NEAR-INFRARED PHOTOMETRY OF Y DWARFS: LOW AMMONIA ABUNDANCE AND THE ONSET OF WATER CLOUDS
NEAR-INFRARED PHOTOMETRY OF Y DWARFS: LOW AMMONIA ABUNDANCE AND THE ONSET OF WATER CLOUDS
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Y矮星的近红外光度测量：低氨丰度和水云的出现
DOI：
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发表时间：
2014
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通讯作者：
D. Saumon
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作者：
S. Leggett;C. Morley;M. Marley;D. Saumon
本文介绍了在双子座天文台获得的7颗晚型T矮星和9颗Y矮星的近红外测光结果，以及第10颗Y矮星的下限星等。我们还提出了一个重新分析的H波段成像数据从凯克天文台档案，第11 Y矮星。这些数据与早期的MKO系统测光，斯皮策和WISE中红外测光，以及可用的三角测量相结合，创建了一个晚型棕矮星的样本，其中包括10个T9-T9.5矮星或矮星系统，以及16个Y矮星。我们将数据与我们的模型进行比较，其中包括更新的H2和NH3不透明度以及低温冷凝云。模型定性地再现了观测到的颜色趋势;然而，Y 0-Y1矮星的通量存在大约两倍的差异，Teff在350-400 K之间。在Teff = 400 K时，可以通过显著减少NH3吸收来解决问题，例如通过可能通过垂直混合将NH3的丰度减半。在Teff = 350 K时，这种差异可以通过加入厚的水云来解决。这些云的开始可能发生在Teff的一个狭窄范围内，正如所观察到的J-W2颜色变化很大的5 μm通量变化所表明的那样。在已知的Y矮星中，J −W2中最红的是WISEP J182831.08+265037.8和WISE J085510.83−071442.5。我们将前者解释为一对相同的300-350 K矮星，后者解释为一个250 K矮星。如果这两个天体的年龄是103 Gyr，它们的质量分别是100和105 Gyr。
We present new near-infrared photometry for seven late-type T dwarfs and nine Y-type dwarfs, and lower limit magnitudes for a tenth Y dwarf, obtained at Gemini Observatory. We also present a reanalysis of H-band imaging data from the Keck Observatory Archive, for an 11th Y dwarf. These data are combined with earlier MKO-system photometry, Spitzer and WISE mid-infrared photometry, and available trigonometric parallaxes, to create a sample of late-type brown dwarfs that includes 10 T9–T9.5 dwarfs or dwarf systems, and 16 Y dwarfs. We compare the data to our models, which include updated H2 and NH3 opacity, as well as low-temperature condensate clouds. The models qualitatively reproduce the trends seen in the observed colors; however, there are discrepancies of around a factor of two in flux for the Y0–Y1 dwarfs, with Teff ≈ 350–400 K. At Teff ∼ 400 K, the problems could be addressed by significantly reducing the NH3 absorption, for example by halving the abundance of NH3 possibly by vertical mixing. At Teff ∼ 350 K, the discrepancy may be resolved by incorporating thick water clouds. The onset of these clouds might occur over a narrow range in Teff, as indicated by the observed small change in 5 μm flux over a large change in J − W2 color. Of the known Y dwarfs, the reddest in J −W2 are WISEP J182831.08+265037.8 and WISE J085510.83−071442.5. We interpret the former as a pair of identical 300–350 K dwarfs, and the latter as a 250 K dwarf. If these objects are ∼3 Gyr old, their masses are ∼10 and ∼5 Jupiter-masses, respectively.