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arXiv 2204.08487astro-ph.SRastro-ph.GAphysics.plasm-ph
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利用温盐模式的约化密度比表征红巨星中观测到的额外混合趋势

Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models

Adrian E. Fraser, Meridith Joyce, Evan H. Anders, Jamie Tayar, Matteo Cantiello

备注：
22 pages, 7 figures, accepted for publication in ApJ. MESA histories available on Zenodo at https://doi.org/10.5281/zenodo.7152746 . MESA inlists available on GitHub at https://github.com/afraser3/Empirical-Magnetic-Thermohaline
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摘要：

观测结果显示，在低质量的上巨分支恒星中，几乎无处不在地存在着额外的混合。最常见的解释是温盐混合。一维恒星演化模型包括各种热盐混合的处方，但使用观测数据直接区分热盐处方迄今为止是有限的。在这里，我们提出了一个新的框架，以促进直接比较：使用碳-氮测量从SDSS-IV APOGEE调查作为探针的混合和流体参数称为约化密度比从一维恒星演化程序，我们比较观察到的额外混合量的上巨分支预测的趋势从三维流体动力学模拟。使用这种方法，我们能够凭经验约束混合效率应如何随约化密度比而变化。我们发现所观察到的额外混合量与约化密度比密切相关，并且约化密度比和基本恒星参数之间的趋势在建模处方的选择中是稳健的。我们发现，可用的混合数据的恒星往往有相对较低的密度比，这应该告知未来的模拟工作所选择的制度。最后，我们表明，有增加混合在低的约化密度比，这是符合目前的流体动力学模型的温盐混合。这个框架的引入为理论建模工作设定了一个新的标准，因为不仅可以验证额外混合的量，而且可以验证额外混合程度与基本恒星参数之间的趋势。

英文摘要：

Observations show an almost ubiquitous presence of extra mixing in low-mass upper giant branch stars. The most commonly invoked explanation for this is thermohaline mixing. One-dimensional stellar evolution models include various prescriptions for thermohaline mixing, but the use of observational data directly to discriminate between thermohaline prescriptions has thus far been limited. Here, we propose a new framework to facilitate direct comparison: Using carbon-to-nitrogen measurements from the SDSS-IV APOGEE survey as a probe of mixing and a fluid parameter known as the reduced density ratio from one-dimensional stellar evolution programs, we compare the observed amount of extra mixing on the upper giant branch to predicted trends from three-dimensional fluid dynamics simulations. Using this method, we are able to empirically constrain how mixing efficiency should vary with the reduced density ratio. We find the observed amount of extra mixing is strongly correlated with the reduced density ratio and that trends between reduced density ratio and fundamental stellar parameters are robust across choices for modeling prescription. We show that stars with available mixing data tend to have relatively low density ratios, which should inform the regimes selected for future simulation efforts. Finally, we show that there is increased mixing at low reduced density ratios, which is consistent with current hydrodynamical models of thermohaline mixing. The introduction of this framework sets a new standard for theoretical modeling efforts, as validation for not only the amount of extra mixing, but trends between the degree of extra mixing and fundamental stellar parameters is now possible.

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