[论文解读] Fundamental parameters of Galactic luminous OB stars IV. The upper HR diagram

A. Herrero, J. Puls+1 more|arXiv (Cornell University)|Dec 9, 1999

Stellar, planetary, and galactic studies参考文献 1被引用 7

一句话总结

本研究利用非局部热动平衡（NLTE）平面平行和球面对称模型大气，分析了七颗早期型银河系O型星，以确定其基本参数。研究发现，紫外金属线遮蔽效应在准确建模中至关重要，且包含质量损失的球面对称模型可改善光谱拟合，部分解决了光谱质量与演化质量之间的质量差异，部分恒星的初始质量可能接近或超过100 M⊙。

ABSTRACT

We present observations and analyses of seven Galactic O stars of type O6 and earlier. Analyses are carried out using NLTE plane-parallel, hydrostatic models as well as NLTE spherical models with mass-loss. With detailed calculations for the former and simulations for the latter, it is shown that the flux blocking due to UV metal lines is important for these objects and the way the mechanism operates is explained. We find that the plane-parallel, hydrostatic unblanketed model atmospheres have increasing difficulties in fitting the early-type spectra of massive stars, and for 50000 K and above a fit seems to be impossible. We then perform an analysis using spherical models with mass-loss. Gravities should be increased by 0.1-0.25 dex, reducing, but not solving, the mass discrepancy. We show that spectroscopic masses are in better agreement with the theory of radiatively driven winds than evolutionary masses are. A helium abundance larger than solar is obtained for most objects. Some additional effects (partly related to present approximations) that have an influence in our analyses are studied. Some stars of our sample have such high mass-loss rates that the derivation of gravities from the wings of Balmer lines, in particular Hgamma, becomes doubtful. From the point of view of individual stars, we have analysed some of the most massive and luminous stars in the Milky Way. According to our analysis, three of them (Cyg OB2-7, HD 15570 and HD15558) have particularly large initial masses, close to or in excess of 100 Msun. HD 5689 could have been erroneously assigned to Cas OB7 and might be a runaway star.

研究动机与目标

• 利用先进的大气模型，确定银河系中最明亮O型星的基本参数。
• 解决大质量恒星中长期存在的光谱质量与演化质量之间的质量差异问题。
• 评估紫外金属线遮蔽效应和质量损失对光谱能量分布及重力测定的影响。
• 评估从极端明亮恒星的巴耳末线翼部推导出的重力与质量估计的可靠性。
• 研究氦元素丰度，并探讨逃逸星或大质量恒星的潜在误分类问题。

提出的方法

• 应用NLTE平面平行、静态模型大气，分析早型O型星的观测光谱。
• 采用NLTE球面对称模型并引入质量损失，以考虑辐射加速度和星风效应。
• 在模型计算中引入详细的紫外金属线遮蔽效应，以提高流量分布的准确性。
• 将观测到的巴耳末线轮廓（特别是Hgamma）与模型生成的合成轮廓进行比较，以推导表面重力。
• 利用模型大气生成的合成光谱进行光谱分析，以推导有效温度、光度和质量。
• 评估光谱质量与理论辐射驱动星风模型的一致性。

实验结果

研究问题

• RQ1 紫外金属线遮蔽效应对最明亮O型星的光谱能量分布及其推导参数有何影响？
• RQ2 与平面平行、静态模型相比，包含质量损失的球面对称模型在多大程度上改善了对观测光谱的拟合？
• RQ3 为何在最质量最大的O型星中，光谱质量系统性地超过演化质量？
• RQ4 高质损失率对从巴耳末线翼部推导重力估计的可靠性有何影响？
• RQ5 某些高光度O型星（如HD 56890）是否被误分类或为逃逸星，这如何影响其质量估计？

主要发现

• 在赫罗图上部区域，紫外金属线遮蔽效应占主导地位，显著改变流量分布，必须在模型大气中予以包含。
• 平面平行、静态、未考虑遮蔽的模型无法拟合有效温度T_eff ≥ 50,000 K恒星的光谱，表明其存在根本性局限。
• 包含质量损失的球面对称模型使推导的重力值提高0.1–0.25 dex，虽减少但未完全消除质量差异。
• 光谱质量与理论辐射驱动星风模型更为一致，而非演化质量。
• 大多数恒星的氦丰度高于太阳，表明可能存在富集或初始过量。
• 三颗恒星——Cyg OB2-7、HD 15570和HD 15558——的初始质量接近或超过100 M⊙，暗示极端质量形成或演化过程。