SYNTHETIC AGB EVOLUTION .1. A NEW MODEL
SYNTHETIC AGB EVOLUTION .1. A NEW MODEL
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DOI：
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发表时间：
1993
影响因子：
6.5
通讯作者：
M. Groenewegen;T. Dejong
中科院分区：
文献类型：
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作者：
M. Groenewegen;T. Dejong
我们建立了一个模型，综合计算了渐近巨星分支（AGB）上恒星的演化。演化开始于第一个热脉冲（TP），当包层质量由于质量损失而损失或当核心质量达到Ekrasekhar质量时终止。我们的模型比以前的合成进化模型更现实，因为它包含了更多的物理过程。在脉冲间隔期间的光度的变化被考虑到，以及事实上，最初，前几个脉冲还没有在全振幅和光度低于标准的核心质量光度关系。大多数使用的关系是金属量依赖于能够作出现实的比较与恒星的不同金属量。考虑了第一次、第二次和第三次疏浚的影响。热底部燃烧（HBB）的影响，包括在一个近似的方式。AGB上的质量损失通过Reimers定律包括在内。我们还包括AGB之前的质量损失。我们计算中的自由参数是疏浚的最小核心质量（M（c）min）、第三疏浚效率（lambda）和三个质量损失标度参数（eta（RGB）、eta（EAGB）、eta（AGB））。该模型已被应用到LMC使用最近确定的年龄金属丰度和星星形成率（SFR）的LMC。观测到的碳星星光度函数和观测到的富氧与富碳AGB星的比例作为模型的约束条件。几个模型进行计算，以证明各种参数的影响。M（c）min = 0.58M的模型，λ = 0.75，eta（RGB）= 0-86，eta（AGB）= eta（EAGB）= 5，包括HBB，很好地再现了观测结果。在TP之后形成的碳的量可能高于X12 = 0.22的标准值。只要X12 =0.165，模型就能拟合观测值。很难区分较高的X12和较高的λ。第三，挖掘需要更有效，必须从比通常预测的更低的核心质量开始，以解释观察到的碳星星LF。有人建议，进化计算已经进行了混合长度参数太小。所采用的质量损失率系数对应于0.20M的预AGB质量损失。一个1 M。1.8M。一个5 M。星星。低质量的恒星在RGB上失去了这一点，而高质量的恒星在TP-AGB之前达到高亮度时处于核心氦燃烧阶段。AGB上的Reimers系数（eta（AGB）= 5）对应于1.0 × 10（-6）M的质量损失速率。yr-1在第一个TP为最初的1 M。具有LMC丰度的星星。这些高的质量损失率是必要的，以适应初末质量关系和高亮度的碳星星LF的尾巴。具有这些高质量损失率的大质量恒星的寿命与观测到的大质量AGB恒星及其祖先造父变星的数量非常一致。我们认为，在第一TP的大质量恒星的核心质量以前被高估，因为它们的演化计算忽略前AGB的质量损失。在观测上，LMC中的C-13富集碳星呈双峰分布。有少量（约0.1%）的高光度（M（bol）- 4.75）J型恒星。这两个分布区相对数量的差异以及光度的差距表明了不同的演化起源。低光度的J型星可能与银河核球中的R型星有关，它们的光度在0小于或接近M（bol）小于或接近-3之间，表明它们的起源在AGB以下。少量的高光度J型碳星可以用HBB来解释。考虑到观测到的LF和我们对六溴代二苯的近似处理的不确定性，协议是好的。我们预测大约1%的M和S星富含C-13（和N-14）。我们考虑了“遮蔽”的影响，当恒星失去如此多的质量时，它们变得光学不可见。根据Reid等（1990）的V波段、I波段和IRAS数据，结合辐射传输模型，我们发现，在所有亮度大于M（bol）=-6的碳星中，最多有3%可能在光学巡天中被遗漏。使用我们的模型，我们得出的碳星的遮蔽的整体效果是可以忽略不计的（约0.1%）。预测的平均最终质量（M（f）= 0.59 M.）与实测值（0.60 +/- 0.02M.）我们预测我们的LMC模型，恒星的初始质量大于1.2- 1.4 M。直接转变成碳星，这些恒星最初的质量大于1.5 M。在成为碳星之前会经过中间的S星阶段。这与在大麦哲伦星系团中观测到的碳星和S星是一致的。AGB星的出生率与造父变星和丛星的死亡率一致。行星状星云（PN）的诞生率比AGB星的死亡率低2倍，这表明低质量恒星（M小于或接近1.1 M）不能成为PN。
We have constructed a model to calculate in a synthetic way the evolution of stars on the asymptotic giant branch (AGB). The evolution is started at the first thermal pulse (TP) and is terminated when the envelope mass has been lost due to mass loss or when the core mass reaches the Chandrasekhar mass. Our model is more realistic than previous synthetic evolution models in that more physics has been included. The variation of the luminosity during the interpulse period is taken into account as well as the fact that, initially, the first few pulses are not yet at full amplitude and that the luminosity is lower than given by the standard core-mass-luminosity relations. Most of the relations used are metallicity dependent to be able to make a realistic comparison with stars of different metallicity. The effects of first, second and third dredge-up are taken into account. The effect of hot bottom burning (HBB) is included in an approximate way. Mass loss on the AGB is included through a Reimers Law. We also included mass loss prior to the AGB. The free parameters in our calculations are the minimum core mass for dredge-up (M(c)min), the third dredge-up efficiency (lambda) and three mass loss scaling parameters (eta(RGB), eta(EAGB), eta(AGB). The model has been applied to the LMC using a recent determination of the age-metallicity and star formation rate (SFR) for the LMC. The observed carbon star luminosity function and the observed ratio of oxygen-rich to carbon-rich AGB stars in the LMC acted as constraints to the model. Several models are calculated to demonstrate the effects of the various parameters. A model with M(c)min = 0.58M., lambda = 0.75, eta(RGB) = 0-86, eta(AGB) = eta(EAGB) = 5, including HBB reproduces the observations quite well. It is possible that the amount of carbon formed after a TP is higher than the standard value of X12 = 0.22. As long as lambdaX12=0.165 the model fits the observations. It is difficult to discriminate between a higher X12 and a higher lambda. Third dredge-up needs to be more efficient and must start at lower core masses than commonly predicted to account for the observed carbon star LF. It is suggested that evolutionary calculations have been performed with mixing-length parameters which are too small. The adopted mass loss rate coefficients correspond to a pre-AGB mass loss of 0.20M. for a 1M. and 1.8M. for a 5M. star. The low mass stars lose this on the RGB, the high mass stars in the core helium burning phase when they reach high luminosities before the TP-AGB. The Reimers coefficient on the AGB (eta(AGB) = 5) corresponds to a mass loss rate of 1.0 10(-6) M. yr-1 at the first TP for an initially 1 M. star with LMC abundances. These high mass loss rates are necessary to fit the initial-final mass relation and the high luminosity tail of the carbon star LF. The lifetime of the massive stars with these high mass loss rates are in good agreement with the observed number of massive AGB stars and their progenitors, the Cepheid variables. We suggest that the core mass at the first TP for massive stars has previously been overestimated because their evolution was calculated neglecting pre-AGB mass loss. Observationally the distribution of C-13 enriched carbon stars in the LMC is bimodal. There is a small number (approximately 0.1%) of high-luminosity (M(bol) - 4.75) J-type stars. The difference in relative numbers as well as the gap in luminosity between the two distributions suggst a different evolutionary origin. The low-luminosity J-type stars may be related to the R-stars in the Galactic bulge which have luminosities between 0 less than or similar to M(bol) less than or similar to - 3 indicating an origin at luminosities below the AGB. The small number of high-luminosity J-type carbon stars can be explained by HBB. Given the uncertainty in the observed LF and our approximate treatment of HBB the agreement is good. We predict that about 1 % of M and S stars are enriched in C-13 (and N-14). We considered the effect of ''obscuration'', when stars lose so much mass that they become optically invisible. Based on V-band, I-band and IRAS data of Reid et al. (1990) and a radiative transfer model we find that at most 3% of all carbon stars brighter than M(bol) = - 6 could have been missed in optical surveys. Using our model we derive that the overall effect of obscuration of carbon stars is negligible (approximately 0.1%). The predicted average final mass (M(f) = 0.59M.) is in good agreement with the observed value (0.60 +/- 0.02M.). We predict for our LMC model that stars with initial masses larger than 1.2-1.4M. turn into carbon stars directly and that stars initially more massive than about 1.5M. pass through an intermediate S-star phase before becoming carbon stars. This is consistent with the observation of carbon stars and S-stars in LMC clusters. The predicted birth rate of AGB stars is found to be in agreement with the death rate of Cepheids and the clump stars. The birth rate of planetary nebulae (PN) is a factor of 2 lower than the death rate of AGB stars suggesting that low mass stars (M less than or similar 1.1 M.) may not become PN.