共查询到20条相似文献，搜索用时 250 毫秒
1.
中等质量富金属AGB长周期变星的振动研究 总被引：1，自引：0，他引：1
本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 相似文献
2.
ＡＧＢ长周期变星是中小质量恒星演化到晚期时形成的，由于它们存在强烈的振动和巨大的星风物质损失，研究这类变星对于了解恒星振动性质，星风产生机制及恒星晚期演化都有十分重要的意义，本文比较广泛地考查了最近十几年来国际上对于ＡＧＢ长周期变星观测和理论研究工作的现状，讨论了ＡＧＢ长周期变星的主要观测性质以及恒星振动和演化理论工作中所取得的进展，同时也分析了目前研究工作中存在的一些问题和困难，指出今后研究工作 相似文献
3.
至今国际上对ＡＧＢ星的超星风、热脉动及振动的理论研究工作中都忽略了湍流压的存在，然而对于一个整个外壳都处于完全对流的ＡＧＢ星，湍流压是存在的，并且由于湍流压的存在将对气体压强、密度、温度以及各热力学量产生影响，因而对ＡＧＢ星的超星风、热脉动及振动产生影响。本文以恒星结构演化理论中常用的混合程对流理论为基础，给出了湍流压以及考虑湍流压情况下恒星内部物态方程和各热力学量的表达式，并给出了判别恒星外壳产生动力学非稳定性的判据，并在此基础上研究了质量为２．８Ｍ⊙和７Ｍ⊙的恒星从主序到ＡＧＢ星的演化。目的是研究有湍流压情况下ＡＧＢ星的超星风及热脉动的机理。结果表明，对于７Ｍ⊙恒星，在红巨星及ＡＧＢ阶段，湍流压可以达到总压强的３０％，并且对恒星的结构产生重要影响，对于２．８Ｍ⊙恒星，在红巨星及ＡＧＢ阶段影响较小。由于湍流压的作用，２．８Ｍ⊙和７Ｍ⊙恒星在ＡＧＢ阶段和红巨星阶段，靠近表面的区域会出现动力学非稳定性，造成物质向外喷流，这就是ＡＧＢ星和红巨星产生强大星风的物质损失的原因。研究还表明，湍流压效应造成７Ｍ⊙ＡＧＢ星的氢燃烧壳层产生主周期为５０年的脉动现象，同时使氦燃烧壳层的热脉动提前发生和周期变短 相似文献
4.
中等质量恒星在赫罗图中由E-AGB星进入TP-AGB星的分界点 总被引：1，自引：0，他引：1
通过对3～10 M_☉恒星在赫罗图上演化轨迹的研究,分析恒星内部氦壳层燃烧峰值处能量、密度、温度、氦壳层表面光度与恒星表面光度比及恒星半径的变化,给出了中等质量恒星由早期AGB星演化至热脉冲AGB星阶段在赫罗图上的分界点,与119颗碳星的观测结果吻合得相当好.同时提出:在恒星演化至该分界点之后,其星风物质损失公式可能需要引入一个与表面光度无关的量以主导超星风的形成.在此基础上,通过对考虑湍流压效应下5 M_☉恒星的结构和演化及星风物质损失率的分析,发现湍流压在热脉冲AGB星阶段对星风物质损失影响较大,从而使得热脉冲AGB星的湍流压不可忽略,进而提出了影响热脉冲AGB星星风物质损失的可能的物理因素. 相似文献
5.
本文提供了一个判别红巨星和ＡＧＢ星对流外壳中是否会产生动力学非稳定性的条件．根据这个条件和对２８Ｍ及７Ｍ恒星从主序到巨星阶段和ＡＧＢ阶段的演化计算,可以证明,湍流压效应是导致红巨星和ＡＧＢ星靠近表面区域产生动力学非稳定性,从而造成物质向外喷流的原因．并认为它就是造成红巨星和ＡＧＢ星表面温度很低（因而辐射压很低）从而产生强大星风物质损失的物理原因． 相似文献
6.
在考虑引力对星风物质损失的制约效果及辐射压与湍流压对其扰动效果的同时,通过引入可压缩流机制,考虑了恒星外壳对流区对星风物质损失的促进作用,从而建立了新的星风物质损失公式.之后,通过对3～5 M_⊙恒星理论模型的星风物质损失率计算,发现:从主序直至中心氦核燃烧结束阶段,新的星风物质损失公式计算所得星风物质损失率与经典星风物质损失公式计算结果几乎一致;而在TP-AGB阶段,应用新的星风物质损失公式计算的恒星模型则不受光度影响产生了持续的较大的星风物质损失,比较符合实际观测结果. 相似文献
7.
本提供了一个判别红巨星和ＡＧＢ星对流外壳中是否会产生动力学非稳定性的条件。根据这个条件和对２．８Ｍ⊙及７Ｍ⊙恒星从主序列到巨星阶段和ＡＧＢ阶段的演化计算,可以证明,湍流压效应是导致红居星和ＡＢＧ星靠近表达区域产生动力学非稳定性,从而造成质向外喷流的原因,并认为它就是造成红巨星和ＡＧＢ星表面温度很低（因而辐射压很低）从而产生强大星风物质损失的物理原因。 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.
AGB长周期变星是中小质量恒星演化到晚期时形成的，由于它们存在强烈的振动和巨大的星风物质损失，研究这类变星对于了解恒星振动性质、星风产生机制及恒星晚期演化都有十分重要的意义。本文比较广泛地考查了最近十几年来国际上对于AGB长周期变星观测和理论研究工作的现状，讨论了AGB长周期变星的主要观测性质以及恒星振动和演化理论工作中所取得的进展，同时也分析了目前研究工作中存在的一些问题和困难，指出今后研究工作中需要逐步加以解决的一些问题。 相似文献
10.
以＾１３Ｃ（α,ｎ）＾１６Ｏ及＾２２Ｎｅ（α,ｎ）＾２５Ｍｇ作为双脉冲中子源,对于低质量ＡＧＢ星,采用无分叉ｓ－过程反应通道,结合最近恒星演化的计算结果,在各物理参量合理取值范围内,计算了了ＡＧＢ星Ｈｅ壳层内,表面重核素的丰度,在此基础上将星风吸各模型同内禀ＡＧＢ星核合成模型结合起来计算外赋ＡＧＢ星重元素的超丰。 相似文献
11.
We briefly discuss the current status of our radiation-hydrodynamical models of pulsation in various stellar objects and of dust-driven mass loss in LPVs. We emphasize the importance of a future combined modelling of pulsation and mass loss in AGB stars which has to be based on reliable physical and numerical methods. 相似文献
12.
Recent observations of long-period variable stars at spatial resolutions from approximately 1 arcsec to several milli-arcsecs have provided new insights into pulsation, dust formation, and mass-loss of AGB stars. These insights have come from long baseline interferometric observations obtained across a wide range of wavelengths, from the optical, through the infrared, to wavelengths as long as several millimeters. The present status and recent results from long baseline interferometry, particularly at optical and infrared wavelengths, are discussed. Such results include diameters and limb-darkening, surface features, mode of pulsation, location of SiO masers, inner radii of dust shells, physical conditions in the dust formation zone and of the inner regions of the dust shells. The results are interpreted in terms of present models of dust formation and mass-loss. 相似文献
13.
P. R. Wood 《Astrophysics and Space Science》1994,217(1-2):121-130
The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar. 相似文献
14.
Based on a search for multi‐periodic variability among the semi‐regular red variable stars in the database of the All Sky Automated Survey (ASAS), a sample of 72 typical examples is presented. Their period analysis was performed using the Discrete Fourier Transform. In 41 stars we identified two significant periods each, simultaneously present, while the remaining 31 cases revealed even three such periods per star. They occur in a range roughly between 50 and 3000 days. Inter‐relationships between these periods were analyzed using the “double period diagram” which compares adjacent periods, and the so‐called “Petersen diagram”, the period ratio vs. the shorter period. In both diagrams we could identify six sequences of accumulation of the period values. For five of these sequences (containing 97 % of all data points) we found an almost perfect coincidence with those of previous studies which were based on very different samples of semiregular red variables. Therefore, existence and locations of these sequences in the diagrams seem to be universal features, which appear in any data set of semi‐regularly variable red giants of the AGB; we conclude that they are caused by different pulsation modes as the typical and consistent properties of similar stellar AGB configurations. Stellar pulsations can be considered as the principal cause of the observed periodic variability of these stars, and not binary, rotation of a spotted surface or other possible reasons suggested in the literature. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
J. A. McSaveney P. R.Wood M. Scholz J. C. Lattanzio K. H. Hinkle 《Monthly notices of the Royal Astronomical Society》2007,378(3):1089-1100
High-dispersion near-infrared spectra have been taken of seven highly evolved, variable, intermediate-mass (4–6 M⊙ ) asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible. 相似文献
16.
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
17.
The semi‐regular variable star RU Vulpeculae (RU Vul) is being observed visually since 1935. Its pulsation period and amplitude are declining since ∼1954. A leading hypothesis to explain the period decrease in asymptotic giant branch (AGB) stars such as RU Vul is an ongoing flash of the He‐burning shell, also called a thermal pulse (TP), inside the star. In this paper, we present a CCD photometric light curve of RU Vul, derive its fundamental parameters, and test if the TP hypothesis can describe the observed period decline. We use CCD photometry to determine the present‐day pulsation period and amplitude in three photometric bands, and high‐resolution optical spectroscopy to derive the fundamental parameters. The period evolution of RU Vul is compared to predictions by evolutionary models of the AGB phase. We find that RU Vul is a metal‐poor star with a metallicity [M/H] = –1.59 ± 0.05 and an effective surface temperature of Teff = 3634 ± 20 K. The low metallicity of RU Vul and its kinematics indicate that it is an old, low‐mass member of the thick disc or the halo population. The present day pulsation period determined from our photometry is ∼108 d, the semiamplitude in the V ‐band is 0.39 ± 0.03 mag. The observed period decline is found to be well matched by an evolutionary AGB model with stellar parameters comparable to those of RU Vul. We conclude that the TP hypothesis is in good agreement with the observed period evolution of RU Vul. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
C. Simon Jeffery Hideyuki Saio 《Monthly notices of the Royal Astronomical Society》1999,308(1):221-227
Non-adiabatic linear pulsation models have been calculated for low-mass stars with effective temperatures between 16 000 and 35 000 K, and with surface gravities in the range 3⊙ , X =0.00, Z =0.02. It is shown that the Z -bump instability persists to low masses ( M ∼0.4 M⊙ ) but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected result is the discovery that Z -bump instability persists at hydrogen abundances X >0.3, although the position of the red edge is sensitive to X . We have found that non-radial pulsations are also excited in the same instability region as radial pulsations.
The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z -bump driven pulsations (radial and/or non-radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by the prototype V652 Her. 相似文献
The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z -bump driven pulsations (radial and/or non-radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by the prototype V652 Her. 相似文献
19.
New Variable Stars Discovered as By-product of the Beijing Astronomical Observatory Supernova Survey
The Beijing Astronomical Observatory (BAO) 0.6 m telescope has been used for nearby supernova survey in more than 3000 fields, covering a total area of 235 deg2. More than 260000 CCD images have been collected since April 1996, with 45 supernovae discovered. We searched for variables in about 90000 images taken during 1996-1998. For the fields in which long period variables (LPVs) were discovered, we reduced further images taken from 1999 to 2000, for the period estimation. Among the 280000 stars selected from the survey fields, i.e., brighter than 18 mag, we discovered seven new LPVs and reconfirmed three known LPVs. Additionally, we found 146 variable star candidates, and reconfirmed about 20 previously known or suspected objects. 相似文献
20.
I. S. Glass S. Matsumoto B. S. Carter K. Sekiguchi 《Monthly notices of the Royal Astronomical Society》2001,321(1):77-95
We report here the results of a 4-yr K -band (2.2 μm) survey for large-amplitude variable stars in a area centred on the Galactic Centre. A total of 409 likely long-period variables (LPVs) were detected, for which positions, amplitudes, average magnitudes and periods were obtained whenever possible. The surface density of LPVs is more than ten times greater than in the Sgr I Baade window at
The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction which affects the probability of detecting variables, occurs in less than 25 per cent of the area.
Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K -band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters. 相似文献
The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction which affects the probability of detecting variables, occurs in less than 25 per cent of the area.
Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K -band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters. 相似文献