[论文解读] The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets

M. Mayor, M. Marmier+12 more|arXiv (Cornell University)|Sep 12, 2011

Stellar, planetary, and galactic studies参考文献 1被引用 342

一句话总结

本研究分析了8年HARPS径向速度数据，以表征围绕类太阳恒星的超级-Earth和海王星质量行星。结果表明，超过50%的此类恒星至少拥有一颗轨道周期小于100天的低质量行星，其质量分布从15至30 M⊕急剧上升，且未发现对金属丰度较高的宿主恒星有偏好——这与气态巨行星形成鲜明对比，后者与高金属丰度强烈相关，并表现出高达0.9的偏心率。

ABSTRACT

We report on the results of an 8-year survey carried out at the La Silla Observatory with the HARPS spectrograph to detect and characterize planets in the super-Earth and Neptune mass regime. The size of our star sample and the precision achieved with HARPS have allowed the detection of a sufficiently large number of low-mass planets to study the statistical properties of their orbital elements, the correlation of the host-star metallicity with the planet masses, as well as the occurrence rate of planetary systems around solar-type stars. A robust estimate of the frequency of systems shows that more than 50% of solar-type stars harbor at least one planet of any mass and with period up to 100 days. Different properties are observed for the population of planets less massive than about 30M-Earth compared to the population of gaseous giant planets. The mass distribution of Super-Earths and Neptune-mass planets (SEN) is strongly increasing between 30 and 15M-Earth. The SEN occurence rate does not exhibit a preference for metal rich stars. Most of the SEN planets belong to multi-planetary systems. The orbital eccentricities of the SEN planets seems limited to 0.45. At the opposite, the occurence rate of gaseous giant planets is growing with the logarithm of the period, and is strongly increasing with the host-star metallicity. About 14% of solar-type stars have a planetary companion more massive than 50M-Earth? on an orbit with a period shorter than 10 years. Orbital eccentricities of giant planets are observed up to 0.9 and beyond. The precision of HARPS-type spectrographs opens the possibility to detect planets in the habitable zone of solar-type stars. Identification of a significant number of super-Earths orbiting solar-type of the Sun vicinity is achieved by Doppler spectroscopy. 37 newly discovered planets are announced in the Appendix of this paper, among which 15 Super-Earths.

研究动机与目标

• 利用长期径向速度监测，确定类太阳恒星周围低质量行星（超级-Earth和海王星质量）的出现率。
• 研究超级-Earth/海王星（SEN）行星群体中行星质量、轨道周期和偏心率的统计分布。
• 比较超级-Earth/海王星行星与气态巨行星（GGP）在金属丰度依赖性和轨道特性方面的出现率。
• 评估宿主恒星金属丰度在不同行星群体形成中的作用。
• 评估利用HARPS等高精度仪器通过多普勒光谱法探测宜居带行星的潜力。

提出的方法

• 在拉西拉天文台的ESO 3.6米望远镜上，使用HARPS光谱仪进行了为期8年的径向速度巡天。
• 以亚米每秒级别的高精度分析径向速度测量值，以探测低质量行星信号。
• 应用统计方法估算行星出现率，校正检测偏差和巡天完备性。
• 通过轨道解法拟合，从径向速度曲线中确定行星质量（m₂sin i）、周期和偏心率。
• 将宿主恒星金属丰度（Fe/H）与行星质量及出现频率相关联，以评估形成依赖性。
• 区分单行星系统与多行星系统，聚焦于多行星系统中最重的行星进行对比分析。

实验结果

研究问题

• RQ1 类太阳恒星周围轨道周期不超过100天的超级-Earth和海王星质量行星的出现率是多少？
• RQ2 在15至30 M⊕范围内，低质量行星的质量分布如何变化，这暗示了何种检测偏差？
• RQ3 与气态巨行星相比，宿主恒星金属丰度与超级-Earth/海王星质量行星出现率之间是否存在相关性？
• RQ4 超级-Earth/海王星与气态巨行星的典型轨道偏心率分布有何不同？
• RQ5 多行星系统在低质量行星群体中占主导地位的程度如何，这暗示了何种形成机制？

主要发现

• 超过50%的类太阳恒星至少拥有一颗质量小于30 M⊕、轨道周期不超过100天的行星。
• 超级-Earth和海王星质量行星的质量分布从15至30 M⊕之间急剧上升，表明该范围内更为普遍。
• 超级-Earth和海王星质量行星的出现率对金属丰度较高的宿主恒星无显著偏好，中位金属丰度为Fe/H ≈ -0.1 dex。
• 超级-Earth和海王星质量行星的轨道偏心率上限为0.45，而气态巨行星的偏心率可高达0.9。
• 气态巨行星（m₂sin i > 50 M⊕，P < 10年）的出现率约为14%，且随宿主恒星金属丰度显著增加。
• 低质量行星群体在轨道周期40至80天之间出现频率达到峰值，经巡天偏差校正后依然显著。