T型矮星系Indi Ba的K-H_2准分子吸收
K-H_2 Quasi-molecular absorption detected in the T-dwarf epsilon Indi Ba
摘要：
T型矮星呈现出以6950 A为中心的宽而浅的吸收特征，位于0.77微米处的K双峰的蓝翼中，其在深度和形状上类似于通过详细的碰撞增宽轮廓预测的卫星吸收。然而，在我们之前的工作中，与观测到的特征相比，预测的线卫星位置有点太蓝了。本文研究了K-H_2系统势能面的新计算，包括自旋-轨道耦合，是否能使卫星位置与观测位置更接近。我们还调查了CaH吸收带的功能在多大程度上作出贡献。我们提出了模型大气和合成光谱，包括重力沉降，以改进对难熔元素丰度随深度变化的描述，并基于新的K-H_2谱线轮廓，使用改进的相互作用势。通过与T1矮星系Indi Ba的高信噪比光谱的比较，我们发现这些新模型确实再现了观测到的特征，而CaH对所考虑的大气参数没有贡献。我们还发现，CaH是解决了如此之深的大气层，即使湍流垂直混合似乎不足以使大量的CaH的光球在矮星晚于~L5。我们的结论是，以前的识别功能在这个位置上的T和晚L矮星光谱与钙是错误的，正如预期的物理基础：钙凝聚到颗粒在早期的L矮星，因此应该解决了光球在较冷的褐矮星。这一发现推翻了云清除理论的一项观察验证：早期T矮星的云层逐渐清除。
英文摘要：
T-type dwarfs present a broad and shallow absorption feature centred around 6950 A in the blue wing of the K doublet at 0.77 micron which resembles in depth and shape the satellite absorption predicted by detailed collisional broadening profiles. In our previous work, the predicted line satellite position was however somewhat too blue compared to the observed feature. In this paper we investigate whether new calculations of the energy surfaces of the potentials in the K-H_2 system, including spin-orbit coupling, result in a closer coincidence of the satellite with the observed position. We also investigate the extent to which CaH absorption bands contribute to the feature. We present model atmospheres and synthetic spectra, including gravitational settling for an improved description of depth-dependent abundances of refractory elements, and based on new K-H_2 line profiles using improved interaction potentials. By comparison with a high signal-to-noise optical spectrum of the T1 dwarf epsilon Indi Ba, we find that these new models do reproduce the observed feature, while CaH does not contribute for the atmospheric parameters considered. We also find that CaH is settled out so deep into the atmosphere that even turbulent vertical mixing would appear insufficient to bring significant amounts of CaH to the photosphere in dwarfs later than ~L5. We conclude that previous identification of the feature at this location in T and late L dwarf spectra with CaH was erroneous, as expected on physical grounds: calcium condenses onto grains in early L dwarfs and thus should have settled out of the photosphere in cooler brown dwarfs. This finding revokes one observational verification for the cloud-clearing theory: a gradual clearing of the cloud cover in early T dwarfs.
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