[论文解读] Are isolated planetary-mass objects really isolated? A brown dwarf-exoplanet system candidate in the sigma Orionis cluster

J. A. Caballero, E. L. Martı́n+2 more|ArXiv.org|Aug 30, 2006

Astrophysics and Star Formation Studies参考文献 43被引用 24

一句话总结

本研究在σ Orionis星协中发现了一个候选棕矮星-系外行星系统，其中M5–6型棕矮星（SE 70）与一颗行星质量天体（S Ori 68）之间的投影分离距离为1,700 ± 300 AU。光谱学证据，包括锂吸收线和Hα发射线，支持SE 70属于该星协，表明某些‘孤立’的行星质量天体实际上可能是宽系外行星系统的成员。

ABSTRACT

The recent detection by direct imaging of three giant planets at wide separation (50-250 AU) from their primaries has raised the question about the ``true isolation'' of planetary-mass objects in clusters. Our goal was to test the possibility that some free-floating planetary-mass object could in fact be part of wide planetary systems. We searched in the literature for stellar and brown-dwarf member candidates of the sigma Orionis cluster at small angular separations from published candidate planetary-mass objects. We found one candidate planetary system, SE 70, composed of an X-ray source and a planetary-mass object, namely S Ori 68, separated by only 4.6 arcsec. In order to assess the cluster membership of the X-ray source, we obtained mid-resolution optical spectroscopy using ISIS on the William Herschel Telescope. We also compiled additional data on the target from available astronomical catalogues. We have found that SE 70 follows the spectrophotometric sequence of the cluster and displays spectroscopic features of youth, such as lithium in absorption and chromospheric Halpha emission. The radial velocity is consistent with cluster membership. Hence, SE 70 is very probably a sigma Orionis cluster member. The projected physical separation between SE 70 and S Ori 68 is 1700+/-300 AU at the distance of the cluster. If the common proper motion is confirmed in the near future, the system would be composed of an M5-6 brown dwarf with an estimated mass of ~45 M_Jup and an L5+/-2 giant planet with an estimated mass of ~5 M_Jup. It would be the widest and one of the lowest-mass planetary systems known so far.

研究动机与目标

• 调查年轻星协中自由漂浮的行星质量天体（PMOs）是否真正孤立，或是否属于宽系外行星系统。
• 检验某些PMOs可能在大距离上与低质量恒星或棕矮星伴星束缚的假设。
• 通过光谱学和测光分析，评估候选行星系统SE 70–S Ori 68的星协成员资格。
• 评估该系统作为束缚宽双星系统的动力学与运动学一致性。

提出的方法

• 在σ Orionis星协中，针对已知PMO候选体周围15角秒范围内的恒星和棕矮星候选体进行文献搜索。
• 利用威廉·赫歇尔望远镜上的ISIS仪器，获取X射电源SE 70的中分辨率光学光谱。
• 从天文星表中汇编多波段数据，包括X射线、光学和近红外星等。
• 通过分析锂吸收线和Hα发射线等光谱特征，推断其年轻性与星协成员资格。
• 利用星协距离（360 pc）估算SE 70与S Ori 68之间的投影物理分离距离。
• 通过径向速度一致性与空间分布，评估共同自行运动和动力学束缚的可能性。

实验结果

研究问题

• RQ1 行星质量天体S Ori 68是否真正孤立，还是可能与伴星束缚？
• RQ2 X射电源SE 70是否表现出指示年轻性和星协成员资格的光谱特征？
• RQ3 SE 70与S Ori 68之间的投影物理分离距离是多少，是否与宽系外行星系统一致？
• RQ4 基于其空间分布，SE 70与S Ori 68形成真实物理系统的概率是多少？
• RQ5 径向速度与自行测量能否证实该系统的动力学束缚？

主要发现

• SE 70表现出锂吸收线和Hα发射线，表明其年轻，并强烈支持其属于σ Orionis星协。
• SE 70的径向速度与星协的平均径向速度一致，进一步支持其成员资格。
• 在360 pc的星协距离下，SE 70与S Ori 68之间的投影物理分离距离为1,700 ± 300 AU。
• 基于其空间分布，SE 70与S Ori 68形成真实物理系统的概率约为99%。
• 若得到证实，该系统将由一颗M5–6型棕矮星（约45 M_Jup）和一颗L5 ± 2型行星质量天体（约5 M_Jup）组成，成为目前已知最宽、质量最低的系外行星系统之一。
• 需要低于1 km s⁻¹的高精度径向速度测量以及未来的自行研究，以确认共同自行运动与动力学束缚。