用动力学质量研究晚M矮星的物理多样性
Studying the Physical Diversity of Late-M Dwarfs with Dynamical Masses
摘要：
我们提出了一个系统的研究晚M矮星的物理性质的基础上高质量的动态质量测量和近红外光谱（NIR）。我们使用天体测量凯克NGS和LGS AO成像，以确定晚M双星的轨道。我们发现LP 349-25（M7.5+M8）是一对年轻的棕矮星（Mtot = 0.120 Msun），Lyon和Tucson演化模型联合预测其年龄为1.4亿+/-3000万年。这与昴星团的年龄相一致，但至少LP 349- 25 A违反了昴星团的经验锂耗尽边界，这意味着该系统实际上更古老，并且演化模型低估了其组成部分的光度。我们发现LHS 1901 AB（M6.5+M6.5）是一对质量非常低的恒星（Mtot = 0.194 Msun），模型推导的年龄与其缺乏活动（> 6 Gyr）的限制一致。我们对Gl 569 Bab（M8.5+M9）的改进轨道导致该双星的质量（Mtot = 0.140 Msun）比以前的工作（0.125 Msun）更高。我们使用这些质量以及我们发表的2 MASS J2206- 2047 AB（M8+M8）的结果来测试目前使用的四组超冷模型大气。拟合这些模型，我们的近红外积分光光谱提供温度估计温暖约250 K比那些独立从尘埃演化模型给出的测量质量和光度。我们认为模型大气更有可能是这种差异的来源，因为在进化模型的背景下，很难解释在如此广泛的质量，年龄和活动水平上的均匀温度偏移。我们的结果与Konopacky等人的结果形成对比，因为当我们采用他们的模型测试方法时，我们发现与他们报告的相反且较小的质量差异，因为我们从拟合光谱中估计的Teff比他们单独拟合宽带光度法高约650 K。
英文摘要：
We present a systematic study of the physical properties of late-M dwarfs based on high-quality dynamical mass measurements and near-infrared (NIR) spectra. We use astrometry from Keck NGS and LGS AO imaging to determine orbits for late-M binaries. We find that LP 349-25 (M7.5+M8) is a pair of young brown dwarfs (Mtot = 0.120 Msun) for which Lyon and Tucson evolutionary models jointly predict an age of 140+/-30 Myr. This is consistent with the age of the Pleiades, but at least LP 349-25A defies the empirical Pleiades lithium depletion boundary, implying that the system is in fact older and that evolutionary models underpredict the component luminosities. We find that LHS 1901AB (M6.5+M6.5) is a pair of very low-mass stars (Mtot = 0.194 Msun) with model-derived ages consistent with limits from its lack of activity (> 6 Gyr). Our improved orbit for Gl 569Bab (M8.5+M9) results in a higher mass for this binary (Mtot = 0.140 Msun) compared to previous work (0.125 Msun). We use these masses along with our published results for 2MASS J2206-2047AB (M8+M8) to test four sets of ultracool model atmospheres currently in use. Fitting these models to our NIR integrated-light spectra provides temperature estimates warmer by ~250 K than those derived independently from Dusty evolutionary models given the measured masses and luminosities. We propose that model atmospheres are more likely to be the source of this discrepancy, as it would be difficult to explain a uniform temperature offset over such a wide range of masses, ages, and activity levels in the context of evolutionary models. Our results contrast those of Konopacky et al. as we find an opposite and smaller mass discrepancy from what they report when we adopt their model-testing approach since our Teff estimates from fitting spectra are ~650 K higher than from their fitting of broadband photometry alone.
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