Keck Laser引导星自适应光学系统对M8 + L7双星LHS 2397 aAB的监测：L/T跃迁时的首次动态质量基准
Keck Laser Guide Star Adaptive Optics Monitoring of the M8+L7 Binary LHS 2397aAB: First Dynamical Mass Benchmark at the L/T Transition
摘要：
我们提出了凯克LGS AO成像和孔径掩蔽观测的M8+L7二进制LHS 2397 aAB。再加上档案HST，Gemini-North和VLT数据，我们的观测跨越了11.8年的双星14.2年轨道周期。我们确定的总动力学质量为0.146+/-0.014 MSun（153+/-15 MJup）。使用总质量和单个光度的综合观测约束，图森（里昂）演化模型给出了系统的年龄为1.6 Gyr（1.8 Gyr），与其空间运动一致。我们还使用这些模型来确定质量比，LHS 2397 aA的单个质量为0.0839+/-0.0011 MSun（0.0848+/-0.0011 MSun），LHS 2397 aB的单个质量为0.061+/-0.013 MSun（0.060+/-0.010 MSun）。由于LHS 2397 aB非常接近锂燃烧的理论质量极限，测量它的锂消耗将独特地测试亚恒星模型。我们估计LHS 2397 aB的光谱类型为L7+/-1，使其成为第一个具有动力学质量测定的L/T过渡天体。这使得图森（里昂）模型能够精确估计其有效温度为1460+/-40 K（1440+/-40 K），比年轻的晚L型伴星高出200 K，但与较老的晚L型场矮星一致，支持了L/T转变的温度依赖于表面引力的观点。将我们对LHS 2397 aB的温度估计与类似天体的光谱合成模型进行比较，揭示了L/T过渡阶段演化模型和大气模型之间的一致性，尽管目前对亚恒星演化这一阶段的理解有限。未来的动态质量的L/T双星跨越一系列的表面重力和年龄将提供下一个关键的测试亚恒星模型在L/T过渡。
英文摘要：
We present Keck LGS AO imaging and aperture masking observations of the M8+L7 binary LHS 2397aAB. Together with archival HST, Gemini-North, and VLT data, our observations span 11.8 years of the binary's 14.2-year orbital period. We determine a total dynamical mass of 0.146+/-0.014 MSun (153+/-15 MJup). Using the combined observational constraints of the total mass and individual luminosities, the Tucson (Lyon) evolutionary models give an age for the system of 1.6 Gyr (1.8 Gyr), consistent with its space motion. We also use these models to determine the mass ratio, giving individual masses of 0.0839+/-0.0011 MSun (0.0848+/-0.0011 MSun) for LHS 2397aA and 0.061+/-0.013 MSun (0.060+/-0.010 MSun) for LHS 2397aB. Because LHS 2397aB is very close to the theoretical mass-limit of lithium burning, measuring its lithium depletion would uniquely test substellar models. We estimate a spectral type of L7+/-1 for LHS 2397aB, making it the first L/T transition object with a dynamical mass determination. This enables a precise estimate of its effective temperature from Tucson (Lyon) models of 1460+/-40 K (1440+/-40 K), which is 200 K higher than estimates for young late-L companions but consistent with older late-L field dwarfs, supporting the idea that the temperature of the L/T transition is surface gravity dependent. Comparing our temperature estimate for LHS 2397aB to those derived from spectral synthesis modeling for similar objects reveals consistency between evolutionary and atmospheric models at the L/T transition, despite the currently limited understanding of this phase of substellar evolution. Future dynamical masses for L/T binaries spanning a range of surface gravity and age will provide the next critical tests of substellar models at the L/T transition.
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