Y矮星的近红外测光：低氨丰度和水云的开始
Near-infrared photometry of Y dwarfs: low ammonia abundance and the onset of water clouds
摘要：
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，第十一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样本，其中包括10个T9-T9.5矮星或矮星系统和16个Y矮星。我们比较的数据，我们的模式，其中包括更新的H_2和NH_3不透明度，以及低温凝结云。模型定性地再现了观测到的颜色趋势，但是对于Y 0-Y1矮星，T_eff~350- 400 K，通量存在大约两倍的差异。在T_eff~ 400 K时，可以通过显著减少NH_3的吸收来解决问题，例如通过垂直混合使NH_3的丰度减半。在T_eff~ 350 K时，可以通过加入厚的水云来解决这种差异。这些云的开始可能发生在一个狭窄的T_eff范围内，如所观察到的J-W_2颜色的大变化上的5 μ m通量的小变化所指示的。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。如果这些天体的年龄为~3 Gyrs，则它们的质量分别为~10和~5千万亿质量。
英文摘要：
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 eleventh 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 which includes ten T9-T9.5 dwarfs or dwarf systems, and sixteen Y dwarfs. We compare the data to our models which include updated H_2 and NH_3 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 T_eff~350-400K. At T_eff~400K, the problems could be addressed by significantly reducing the NH_3 absorption, for example by halving the abundance of NH_3 possibly by vertical mixing. At T_eff~350K, the discrepancy may be resolved by incorporating thick water clouds. The onset of these clouds might occur over a narrow range in T_eff, as indicated by the observed small change in 5um 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 300-350K dwarfs, and the latter as a 250K dwarf. If these objects are ~3 Gyrs old, their masses are ~10 and ~5 Jupiter-masses respectively.
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