Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models
Characterizing Observed Extra Mixing Trends in Red Giants using the Reduced Density Ratio from Thermohaline Models
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DOI：
10.3847/1538-4357/aca024
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发表时间：
2022-04
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影响因子：
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通讯作者：
A. Fraser;M. Joyce;E. H. Anders;J. Tayar;M. Cantiello
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作者：
A. Fraser;M. Joyce;E. H. Anders;J. Tayar;M. Cantiello
观测表明，低质量上巨星分支恒星中几乎普遍存在额外混合。对此最常引用的解释是温盐混合。一维恒星演化模型包括温盐混合的各种规定，但迄今为止直接使用观测数据来区分温盐规定的方法还很有限。在这里，我们提出了一个新的框架来促进直接比较：使用斯隆数字天空巡天-IV APOGEE巡天的碳氮测量作为混合探针和一维恒星演化程序中称为折合密度比的流体参数，我们将上巨星分支上观察到的额外混合量与三维流体动力学模拟的预测趋势进行比较。使用这种方法，我们能够根据经验限制混合效率应如何随密度比降低而变化。我们发现观察到的额外混合量与折合密度比密切相关，并且折合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。我们表明，具有可用混合数据的恒星往往具有相对较低的密度比，这应该为未来模拟工作选择的制度提供信息。最后，我们表明在低折合密度比下混合增加，这与当前温盐混合的流体动力学模型一致。该框架的引入为理论建模工作设立了新标准，因为现在不仅可以验证额外混合的量，还可以验证额外混合程度与基本恒星参数之间的趋势。
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 Sloan Digital Sky Survey-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.