[论文解读] Early-type stars in the young open cluster IC1805. II. The probably single stars HD15570 and HD15629, and the massive binary/triple system HD15558

M. De Becker, G. Rauw+2 more|ArXiv.org|Jun 15, 2006

Stellar, planetary, and galactic studies参考文献 31被引用 28

一句话总结

本研究通过径向速度测量和光谱分解，调查了年轻开放星团IC 1805中三颗早型星的多重性。结果确认HD 15570和HD 15629为可能的单星，同时首次获得大质量双星HD 15558的SB2轨道解，揭示其质量比极高，最小质量分别约为150 M⊙和50 M⊙，暗示可能存在三合星系统，以调和主星质量与其光谱型之间的矛盾。

ABSTRACT

Aims: We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC1805, namely HD15570, HD15629 and HD15558. Methods: For the three stars, we measured the radial velocity by fitting Gaussian curves to line profiles in the optical domain. In the case of the massive binary HD15558, we also used a spectral disentangling method to separate the spectra of the primary and of the secondary in order to derive the radial velocities of the two components. These measurements were used to compute orbital solutions for HD15558. Results: For HD15570 and HD15629, the radial velocities do not present any significant trend attributable to a binary motion on time scales of a few days, nor from one year to the next. In the case of HD15558 we obtained an improved SB1 orbital solution with a period of about 442 days, and we report for the first time on the detection of the spectral signature of its secondary star. We derive spectral types O5.5III(f) and O7V for the primary and the secondary of HD15558. We tentatively compute a first SB2 orbital solution although the radial velocities from the secondary star should be considered with caution. The mass ratio is rather high, i.e. about 3, and leads to very extreme minimum masses, in particular for the primary object. Minimum masses of the order of 150 \pm 50 and 50 \pm 15 M_\odot are found respectively for the primary and the secondary. Conclusions: We propose that HD15558 could be a triple system. This scenario could help to reconcile the very large minimum mass derived for the primary object with its spectral type. In addition, considering new and previously published results, we find that the binary frequency among O-stars in IC1805 has a lower limit of 20%, and that previously published values (80%) are probably overestimated.

研究动机与目标

• 确定IC 1805中三颗最亮的早型星（HD 15570、HD 15629和HD 15558）的多重性状态。
• 重新评估此前报告的IC 1805中O型星80%的双星频率，该数值可能被高估。
• 探究HD 15558主星的极高最小质量是否能与其O5.5III(f)光谱型相协调。
• 探讨HD 15558可能为三合星系统的可能性，以解释其极端质量与光谱型。

提出的方法

• 通过在法国上普罗旺斯天文台获取的光学光谱中对吸收线拟合高斯轮廓，获得径向速度测量值。
• 对HD 15558应用光谱分解方法，以分离复合光谱中主星与次星的贡献。
• 利用跨越数年多个历元收集的径向速度数据，计算轨道解。
• 基于线轮廓特征、等效宽度以及He ii λ4686线的P-Cygni轮廓，分配光谱型。
• 从轨道解中推导质量比与最小质量，不确定性由径向速度误差传播得出。
• 通过考虑主星质量相对于其光谱型的极端性，评估三合星系统的可能性。

实验结果

研究问题

• RQ1 HD 15570和HD 15629是否为单星，或其轨道周期在数天至一年基线内可探测到的不可见伴星？
• RQ2 大质量双星HD 15558的轨道解为何？是否支持双谱分光双星（SB2）系统？
• RQ3 能否调和HD 15558主星约150 M⊙的最小质量与其O5.5III(f)光谱型之间的矛盾？
• RQ4 HD 15558中观测到的高质质量比（q ≈ 3）在物理上是否可能？还是暗示更复杂的系统（如三合星）？
• RQ5 在重新评估先前结论的前提下，IC 1805中O型星的真实双星频率是多少？

主要发现

• HD 15570和HD 15629在数天至一年的时间基线上未表现出显著的径向速度变化，支持其为可能单星的分类。
• 获得了HD 15558的改进SB1轨道解，周期为442 ± 12天，偏心率约为0.4。
• 光谱分解方法成功揭示了HD 15558的次星成分，证实其为双谱分光双星（SB2）。
• 基于线轮廓和等效宽度，为主星和次星分别分配了O5.5III(f)和O7V的光谱型。
• 基于轨道解，主星和次星的最小质量分别为150 ± 50 M⊙和50 ± 15 M⊙。
• 高质质量比（q ≈ 3）导致极端最小质量，暗示HD 15558可能为三合星系统，以调和主星质量与其光谱型之间的矛盾。