arXiv 1609.058502016-09-20astro-ph.SRastro-ph.EP

斯皮策太空望远镜对已知最冷的褐矮星的光度监测

Photometric Monitoring of the Coldest Known Brown Dwarf with the Spitzer Space Telescope

Taran Esplin, Kevin Luhman, Michael Cushing, Kevin Hardegree-Ullman, Jessica Trucks, Adam Burgasser, Adam Schneider

备注：: 5 pages, 3 figures, 2 machine readable tables avaiable at https://www.dropbox.com/s/c8vbzcnk5ut8t64/tab1.txt?dl=0 and https://www.dropbox.com/s/olop66rke57fb2m/tab2.txt?dl=0 , respectively. Accepted for publication in The Astrophysical Journal
DOI：: 10.3847/0004-637X/832/1/58

摘要：

由于WISE J085510.83 $-$ 071442.5（以下简称WISE 0855-0714）是已知最冷的棕矮星（$\sim250$ K），也是太阳最近的邻居之一（2.2 pc），它提供了一个独特的机会来研究一个未被探索的温度制度下的行星状大气。为了检测和表征其大气中的不均匀性（例如，我们用斯皮策太空望远镜对WISE 0855-0714进行了3.6和4.5微米的时间序列测光监测，监测周期为23小时，间隔数月。对于这两个波段，我们已经检测到的变化与峰-峰幅度的4-5%和3-4%，在第一和第二时期，分别。两个波段的光变曲线在第一个历元都是半周期性的，但在第二个历元中更不规则。斑片状云的模型已经预测了在$T_{\rm eff}<$ 375 K出现水冰云时中红外变化幅度的大幅增加（对于给定的云覆盖率），因此如果这种云是WISE 0855-0714变化的原因，那么它的小变化幅度表明半球之间云覆盖率的偏差非常小。或者，WISE 0855-0714与稍微温暖的T和Y矮星之间的中红外变化幅度的相似性可能表明它们的变化具有共同的起源（即，不是水云）。除了我们的变率数据，我们还研究了WISE 0855-0714大气中水冰云存在的其他限制，包括最近从\citet{ske16}获得的中红外光谱。我们发现，这种云的有力证据还没有。

英文摘要：

Because WISE J085510.83$-$071442.5 (hereafter WISE 0855-0714) is the coldest known brown dwarf ($\sim250$ K) and one of the Sun's closest neighbors (2.2 pc), it offers a unique opportunity for studying a planet-like atmosphere in an unexplored regime of temperature. To detect and characterize inhomogeneities in its atmosphere (e.g., patchy clouds, hot spots), we have performed time-series photometric monitoring of WISE 0855-0714 at 3.6 and 4.5 micron with the Spitzer Space Telescope during two 23~hr periods that were separated by several months. For both bands, we have detected variability with peak-to-peak amplitudes of 4-5% and 3-4% in the first and second epochs, respectively. The light curves are semi-periodic in the first epoch for both bands, but are more irregular in the second epoch. Models of patchy clouds have predicted a large increase in mid-IR variability amplitudes (for a given cloud covering fraction) with the appearance of water ice clouds at $T_{\rm eff}<$375 K, so if such clouds are responsible for the variability of WISE 0855-0714, then its small amplitudes of variability indicate a very small deviation in cloud coverage between hemispheres. Alternatively, the similarity in mid-IR variability amplitudes between WISE 0855-0714 and somewhat warmer T and Y dwarfs may suggest that they share a common origin for their variability (i.e., not water clouds). In addition to our variability data, we have examined other constraints on the presence of water ice clouds in the atmosphere of WISE 0855-0714, including the recent mid-IR spectrum from \citet{ske16}. We find that robust evidence of such clouds is not yet available.

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英文摘要：

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