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arXiv 1008.4582astro-ph.EP
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太阳系外行星HR 8799 b的近红外光谱

Near-Infrared Spectroscopy of the Extrasolar Planet HR 8799 b

Brendan P. Bowler, Michael C. Liu, Trent J. Dupuy, Michael C. Cushing

摘要:

[摘要]介绍了太阳系外行星HR 8799 b的2.12 - 2.23 μ m高对比度积分场光谱。我们的观测是用凯克II望远镜上的OSIRIS获得的,并对2.2 μ m的CH4特征进行了采样,这对光谱分类和作为超冷物体的温度诊断是有用的。HR 8799 b的光谱相对来说没有什么特征,只有很少或没有甲烷吸收,也没有类似绝对星等的T矮星所具有的强烈甲烷吸收。总体而言,我们发现HR 8799 b具有与L5-T2一致的光谱型,尽管与大多数场对象相比,它的SED是非典型的。我们使用Hubeny & Burrows,Burrows等人,和埃姆斯-尘埃模型大气网格,其中包括非平衡化学,非太阳金属丰度,以及清晰和多云的变体。没有模型与所有数据一致,但那些具有中间云的模型产生了更好的拟合。最大的差异发生在J波段,在HR 8799 b中受到高度抑制。最佳拟合有效温度范围为1300 - 1700 K,半径范围为~0.3 - 0.5 RJup。这些数值与基于HR 8799 b光度和HR 8799年龄的演化模型得出的数值(800 - 900 K和1.1 - 1.3 RJup)不一致,这种差异可能是由于不完善的大气模型或模型所涵盖的物理参数范围有限。从演化模型推断的低温表明HR 8799 b比场L/T转变天体冷约400 K,进一步证明了L/T转变是重力依赖的。HR 8799 b的光球层异常多尘,光谱型的光度异常低,次级物理参数异常极端,这使得它看起来不像目前已知的任何一类褐矮星。

英文摘要:

[Abridged] We present 2.12-2.23 um high contrast integral field spectroscopy of the extrasolar planet HR 8799 b. Our observations were obtained with OSIRIS on the Keck II telescope and sample the 2.2 um CH4 feature, which is useful for spectral classification and as a temperature diagnostic for ultracool objects. The spectrum of HR 8799 b is relatively featureless, with little or no methane absorption, and does not exhibit the strong CH4 seen in T dwarfs of similar absolute magnitudes. Overall, we find that HR 8799 b has a spectral type consistent with L5-T2, although its SED is atypical compared to most field objects. We fit the 2.2 um spectrum and the infrared SED using the Hubeny & Burrows, Burrows et al., and Ames-Dusty model atmosphere grids, which incorporate nonequilibrium chemistry, non-solar metallicities, and clear and cloudy variants. No models agree with all of the data, but those with intermediate clouds produce significantly better fits. The largest discrepancy occurs in the J-band, which is highly suppressed in HR 8799 b. The best-fitting effective temperatures range from 1300-1700 K with radii between ~0.3-0.5 RJup. These values are inconsistent with evolutionary model-derived values of 800-900 K and 1.1-1.3 RJup based on the luminosity of HR 8799 b and the age of HR 8799, a discrepancy that probably results from imperfect atmospheric models or the limited range of physical parameters covered by the models. The low temperature inferred from evolutionary models indicates that HR 8799 b is ~400 K cooler than field L/T transition objects, providing further evidence that the L/T transition is gravity-dependent. With an unusually dusty photosphere, an exceptionally low luminosity for its spectral type, and hints of extreme secondary physical parameters, HR 8799 b appears to be unlike any class of field brown dwarf currently known.

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