Dredge-up and Envelope Burning in Intermediate-Mass Giants of Very Low Metallicity
Dredge-up and Envelope Burning in Intermediate-Mass Giants of Very Low Metallicity
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DOI：
10.1086/382200
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发表时间：
2003-12
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影响因子：
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通讯作者：
F. Herwig
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作者：
F. Herwig
详细研究了中等质量极低金属丰度恒星在最后热脉冲渐近巨星分支（AGB）阶段的演化。作为代表性的例子，讨论了初始质量为4和5 M的中子，金属丰度为Z = 0.0001（[Fe/H] ~ -2.3）的模型。一维恒星结构和演化模型包括由流体动力学模拟激发的时间和深度相关的超调，以及完整的核网络和时间相关的混合。特别注意的是，高的时间和空间分辨率，以避免与第三次疏浚和热底燃烧预测的数值文物。模型计算预测非常有效的第三次挖掘，混合信封与整个壳层间层或其大部分，并在某些情况下渗透到C/O核心以下的He壳。在所有情况下，初级氧气都混合到外壳中。该模型预测有效的包络线燃烧在脉冲间阶段。根据燃烧温度的不同，氧气会被破坏到不同的程度。在本研究中考虑的任何情况下，疏浚和信封燃烧的综合影响不会导致任何显着的氧气消耗。在我们的模型中，大的挖掘效率与H壳层在低金属丰度非常贫金属的恒星的挖掘阶段的特殊性质密切相关，这是在许多热脉冲之后。在疏浚阶段，温度刚好低于对流边界是大到足以让质子剧烈燃烧时，他们被带到富C环境下的对流边界的时间和深度依赖的过冲。产生105 ~2 × 106 L的氢燃烧光度。C和较小程度的O在这个挖掘的过冲层中转化为N，并进入包层。全球对CNO丰度的影响类似于热底部燃烧。如果过冲效率更大，则挖掘H燃烧导致挖掘效率的进一步增加。在一些热脉冲之后，挖掘继续通过He壳层进入下面的CO核心。在脉冲间隔阶段，由于13 C（α，n）16 O反应中α粒子的缺乏，在辐射条件下13 C可能不会释放中子。定性地讨论了s-过程的条件。描述了H、He、C、N、O和Na的丰度演化。最后，钠和氧的模型预测与观测到的丰度进行了比较。大质量AGB星是球状星团巨星中O-Na丰度关系的起源的观点与本研究的模型预测不一致。讨论了富碳极贫金属双星LP 625-44、CS 29497-030和HE 0024-2523的丰度。
The evolution of intermediate-mass stars at very low metallicity during their final thermal pulse asymptotic giant branch (AGB) phase is studied in detail. As representative examples, models with initial masses of 4 and 5 M☉ and with a metallicity of Z = 0.0001 ([Fe/H] ~ -2.3) are discussed. The one-dimensional stellar structure and evolution model includes time- and depth-dependent overshooting motivated by hydrodynamic simulations, as well as a full nuclear network and time-dependent mixing. Particular attention is given to high time and space resolution to avoid numerical artifacts related to third dredge-up and hot bottom burning predictions. The model calculations predict very efficient third dredge-up that mixes the envelope with the entire intershell layer or a large fraction thereof and in some cases penetrates into the C/O core below the He shell. In all cases primary oxygen is mixed into the envelope. The models predict efficient envelope burning during the interpulse phase. Depending on the envelope-burning temperature, oxygen is destroyed to varying degrees. The combined effect of dredge-up and envelope burning does not lead to any significant oxygen depletion in any of the cases considered in this study. The large dredge-up efficiency in our model is closely related to the particular properties of the H shell during the dredge-up phase in low-metallicity very metal-poor stars, which is followed here over many thermal pulses. During the dredge-up phase, the temperature just below the convective boundary is large enough for protons to burn vigorously when they are brought into the C-rich environment below the convection boundary by the time- and depth-dependent overshooting. H-burning luminosities of 105 to ~2 × 106 L☉ are generated. C, and to lesser degree O, is transformed into N in this dredge-up overshooting layer and enters the envelope. The global effect on the CNO abundance is similar to that of hot bottom burning. If the overshoot efficiency is larger, then dredge-up H burning causes a further increase in the dredge-up efficiency. After some thermal pulses, the dredge-up proceeds through the He shell and into the CO core beneath. Then neutrons may not be released from 13C in radiative conditions during the interpulse phase because of the scarcity of α-particles for the 13C(α,n)16O reactions. Conditions for the s-process are discussed qualitatively. The abundance evolution of H, He, C, N, O, and Na is described. Finally, the model predictions for sodium and oxygen are compared with observed abundances. The notion that massive AGB stars are the origin of the O-Na abundance anticorrelation in globular cluster giants is not consistent with the model predictions of this study. The abundance of the C-rich extremely metal-poor binaries LP 625-44, CS 29497-030, and HE 0024-2523 is discussed.