We combine Keck/HIRES radial velocities, imaging with HiCIAO/Subaru and the Hubble Space Telescope, and absolute astrometry from Hipparcos and Gaia to measure a dynamical mass of $70 \pm 5$ Jupiter masses for the brown dwarf companion to Gl 229. Gl 229B was the first imaged brown dwarf to show clear signs of methane in its atmosphere. Cooling models have been used to estimate a mass in the range of 20 - 55 Jupiter masses, much lower than our measured value. We argue that our high dynamical mass is unlikely to be due to perturbations from additional unseen companions or to Gl 229B being itself a binary, and we find no evidence of a previously claimed radial velocity planet around Gl 229A. Future Gaia data releases will confirm the reliability of the absolute astrometry, though the data pass all quality checks in both Hipparcos and Gaia. Our dynamical mass implies a very old age for Gl 229, in some tension with kinematic and activity age indicators, and/or shortcomings in brown dwarf cooling models. Gl 229B joins a small but growing list of T dwarfs with masses approaching the minimum mass for core hydrogen ignition.

中文翻译：

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Gliese 229B 的动态质量为 70 ± 5 M Jup，第一个 T 矮人

我们将 Keck/HIRES 径向速度、HiCIAO/Subaru 和哈勃太空望远镜的成像以及来自 Hipparcos 和 Gaia 的绝对天体测量相结合，以测量与 Gl 229 的褐矮星伴星的木星质量为 $70 \pm 5$ 木星质量。Gl 229B是第一个在其大气中显示出明显甲烷迹象的棕矮星。冷却模型已被用于估计 20 - 55 木星质量范围内的质量，远低于我们的测量值。我们认为，我们的高动态质量不太可能是由于来自其他看不见的伴星的扰动，也不太可能是由于 GL 229B 本身就是双星，而且我们没有发现先前声称的围绕 GL 229A 存在径向速度行星的证据。未来的盖亚数据发布将确认绝对天体测量的可靠性，尽管数据通过了 Hipparcos 和盖亚的所有质量检查。我们的动力学质量意味着 GL 229 的年龄非常大，与运动学和活动年龄指标有些紧张，和/或褐矮星冷却模型的缺陷。GL 229B 加入了一个小但不断增长的 T 矮星名单，其质量接近核心氢点火的最小质量。