棕矮星沙漠的绿化。EPIC 212036875 b --一个51 M的$_\mathrm{J}$天体，在围绕F7 V星的5天轨道上运行
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
摘要：
我们的目标是通过在不到20颗凌日棕矮星的小样本中添加一个新的特征明确的天体来研究棕矮星的性质和形成。一颗褐矮星候选者是由KESPRINT联盟在K2太空任务Campaign 16领域寻找系外行星时发现的。我们将K2光度数据与一系列多色光度观测、成像和径向速度测量相结合，以排除假阳性情况并确定系统的基本特性。我们报告的发现和过境褐矮星的特点，在一个5.17天的偏心轨道周围略有演变的F7 V恒星EPIC 212036875。我们发现恒星质量为1.15+/-0.08 M $_\odot$，恒星半径为1.41+/-0.05 R $_\odot$，年龄为5.1+/-0.9 Gyr。伴星的质量和半径分别为51+/-2 MJ和0.83+/-0.03 RJ，相当于平均密度为108+15-13 g cm-3。EPIC 212036875 b是一颗位于棕矮星沙漠中的罕见天体。在行星、棕矮星和恒星的质量密度图中，我们发现所有的巨行星和棕矮星都遵循相同的趋势，从~0.3 MJ到~73 MJ的氢燃烧恒星。EPIC 212036875 b在该图中接近于成熟H/He主导对象的理论模型，这是由内部结构模型以及经验拟合确定的。我们认为，EPIC 212036875 b通过重力盘不稳定性形成的盘的外部部分，随后快速迁移。轨道潮汐循环可能在褐矮星的半径较大的时候就开始了一段短暂的历史。缺乏自旋轨道同步指向一个弱的恒星耗散参数，这意味着一个>23 Gyr的循环时标，或者表明恒星和棕矮星的磁力和潮汐力之间的相互作用。
英文摘要：
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.
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