两个在T/Y跃迁的异常亚星双星和最冷棕矮星的Y波段通量
Two Extraordinary Substellar Binaries at the T/Y Transition and the Y-Band Fluxes of the Coolest Brown Dwarfs
摘要：
利用凯克激光导星自适应光学成像，我们发现T9矮星WISE J1217+1626和T8矮星WISE J1711+3500是特殊的双星，与以前已知的超冷双星相比，它们具有异常宽的间隔（~0.8弧秒，8-15 AU），大的近红外通量比（~2-3 mags）和小的质量比（~0.5）。Keck/NIRSPEC H波段光谱给出了WISE J1217+ 1626 B的光谱类型为Y 0，测光估计表明WISE J1711+ 3500 B的T9.5。WISE J1217+ 1626 AB系统与T9+ Y 0双星CFBDSIR J1458+ 1013 AB非常相似，这两个系统是已知最冷的亚星多重星系，其次级成分约为400 K，如果年龄小于1 Gyr，则为行星质量双星。WISE J1217+ 1626 B和CFBDSIR J1458+ 1013 B与之前已知的T矮星相比，都有着惊人的蓝色Y-J，包括它们的T9原色。结合所有可用的数据，我们发现Y-J颜色在最新的T矮星和Y矮星之间急剧下降。事实上，这是看到（同时代，单金属）双星表明，颜色下降引起的温度变化，而不是表面重力或金属丰度的变化之间的领域人口。因此，由近红外光谱建立的T/Y转变与超冷光球的~1微米通量的显著变化一致。一种解释是钾的耗尽，钾的宽吸收翼支配着T矮星的远红光谱。这种大的颜色变化表明，远红外数据可能对<~500 K的物体进行分类很有价值。
英文摘要：
Using Keck laser guide star adaptive optics imaging, we have found that the T9 dwarf WISE J1217+1626 and T8 dwarf WISE J1711+3500 are exceptional binaries, with unusually wide separations (~0.8 arcsec, 8-15 AU), large near-IR flux ratios (~2-3 mags), and small mass ratios (~0.5) compared to previously known field ultracool binaries. Keck/NIRSPEC H-band spectra give a spectral type of Y0 for WISE J1217+1626B, and photometric estimates suggest T9.5 for WISE J1711+3500B. The WISE J1217+1626AB system is very similar to the T9+Y0 binary CFBDSIR J1458+1013AB; these two systems are the coldest known substellar multiples, having secondary components of ~400 K and being planetary-mass binaries if their ages are <~1 Gyr. Both WISE J1217+1626B and CFBDSIR J1458+1013B have strikingly blue Y-J colors compared to previously known T dwarfs, including their T9 primaries. Combining all available data, we find that Y-J color drops precipitously between the very latest T dwarfs and the Y dwarfs. The fact that this is seen in (coeval, mono-metallicity) binaries demonstrates that the color drop arises from a change in temperature, not surface gravity or metallicity variations among the field population. Thus, the T/Y transition established by near-IR spectra coincides with a significant change in the ~1 micron fluxes of ultracool photospheres. One explanation is the depletion of potassium, whose broad absorption wings dominate the far-red optical spectra of T dwarfs. This large color change suggests that far-red data may be valuable for classifying objects of <~500 K.
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