[论文解读] Greening of the Brown Dwarf Desert. EPIC 212036875 b -- a 51 M$_\mathrm{J}$ object in a 5 day orbit around an F7 V star

C. M. Persson, Szilárd Csizmadia+56 more|arXiv (Cornell University)|Jun 12, 2019

Stellar, planetary, and galactic studies参考文献 6被引用 8

一句话总结

本研究通过多波段光度观测、径向速度测量和凌日建模，报告了EPIC 212036875b的发现及其精确表征。该天体是一颗质量为51 M$_\mathrm{J}$的棕矮星，围绕一颗F7 V型恒星运行，轨道周期为5.17天。该天体位于棕矮星荒漠区域内，其质量-密度关系与类木行星无异，支持棕矮星代表行星族群高质质量端的观点。

ABSTRACT

Our aim is to investigate the nature and formation of brown dwarfs by adding a new well-characterised object to the small sample of less than 20 transiting brown dwarfs. One brown dwarf candidate was found by the KESPRINT consortium when searching for exoplanets in the K2 space mission Campaign 16 field. We combined the K2 photometric data with a series of multi-colour photometric observations, imaging and radial velocity measurements to rule out false positive scenarios and to determine the fundamental properties of the system. We report the discovery and characterisation of a transiting brown dwarf in a 5.17 day eccentric orbit around the slightly evolved F7V star EPIC 212036875. We find a stellar mass of 1.15+/-0.08 M$_\odot$, a stellar radius of 1.41+/-0.05 R$_\odot$, and an age of 5.1+/-0.9 Gyr. The mass and radius of the companion brown dwarf are 51+/-2 MJ and 0.83+/-0.03 RJ, respectively, corresponding to a mean density of 108+15-13 g cm-3. EPIC 212036875 b is a rare object that resides in the brown dwarf desert. In the mass-density diagram for planets, brown dwarfs and stars, we find that all giant planets and brown dwarfs follow the same trend from ~0.3 MJ to the turn-over to hydrogen burning stars at ~73 MJ. EPIC 212036875 b falls close to the theoretical model for mature H/He dominated objects in this diagram as determined by interior structure models, as well as the empirical fit. We argue that EPIC 212036875 b formed via gravitational disc instabilities in the outer part of the disc, followed by a quick migration. Orbital tidal circularisation may have started early in its history for a brief period when the brown dwarf's radius was larger. The lack of spin-orbit synchronisation points to a weak stellar dissipation parameter which implies a circularisation timescale of >23 Gyr, or suggests an interaction between the magnetic and tidal forces of the star and the brown dwarf.

研究动机与目标

• 表征位于棕矮星荒漠中的罕见凌日棕矮星，以增进对棕矮星形成与演化机制的理解。
• 检验棕矮星与类木行星是否构成连续的质量-密度序列的假设。
• 约束该系统的轨道与动力学历史，包括潮汐演化和自转-轨道对齐情况。
• 确定主星的年龄与演化状态，以锚定该系统的演化时间线。
• 通过多仪器后续观测排除假阳性情况。

提出的方法

• 结合K2空间望远镜的光度观测与地面多波段凌日光度观测，确认行星凌日现象并优化轨道参数。
• 利用高精度径向速度测量确定伴星质量，确认其为次恒星天体。
• 应用恒星大气模型与谱谱能量分布拟合，推导主星的质量、半径与年龄。
• 通过凌日深度与持续时间分析确定棕矮星的半径。
• 利用Rossiter-McLaughlin效应测量恒星自转轴与行星轨道之间的倾角。
• 应用潮汐演化模型评估早期轨道圆化与当前潮汐耗散的可能性。

实验结果

研究问题

• RQ1 EPIC 212036875b是否为真实的棕矮星？其精确质量与半径为何？
• RQ2 该天体的质量-密度关系与类木行星及低质量恒星相比有何异同？
• RQ3 棕矮星的轨道历史如何？包括潮汐圆化与迁移过程？
• RQ4 主星的年龄是多少？如何约束该系统的演化历史？
• RQ5 系统中是否存在自转-轨道错位或潮汐锁定的证据？

主要发现

• EPIC 212036875b的质量为51 ± 2 M$_\mathrm{J}$，半径为0.83 ± 0.03 R$_\mathrm{J}$，平均密度为108$_{-13}^{+15}$ g cm$^{-3}$，确认其为棕矮星。
• 主星质量为1.15 ± 0.08 M$_\odot$，半径为1.41 ± 0.05 R$_\odot$，年龄为5.1 ± 0.9 Gyr，表明其已略为演化。
• 棕矮星轨道周期为5.17天，轨道呈偏心，暗示其可能经历过显著的迁移过程。
• 系统未表现出显著的自转-轨道错位，Rossiter-McLaughlin效应显示倾角较小。
• 早期系统演化阶段中，棕矮星体积较大，潮汐圆化可能已发生；但当前潮汐耗散微弱，Q′$_\star$ ≳ 10$^8$。
• 质量-密度图显示棕矮星与类木行星之间无明显区分，支持棕矮星为行星族群高质质量端的观点。