Aims. Our aim is to identify brown dwarf members of the nearby Hyades open star cluster to determine the photometric and spectroscopic properties of brown dwarfs at moderately old ages and extend the knowledge of the substellar mass function of the cluster. Methods. We cross-matched the 2MASS and AllWISE public catalogues and measured proper motions to identify low-mass stars and brown dwarf candidates in an area of radius eight degrees around the central region of the Hyades cluster. We identified objects with photometry and proper motions consistent with cluster membership. For the faintest ( J ?=?17.2 mag) most promising astrometric and photometric low-mass candidate 2MASS?J04183483 + 2131275, with a membership probability of 94.5%, we obtained low-resolution ( R ?=?300 ? 1000) and intermediate-resolution ( R ?=?2500) spectroscopy with the 10.4m Gran Telescopio Canarias. Results. From the low-resolution spectra we determined a L5.0 ± 0.5 spectral type, consistent with the available photometry. In the intermediate dispersion spectrum we detected H α in emission (marginally resolved with a full width half maximum of ~ 2.8 ?) and determined a log?( L _(H α ) / L _(bol)) = ?6.0 dex. From H α we obtained a radial velocity of 38.0 ± 2.9 km?s ~(-1) , which combined with the proper motion leads to space velocities which are fully consistent with membership in the Hyades cluster. We also report a detection in the H _(2) band by the UKIDSS Galactic Plane Survey. Using evolutionary models we determine from the available photometry of the object a mass in the range 0.039 ? 0.055 M _(⊙) . Brown dwarfs with masses below 0.055 M _(⊙) should fully preserve its initial lithium content, and indeed the spectrum at 6708 ? may show a feature consistent with lithium preservation; however, a higher S/N is needed to confirm this point. Conclusions. We have identified a new high-probability L5 brown dwarf member of the Hyades cluster. This is the first relatively old L5 brown dwarf with a well-determined age (500 ? 700 Myr) and measured chromospheric emission.
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机译：目标。我们的目标是识别附近的卫星开放星团的棕色矮人成员，以确定棕色矮人的光度和光谱特性，在中等老年龄段，延长了集群的子弹质量功能的知识。方法。我们交叉匹配2MASS和换货公共目录，并测量了在卫星集群的中心区域周围半径八度的区域中识别低质量恒星和棕色矮小候选的适当动作。我们识别了具有测光和与集群成员符合的适当动作的对象。对于最微弱的（j？= 17.2 mag）最有前途的星形尺寸和光度低质量候选2mass？J04183483 + 2131275，隶属概率为94.5％，我们获得了低分辨率（R？=？300？1000）和中间 - 用10.4M Gran Telescopio Canarias的光谱学（R？= 2500）光谱。结果。从低分辨率光谱，我们确定了一个L5.0±0.5光谱型，与可用度测光一致。在中间色散频谱中，我们在发射中检测到Hα（用全宽半最大〜2.8Ω）的发射（略微解析）并确定了日志？（L _（Hα）/ L _（BOL））=？6.0 dex。来自Hα，我们获得了38.0±2.9 km的径向速度为38.0±2.9公里？S〜（-1），其与适当的运动相结合，导致空间速度与卫星群中的成员身份完全一致。我们还通过UKIDS银行平面调查报告了H _（2）频带中的检测。使用进化模型我们从物体的可用度测光中确定0.039的质量？ 0.055 m _（⊙）。棕色矮人低于0.055 m _（⊙）应完全保持其最初的锂含量，并确实在6708时的频谱？可以显示一致与锂保存一致的功能;但是，需要更高的S / N来确认这一点。结论。我们已经确定了一个新的Hyades集群的高概率L5棕色矮种成员。这是第一个相对较旧的L5棕色矮种，具有良好的年龄（500？700 myr）和测量的色斑排放。
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