距离的OH/IR星来研究他们的光度和质量损失率的演化性质。在本文的研究工作中 ,我们主要讨论了OH/IR星在远红外双色图中的分布规律 ,还发现 相似文献
3.
概述了ＩＲＡＳ升空以来在ＡＧＢ星研究方面的进展和发现的问题,比较详细地报告了６０ｃｍ空间红外望远镜ＩＳＯ携带的探测器及其性能,以及它的成像和光谱观测对研究ＡＧＢ星的演化的影响,尤其是对ＡＧＢ星星周包层的化学环境的研究的重要作用。 相似文献
4.
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
5.
As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity. 相似文献
6.
We collected 55 galactic extreme carbon stars from the published literature in this paper. Observational data from IRAS, 2MASS
and ISO were analyzed. The results show that the infrared properties of extreme carbon stars are quite different to those
for ordinary visual carbon stars. It is shown from IRAS and 2MASS photometric data that extreme carbon stars have much redder
infrared colors not only in the far infrared, but also in the near infrared, hence they have much thicker ciucumstellar envelopes
and mass loss. It is also indicated from IRAS Low-Resolution Spectra (LRS) and ISO Short Wavelength Spectra (SWS) that they
have much redder infrared spectra from 2 μm to 45 μm. The above results are believed to be the signature of undergoing the
last stages of AGB evolution for extreme carbon stars. 相似文献
7.
Arūnas Kučinskas Vladas Vansevičius Toshihiko Tanabé 《Astrophysics and Space Science》2002,280(1-2):151-157
We discuss a possible use of the asymptotic giant branch (AGB) stars for tracing star formation histories on the Galactic
and extragalactic distance scales with the ESA's astrometric space mission GAIA. Extensive numerical simulations demonstrate
that metallicities (Δ [M/H] ≲ 0.3) can be obtained for the AGB stars with GAIA up to the distances of ∼ 200 kpc, if no interstellar
extinction is present. Reliable population ages can be also obtained from the AGB stars if their T
eff are constrained precisely. We show that precise effective temperatures can be obtained by fitting observed spectral energy
distributions of the AGB stars with theoretical fluxes calculated from the synthetic spectra. A combination of the derived
effective temperatures with the bolometric luminosities allows to derive precise population ages for a wide range of ages
and metallicities over the large distance scales. This demonstrates that AGB stars can be employed very effectively for tracing
star formation histories with GAIA, allowing to refine the global evolutionary scenarios of stellar populations in the Milky
Way and the galaxies beyond.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
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
9.
K. Justtanont I. Yamamura T. De Jong L.B.F.M. Waters 《Astrophysics and Space Science》1997,251(1-2):25-30
In this contribution, we present a few highlights of the guaranteed time program to observe AGB stars with differing chemical
compositions and mass loss rates using ISO1 SWS. We briefly discuss C2H2 absorption in C-stars and O-rich stars with the 13 μm dust emission.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
10.
M. Jura 《Astrophysics and Space Science》1997,251(1-2):81-88
We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red
Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks
has a spike with a width near ∼2 km s−1, indicating disk radii of ∼1016 cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter
wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be
many as yet undiscovered disks around AGB stars
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
11.
H.J. Habing 《Astronomy and Astrophysics Review》1996,7(2):97-207
Summary.
Red giants are sometimes surrounded by envelopes, the
result of the ejection of stellar matter at a large rate
(/yr)
and at a low velocity (10 km/s). In this review
the envelopes are discussed and the relation between stars and
envelope: what stars combine with what envelopes?
The envelope emits radiation by various processes and has been
detected at all wavelengths between the visual and the microwave
range. I review the observations of continuum radiation emitted by
dust particles and of rotational transitions of molecules, where
these molecules have been excited by thermal or by non–thermal
(“maser”) processes. I discuss mainly the oxygen–rich
stars, those of spectral type M, and only briefly the closely related
carbon–rich stars.
By and large the density in the envelope is well described by
spherically symmetric outflow at a constant velocity; on the time
scale needed to flow from stellar surface to the outermost layers,
i.e.
yr, the loss of mass is sometimes interrupted suddenly
after which the envelope becomes “detached” from the star. The
temperature decreases when moving outward; heat input is by
friction between dust particles and gas and cooling occurs by line
radiation by various molecules, especially by
HO. The molecular
composition is determined by formation in an equilibrium process
deep in the atmosphere and by destruction in the outer parts of the
outflow by interstellar UV radiation
(H, CO, HO) or by
depletion due to condensation on dust grains (SiO); dust particles
of silicate material solidify where the radiation temperature is
decreased to about 1000 K, and this is at a few stellar radii.
The various continuum spectra produced by the dust particles in
different stars are well modelled by a simple model of the density
and dust temperature distribution plus the assumption that the
particles consist of “dirty silicate”, i.e. silicate with Fe and Al
ions added. A large range of optical depths,
, is observed:
from 0.01 to 10. In envelopes with large optical depth the star
itself can no longer be detected directly. Model calculations also
show that the momentum in the outflow, i.e.
is
provided by radiation pressure on the dust particles followed by
the complete transfer of this momentum to the gas. The mass–loss
rate itself,
, is not determined by radiation pressure but by
dynamic processes in the region below the dust condensation layer.
When
is sufficiently large its measurement, that of the
stellar luminosity,
and that of the outflow velocity,
,
permit the determination of
, i.e. the total
outflow rate, without making assumptions about the abundance of the dust
particles or of the molecular gases. Detached envelopes have been
seen in a few cases.
Thermal molecular radiation is faint compared to the maser emission
but has been measured in distant stars, e.g. in stars near the
galactic center. Different molecules outline different “spheres”
around the star. The largest sphere (a radius of 0.1 pc) is
outlined by an emission line belonging to the
CO()
transition. Higher rotational transitions of CO give smaller
diameters. A comparison of CO
() and () fluxes
in stars with very thick envelopes leads to the conclusion that an
abrupt decrease in the mass–loss rate occurred some ten thousand
years ago.
Three molecules produce each several maser lines: SiO,
HO and
OH. Several new
HO lines have recently been discovered; their
exploration has hardly been started. The high intensity of the
maser lines makes interferometry possible and hence detailed
mapping. The SiO lines are formed deep in the envelope, below the
dust condensation layer. OH maser lines are produced farthest out,
HO lines in
between. The excitation mechanisms for most maser
lines is understood globally, but detailed models are lacking,
largely because the problem is non–linear and the solution of the
radiative transfer equation requires a highly anisotropic geometry.
The geometrical and kinematical properties of the 1612 MHz OH
maser, which in many objects is very strong, are explained by a
thin shell of OH; because the angular diameter of the shell can be
measured directly and the linear diameter can be determined from
the difference in the time of maximum flux of blue and red maser
peaks, the distance of the shell and of the star can be measured.
The presence or absence of individual maser lines appears to depend
on the value of
and is well described by a sequence called
“Lewis' chronology”.
The central star is a long–period variable with a period of 300
days or longer and with a large luminosity amplitude
().
Evidence is given that each star has the
maximum luminosity it will reach during its evolution and that it
is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB)
with a main–sequence mass between 1 and 6
. Stars of the same
main–sequence mass,
,
have different mass–loss rates, in
some cases by a factor of 10. The mass–loss rate probably
increases with time, and the highest mass–loss rates are reached
toward the end of the evolution. Stars with higher
ultimately
reach higher mass–loss rates. The calibration of the
main–sequence mass is reviewed. Most Mira variables with mass loss
have a mass between 1.0 and 1.2
. OH/IR stars with periods
over 1000 days have no counterparts among the carbon stars and thus
have .
Stars as discussed in this review have been
found only in the thin galactic disk and in the bulge.
Finally I review several recently proposed scenarios for TP–AGB
evolution in which mass loss is taken into account. These scenarios
represent the observations quite well; their major short–coming is
the lack of an explanation why the central stars are always
large–amplitude, long–period variables and why such stars are the
ones with high mass–loss rates.
Received: 10 January 1996 相似文献
12.
本文采用恒星演化计算与恒星振动计算相结合的方法，对中等质量富金属恒星演化到渐近巨星分支时的振动性质进行了分析研究，从理论上得出这类恒星的振动方式是处于一阶谐频振动，而振动的激发则是在氢电离区和氦的二次电离区由多种机制共同作用造成的，同时提出很长周期的ＡＧＢ长周期变星只能是由中等质量恒星演化到ＡＧＢ阶段形成的。我们的理论计算结果还比较支持在ＡＧＢ顶端存在巨大星风物质损失的观点，且这种星风物质损失很可能与恒星振动有关。 相似文献
13.
Albert A. Zijlstra 《Astrophysics and Space Science》1995,224(1-2):309-320
Mass loss dominates the stellar evolution on the Asymptotic Giant Branch. The phase of highest mass-loss occurs during the last 1–10% of the AGB and includes the so-called Miras and OH/IR stars. In this review I will discuss the characteristics and evolution of especially Miras, and discuss how they are linked to the mass loss. There are indications that high mass-loss rates are only reached for relatively young stars with massive progenitors. The mass loss rates vary both on long and short time scales: the short-term variations are likely linked to luminosity variations associated with the thermal-pulse cycle. The influence of mass loss in the post-AGB phase is also discussed. 相似文献
14.
G. R. Knapp 《Astrophysics and Space Science》1995,224(1-2):333-333
Copious mass loss on the Asymptotic Giant Branch dominates the late stages of stellar evolution. Maps of extended circumstellar envelopes provide a history of mass loss and trace out anisotropic mass loss. This review concentrates on observations of millimeter wavelength molecular line emission, on high resolution maps of maser emission and on observations of submillimeter, millimeter and radio wavelength continuum emission. Radio continuum observations show that AGB stars are larger at radio than at optical wavelengths. The extended chromospheres indicated by these observations extend to distances from the star large enough for dust to form, thereby initiating mass loss. Molecular line maps have found time-variable mass loss for some stars, including detached shells indicating interrupted mass loss and evidence for a rapid increase in the mass loss rate at the end of the AGB phase. Maps of circumstellar envelopes show evidence of flattening, bipolar outflow and angular variations in both the mass loss rate and the outflow velocity. As stars evolve away from the AGB and planetary nebula formation begins, these structures become more pronounced, and fast bipolar molecular winds are observed. The time scales derived from the dynamical times of these winds and from the expansion rates of the central planetary nebulae are very rapid in some cases, about 100 years, in agreement with the predictions of stellar evolution theory. 相似文献
15.
A. Niedzielski 《Astrophysics and Space Science》2003,286(1-2):237-244
The Wolf-Rayet stars represent an advanced stage of evolution of the most massive stars. Their next immediate stadium will
be supernova explosion. The most striking property of this very rare but exceptionally hot and bright objects is their extreme
mass loss, of the order of 10-5 solar mass per year. In turn of evolution before and during the Wolf-Rayet phase such stars eject a lot of matter (∼ 10 M⊙) with velocity up to 3000 km/s that surrounds the min the form of gas and dust. In the case of binary Wolf-Rayet star such
expanding envelope may interact with a companion (usually hot OB star) wind forming a tail extended for ∼ 100 AU. This spectacular
phenomenon as well as some other connected with Wolf-Rayet stars that can be studied with high spatial resolution instruments
(both astrometric and imaging) are reviewed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
H. Izumiura D. J. M. Kester T. De Jong C. Loup L. B. F. M. Waters Tj. R. Bontekoe 《Astrophysics and Space Science》1995,224(1-2):495-496
We have examined forty-two carbon stars which show excess emission at 60 and/or 100µm by applying maximum-entropy image reconstruction techniques to the IRAS 60µm survey data. Thirteen stars are found to be extended in the reconstructed images. Four of them show a detached ring centered on the stellar position. In particular, U Ant may have a double detached dust shell. The implications of our results are discussed concerning the variation of mass loss on the AGB evolution. 相似文献
17.
18.
O. Hashimoto H. Izumiura D. J. M. Kester Tj. R. Bontekoe 《Astrophysics and Space Science》1995,224(1-2):477-478
Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period. 相似文献
19.
Maria LUGARO Carolyn L. DOHERTY Amanda I. KARAKAS Sarah T. MADDISON Kurt LIFFMAN D. A. GARCÍA‐HERNÁNDEZ Lionel SIESS John C. LATTANZIO 《Meteoritics & planetary science》2012,47(12):1998-2012
Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., 26Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range approximately 7–11 M⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 108 K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce 26Al/27Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 102 to several 103 of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ17O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as 107Pd. 相似文献
20.
New Claret evolutionary model-tracks, constructed for the first time for studying close binary systems (CBS) including tidal
evolution constants, are used to determine the age of 112 eclipsing-variable stars in the Svechnikov-Perevozkina catalog by
the method of isochrones. There is some interest in comparing the calculated ages with previous estimates obtained for these
same close binary systems using evolutionary modeltracks for individual stars taking their mass loss into account. A correlation
of the ages of the principal and secondary components is noted, which is most marked for massive close binaries with principal
components having masses M1 ≥ 3 M⊙. A rejuvenating effect is found to occur for the systems studied here as calculated on the new tracks; it is most distinct
for low-mass close binaries with a total mass M1 + M2 ≤ 3.5 M⊙ and is predicted theoretically in terms of magnetic braking. The calculated broadband grid of isochrones, from zero-age main-sequence
(ZAMS) to the age of the galaxy, can be used for estimating the ages of close binaries from other catalogs. Ages are given
for the 112 eclipsing-variable close binaries with detached components lying within the main sequence.
__________
Translated from Astrofizika, Vol. 50, No. 2, pp. 299–312 (May 2007). 相似文献