HST SPECTRAL MAPPING OF L/T TRANSITION BROWN DWARFS REVEALS CLOUD THICKNESS VARIATIONS
HST SPECTRAL MAPPING OF L/T TRANSITION BROWN DWARFS REVEALS CLOUD THICKNESS VARIATIONS
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DOI：
10.1088/0004-637x/768/2/121
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发表时间：
2013-05-10
影响因子：
4.9
通讯作者：
Jayawardhana, Ray
中科院分区：
文献类型：
--
作者：
Apai, Daniel;Radigan, Jacqueline;Jayawardhana, Ray
大多数直接成像的巨型系外行星比具有相似光谱的褐矮星要暗。为了解释它们的相对低亮度，人们提出了异常多云的大气层。然而，由于任何两个物体之间的多个参数不同，对这一想法进行观测测试仍然是困难的。我们提出了一种新的方法，即灵敏的时间分辨哈勃空间望远镜近红外光谱，用来研究两颗自转的L/T跃迁棕矮星(2M2139和SIMP0136)。这些观测提供了棕矮星云层的空间和光谱分辨率地图。这些数据可以在其他参数不变的情况下研究云结构的变化。我们发现，这两颗褐矮星都表现出相同性质的变化：J-波段和H-波段亮度变化，颜色和光谱变化很小。我们的光曲线模型表明，即使是最简单的表面亮度分布也需要至少三个椭圆点。我们证明，对于每个源，光谱变化可以用两个不同光谱的线性组合来再现，即整个表面被两种不同类型的区域覆盖。对颜色变化和光谱变化进行建模，共同揭示了斑驳的云层，其中包括低亮度、低温厚云和更亮、更薄、更暖的云的空间异质性混合。我们发现，在我们变化的棕矮星目标中看到的相同的厚云团，如果扩展到整个光球层，预测的近红外颜色/等级与直接成像的系外行星所占据的范围相匹配，这些系外行星比具有类似光谱类型的棕矮星更冷、更弱。这支持了这样的模型，即厚厚的云层是这些“弱光”系外行星的近红外特性的原因。
Most directly imaged giant exoplanets are fainter than brown dwarfs with similar spectra. To explain their relative underluminosity, unusually cloudy atmospheres have been proposed. However, with multiple parameters varying between any two objects, it remained difficult to observationally test this idea. We present a new method, sensitive time-resolved Hubble Space Telescope near-infrared spectroscopy, to study two rotating L/T transition brown dwarfs (2M2139 and SIMP0136). The observations provide spatially and spectrally resolved mapping of the cloud decks of the brown dwarfs. The data allow the study of cloud structure variations while other parameters are unchanged. We find that both brown dwarfs display variations of identical nature: J- and H-band brightness variations with minimal color and spectral changes. Our light curve models show that even the simplest surface brightness distributions require at least three elliptical spots. We show that for each source the spectral changes can be reproduced with a linear combination of only two different spectra, i.e., the entire surface is covered by two distinct types of regions. Modeling the color changes and spectral variations together reveal patchy cloud covers consisting of a spatially heterogeneous mix of low-brightness, low-temperature thick clouds and brighter, thin, and warm clouds. We show that the same thick cloud patches seen in our varying brown dwarf targets, if extended to the entire photosphere, predict near-infrared colors/magnitudes matching the range occupied by the directly imaged exoplanets that are cooler and less luminous than brown dwarfs with similar spectral types. This supports the models in which thick clouds are responsible for the near-infrared properties of these "underluminous" exoplanets.