[论文解读] The two components of the evolved massive binary LZ Cep. Testing the effects of binarity on stellar evolution

L. Mahy, F. Martins+3 more|arXiv (Cornell University)|Jun 30, 2011

Stellar, planetary, and galactic studies被引用 15

一句话总结

本研究利用高分辨率NARVAL光谱和Hipparcos光变曲线，对演化中的大质量双星LZ Cep进行了详细的光谱和光度分析，以确定轨道参数、单个恒星属性及质量。关键发现是，系统可能经历了低效的质量转移，即最初质量更大的次星（现约为6.5 M☉）向主星（约16.0 M☉）转移质量，导致形成半分离双星系统，其中次星接近核心氦燃烧状态，尽管其光度分类为主序星，却已化学演化，类似沃尔夫-拉叶星。

ABSTRACT

Aims. We present an in-depth study of the two components of the binary system LZCep in order to constrain the effects of binarity on the evolution of massive stars. Methods. We use a set of high-resolution, high signal-to-noise ratio optical spectra obtained over the orbital period of the system to perform a spectroscopic disentangling and derive an orbital solution. We subsequently determine the stellar properties of each component through an analysis with the CMFGEN atmosphere code. Finally, with the derived stellar parameters, we model the Hipparcos photometric light curve using the program NIGHTFALL to obtain the inclination and the real stellar masses. Results. LZCep is a O9III+ON9.7V binary. It is as a semi-detached system in which either the primary or the secondary star almost fills up its Roche lobe. The dynamical masses are about 16.0 $M_{\odot}$ (primary) and 6.5 M_{\odot}$ (secondary). The latter is lower than the typical mass of late-type O stars. The secondary component is chemically more evolved than the primary (which barely shows any sign of CNO processing), with strong helium and nitrogen enhancements as well as carbon and oxygen depletions. These properties (surface abundances and mass) are typical of Wolf-Rayet stars, although the spectral type is ON9.7V. The luminosity of the secondary is consistent with that of core He-burning objects. The preferred, tentative evolutionary scenario to explain the observed properties involves mass transfer from the secondary - which was initially more massive - towards the primary. The secondary is now almost a core He-burning object, probably with only a thin envelope of H-rich and CNO processed material. A very inefficient mass transfer is necessary to explain the chemical appearance of the primary. Alternative scenarios are discussed but they suffer from more uncertainties.

研究动机与目标

• 通过研究演化双星LZ Cep，约束双星对大质量恒星演化的影响。
• 利用高分辨率光谱确定轨道解及两颗恒星的个体星体参数。
• 通过表面元素丰度和光度分析，研究恒星的演化状态。
• 利用Hipparcos光度光变曲线推导真实质量与系统倾角。
• 将理论双星演化模型与观测性质进行对比，特别是表面元素丰度与质量比。

提出的方法

• 获取了覆盖LZ Cep轨道周期的14天高分辨率、高信噪比NARVAL光谱序列。
• 采用光谱解离法分离出主星与次星的个体光谱。
• 应用CMFGEN大气模型代码，为每颗恒星推导星体参数、风特性及表面元素丰度。
• 使用NIGHTFALL程序建模Hipparcos光度光变曲线，以确定系统倾角。
• 结合光谱轨道解与光度倾角，推导动力学质量，并检验演化情景。
• 评估观测到的表面元素丰度（N、He、C、O）与理论双星演化模型的一致性，特别是后Case A质量转移阶段。

实验结果

研究问题

• RQ1 LZ Cep中两颗恒星的个体星体参数（Teff、log g、vrot、质量、光度）是什么？
• RQ2 观测到的氮、氦、碳、氧表面丰度与单星和双星演化模型的预测相比如何？
• RQ3 系统的轨道倾角与每颗恒星的动力学质量是多少？与光谱质量和演化质量相比如何？
• RQ4 哪种演化情景最能解释观测到的性质，包括次星质量较低但化学演化显著、光度分类为光度级主序星？
• RQ5 该系统与单星演化轨迹的偏离程度如何？这对双星相互作用模型有何启示？

主要发现

• 该系统为半分离双星，轨道周期为3.070508天，偏心率较低但非零（e ≈ 0.031），与近圆形轨道一致。
• 主星分类为O9III，质量约为16.0 M☉；次星为ON9.7V，质量约为6.5 M☉，显著低于典型晚期O型星的质量。
• 次星表现出强烈的氦和氮表面增强，同时碳和氧丰度降低，表明经历了显著的CNO循环处理，正演化至核心氦燃烧阶段。
• 主星表现出极少的化学处理，表明质量转移效率极低，与标准质量转移情景下预期的强富集效应不一致。
• 通过椭球光变曲线变化，系统倾角被约束在约48°，从而可推导出与光谱解一致的真实动力学质量。
• 首选演化情景为：最初质量更大的次星向主星转移质量，导致次星演化为接近核心氦燃烧的天体，具有薄层富含氢、经CNO处理的包层。