AGB星核合成理论的研究进展

综述了近扯为ＡＧＢ星核合成的理论研究情况，包括轻，重核素核合成理论，ＡＧＢ星的分类，ＡＧＢ星的演化特征，ＡＧＢ星内的元素核合成理论的研究及外赋ＭＳ，Ｓ 双星吸积机制的研究情况。     

The Significance of Thermally Pulsing Asymptotic Giant Branch Stars in Post-starburst Galaxies

We measure the significance of thermally pulsing asymptotic giant branch(TP-AGB) stars via the spectral energy distributions(SEDs) of a sample of post-starburst(PSB) galaxies at z = 0.2–0.7. Using ground-and space-based photometry from the 3D-HST catalog, as well as associated near-infrared(NIR) Hubble Space Telescope(HST)slitless grism spectroscopy, we evaluate the importance of TP-AGB stars in the SEDs of 177 PSB galaxies by fitting simple stellar populations with different levels of TP-AGB co...     

Asymptotic Giant Branch Stars as Tracers of Star Formation Histories: the GAIA Context

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.     

Circumstellar Shells of the Mass-Losing Asymptotic Giant Branch Stars: Limits for the Dust-Driven Winds

We investigated whether the radiation pressure on dust grains alone may account for driving winds in semiregular, irregular and non-variable AGB stars. A simple theoretical model is employed to calculate the limits for the radiatively driven winds and to compare theoretical predictions with observations. Present analysis indicates, that for most of the objects in the studied sample of 67 oxygen-rich and 40 carbon-rich mass-losing AGB stars radiation pressure on dust grains alone is not effective enough to drive the observed circumstellar outflows.     

Circumstellar envelopes and Asymptotic Giant Branch stars

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     

Asymptotic Giant Branch variables in the Galaxy and the Local Group

AGB variables, particularly the large amplitude Mira type, are a vital step on the distance scale ladder. They will prove particularly important in the era of space telescopes and extremely large ground-based telescopes with adaptive optics, which will be optimized for infrared observing. Our current understanding of the distances to these stars is reviewed with particular emphasis on improvements that came from Hipparcos as well as on recent work on Local Group galaxies. In addition to providing the essential calibration for extragalactic distances Gaia may also provide unprecedented insight into the poorly understood mass-loss process itself.     

Puzzling Origin of CEMP-r/s Stars: An Interpretation of Abundance and Enrichment of s- and r-Process Elements from Asymptotic Giant Branch Supernovae

CEMP-r/s stars at low metallicity are known as double-enhanced stars that show enhancements of both r-process and s-process elements. The chemical abundances of these very metal-poor stars provide us a lot of information for putting new restraints on models of neutron-capture processes. In this article, we put forward an accreted scenario in which the double enrichment of r-process and s-process elements is caused by a former intermediate-mass Asymptotic Giant Branch (AGB) companion in a detached binary system. As the AGB superwind is only present at the ultimate phase of AGB stars, there is thus a lot of potential that the degenerate-core mass of an intermediate-mass AGB star reaches the Chandrasekhar limit before the AGB superwind. In these circumstances, both s-process elements produced in the AGB shell and r-process elements synthesized in the subsequent explosion would be sprayed contemporaneously and accreted by its companion. Despite similarity to physical conditions of a core-collapse supernova, a major focus in this scenario is the degenerate C–O core surrounded by an envelope of a former intermediate-mass AGB donor that may collapse and explode. Due to the existence of an outer envelope, r-process nucleosynthesis is expected to occur. Hypothesizing the material-rich europium (Eu) accreted by the secondary via the wind from the supernova to be in proportion to the geometric fraction of the companion with respect to the exploding donor star, we find that the estimated yield of Eu (as representative of r-process elements) per AGB supernova event is about 1 × 10^?9 M _⊙ ～ 5 × 10^?9 M _⊙. Using the yields of Eu, the overabundance of r-process elements in CEMP-r/s stars can be accounted for. The calculated results show that the value of parameter f , standing for efficiency of wind pollution from the AGB supernova, will reach about 10⁴, which means that the enhanced factor is much larger than unity due to the impact of gravity of the donor and the result of the gravitational focusing effect of the companion.     

AGB星s-过程核合成相关问题的研究进展

综述了近年来AGB星核合成理论的研究情况,述及AGB星的结构与s-过程核合成有关的中子辐照量分布、人们比较关注的铅星与非铅星、后AGB星元素丰度分布及与AGB星核合成有关的s r星。     

Positions and Spectral Energy Distributions of 41 Star Clusters in M33

We present accurate positions and multi-color photometry for 41 star clusters detected by Melnick & D'odorico in the nearby spiral galaxy M33 as a part of the BATC Color Survey of the sky in 13 intermediate-band filters from 3800 to 10 000A. The coordinates of the clusters are found from the HST Guide Star Catalog. By aperture photometry, we obtain the spectral energy distributions of the clusters. Using the relations between the BATC intermediate-band system and UBVRI broadband system, we derive their V magnitudes and B - V colors and find that most of them are blue, which is consistent with previous findings.     

ISO Observations of Symbiotic Stars

A number of symbiotic stars have been observed with ISO. In addition to a number of emission lines, SWS observations of the symbiotic novae RR Tel and V1016 Cyg reveal prominent, broad 10 & 18 μm silicate dust features. The 10 μm features are similar to the crystalline silicate profiles seen in classical novae. There is some evidence that the silicate brightness in V1016 Cyg varies with Mira-component phase. However, the silicate feature in RR Tel also showed some variation even though observations were made at very similar Mira-component phases. PHT observations of S-type symbiotic stars show the IR emission to be dominated by the red-giant component. However, an excess in the PHT-P filters from 10 to 15 μm is evident in all the stars, and there may be a broad 3.2 μm absorption feature or a broad 3.8 μm emission feature. At this time we have no adequate physical explanations for any of these features. This revised version was published online in August 2006 with corrections to the Cover Date.     

Age and Metallicity Distributions among Galactic Disk Stars

Observational studies of the relations between ages, metalicities and kinematics of disk stars in the solar neighbourhood are discussed with emphasis on the recent survey by Edvardsson et al. (1993), and galactic metallicity gradients inferred from these nearby stars are compared with gradients determined from distant B stars and open clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

ISOPHOT Observations of Vega and Vega-like Stars

Detailed ISOPHOT observations of the far infrared excess of α Lyrae (Vega) are presented. The data comprise photometry in the range 25μm to 200μm and high resolution 60μm scan data to compare with 60μm and 90μm oversampled maps. The dust disk around Vega is clearly resolved. In addition similar maps of HD98800, SAO226057, SAO186777 and α Piscis Austrinus (HR8728) are evaluated, resolving the disks of two Vega-like stars/candidates and providing upper limits for the size of the other two. This revised version was published online in August 2006 with corrections to the Cover Date.     

Isophot Mapping Observations of Carbon Stars

Mapping observations have been made toward five carbon stars in the far-infrared using ISOPHOT, an imaging photo-polarimeter on board the Infrared Space Observatory. Cold, very extended dust shells are clearly revealed in two of them. Y CVn is surrounded by a very extended, detached dust shell, which indicates a sudden decline of the mass-loss by two orders of magnitude in the last (1-2) × 104 years on a short time scale. The Hipparcos parallax reinforces our previous conclusion that Y CVn is a J-type carbon star on the asymptotic giant branch. U Ant shows a double shell structure, a compact dust shell surrounded by a very extended one. The outer shell has a brightness comparable to the dust shell of Y CVn. The structure indicates that there were two different high mass-loss phases separated by a period with a much lower mass-loss rate in between the two. The structure is consistent with the double dust shell proposed for this star by Izumiura et al. (1997) based on a detailed investigation of IRAS survey data. This revised version was published online in August 2006 with corrections to the Cover Date.     

26颗亮射电星的观测结果

本文给出了用云南天文台Ⅱ型光电等高仪从1986年10月至1990年12月观测得到的26颗亮射电星的位置改正。     

FUSE Observations of EC14026 Stars

We present a FUSE abundance analysis of EC14026 stars. We compare the abundances of heavy elements in the atmospheres of EC14026 stars to non-pulsating stars with similar atmospheric parameters, and investigate whether weak stellar winds could explain the coexistence of variable and non-variable sdB stars in the log g – T_eff diagram. We also present preliminary results on time-dependent diffusion calculations of iron in presence of radiative levitation and mass loss, and show how weak stellar winds can affect the diffusive equilibrium between gravitational settling and radiative support.