arXiv 1709.063812017-09-19astro-ph.SR

堆芯氦燃烧模型中混合的处理-- III .用新的超调约束抑制堆芯呼吸脉冲

The treatment of mixing in core helium burning models -- III. Suppressing core breathing pulses with a new constraint on overshoot

Thomas Constantino, Simon W. Campbell, John C. Lattanzio

备注：: Accepted for publication in MNRAS; 11 pages, 6 figures
DOI：: 10.1093/mnras/stx2321

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摘要：

理论预测的核心氦燃烧阶段的恒星演化是高度敏感的不确定性处理的混合在对流边界。在过去的几年里，限制其内部深处的不确定结构的兴趣已经被来自星震学的见解所重新激发。最近，Spruit（2015）基于从对流核心外部摄入的物质的较高浮力，提出了氦燃烧对流核心增长速率的限制。在本文中，我们测试的影响，这样的限制恒星模型的初始质量和金属丰度的范围。我们发现，混合超越史瓦西边界的约束有显着的影响，在核心氦燃烧的演化后期，当核心呼吸脉冲发生和氦的摄入率是最快的。通常，核心呼吸脉冲延长核心氦燃烧寿命到这样一个程度，模型与观测的球状星团人口不一致。在初始恒星质量（$0.83 \leq M/\text{M}_\odot \leq 5$）的广泛范围内，应用Spruit约束减少了核心氦燃烧寿命，因为核心呼吸脉冲被避免或其数量和严重性减少。因此，斯普鲁特提出的约束条件有助于解决观测结果与理论预测之间的重大差异。具体来说，我们发现改进协议$R_2$，观测到的比率渐近巨型分支水平分支恒星的球状星团;这两个群体之间的光度差异;和在asteroseismology，混合模式的周期间隔检测红色团块恒星在\textit{Kepler}领域。

英文摘要：

Theoretical predictions for the core helium burning phase of stellar evolution are highly sensitive to the uncertain treatment of mixing at convective boundaries. In the last few years, interest in constraining the uncertain structure of their deep interiors has been renewed by insights from asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of growth of helium-burning convective cores based on the higher buoyancy of material ingested from outside the convective core. In this paper we test the implications of such a limit for stellar models with a range of initial mass and metallicity. We find that the constraint on mixing beyond the Schwarzschild boundary has a significant effect on the evolution late in core helium burning, when core breathing pulses occur and the ingestion rate of helium is fastest. Ordinarily, core breathing pulses prolong the core helium burning lifetime to such an extent that models are at odds with observations of globular cluster populations. Across a wide range of initial stellar masses ($0.83 \leq M/\text{M}_\odot \leq 5$), applying the Spruit constraint reduces the core helium burning lifetime because core breathing pulses are either avoided or their number and severity reduced. The constraint suggested by Spruit therefore helps to resolve significant discrepancies between observations and theoretical predictions. Specifically, we find improved agreement for $R_2$, the observed ratio of asymptotic giant branch to horizontal branch stars in globular clusters; the luminosity difference between these two groups; and in asteroseismology, the mixed-mode period spacing detected in red clump stars in the \textit{Kepler} field.

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