脉动周期揭示食双星系统中经典造父变星的理论半径与实测半径之间的张力
Pulsation periods reveal tension between theoretical and empirical radii for classical Cepheids in eclipsing binary systems
AI总结：
研究发现用食双星造父变星约束恒星模型时，基于脉动周期与半径的拟合解存在张力，建议用脉动周期而非半径，并需研究大振幅脉动星半径的非线性效应。
AI中文摘要：
背景：拥有精确测定的物理参数的食双星系统中的经典造父变星，常被用于约束恒星演化与脉动理论。为此，在匹配最佳拟合演化模型时，通常会使用它们在赫罗图上的位置、有效温度、光度以及半径。然而，造父变星最精确的观测量——脉动周期，在这类研究中却很少被使用。目的：我们探究将脉动周期作为约束条件匹配演化模型时，对最佳拟合解产生的影响。由于脉动周期遵循周期-平均密度关系，我们检验其是否能提供与基于恒星半径的信息一致的内容。方法：我们对4个包含造父变星的大麦哲伦云（LMC）食双星系统进行建模，使用χ²最小化方法从MESA计算的模型网格中找到最佳匹配的演化模型；这些演化模型补充了由RSP计算的脉动周期，并考虑了非线性周期修正。结果：根据使用半径还是脉动周期选择最佳拟合模型，会得到不一致的解。在基于脉动周期选择的解中，恒星半径系统地低于观测值；相反，对于基于半径选择的解，脉动周期系统地偏长。对于半径精确测定的恒星，这种张力达到几个σ，其中部分源于大振幅脉动星的半径非线性增加，该现象在文献中尚未得到详细研究。结论：当使用食双星系统中的造父变星约束恒星模型时，我们建议使用脉动周期而非半径；需要对大振幅脉动星中非线性效应对恒星半径的影响开展系统性研究。
英文摘要：
Context. With their precisely determined physical parameters, classical Cepheids in eclipsing binary systems are often used to constrain stellar evolution and pulsation theories. To this end, their position in the HR diagram, effective temperature and luminosity, and radius are commonly used when matching best-fitting evolutionary models. However, the pulsation period of Cepheids, the most precise observable, is rarely used in such studies. Aims. We explore how including the pulsation period as a constraint in matching evolutionary models affects the best-fitting solution. As the pulsation period follows the period-mean density relation, we examine whether it provides information consistent with that based on the stellar radius. Methods. We modeled four eclipsing binary systems with Cepheids from the LMC. We used $χ^2$ minimization to find the best-matching evolutionary model from a grid of models computed with MESA. The evolutionary models are supplemented with pulsation periods computed with RSP, with nonlinear period corrections taken into account. Results. Depending on whether the radius or the pulsation period is used to select the best-fitting model, discrepant solutions are obtained. In solutions selected based on the pulsation period, the stellar radius is systematically too low compared with observations. Conversely, for solutions based on the radius, the pulsation period is systematically too long. The tension amounts to a few sigma for stars with precisely determined radii and part of it is traced to a nonlinear increase in radius for large-amplitude pulsators, which has not been studied in detail in the literature. Conclusions. When using Cepheids in eclipsing binary systems to constrain stellar models, we recommend using the pulsation period instead of the radius. A systematic study of nonlinear effects on stellar radius in large-amplitude pulsators is needed.
