ESO超大质量行星调查IX：Luhman 16AB的同位素证据表明近期形成
The ESO SupJup Survey IX: Isotopic evidence of a recent formation for Luhman 16AB
AI总结：
通过同位素分析，研究Luhman 16AB大气化学不平衡及元素组成，揭示其可能的近期形成及年轻年龄
摘要：
直接成像的系外行星和孤立棕矮星不同的形成途径可能在继承的元素和同位素丰度中留下印记，但此类测量需要仔细表征大气层。特别是L-T过渡体表现出驱动其大气层脱离化学平衡的动力学迹象。在本研究中，我们研究了最近的L-T棕矮星Luhman 16A和B，以评估其大气层的化学不平衡，并调查其元素和同位素组成。作为ESO超大质量行星调查的一部分，我们获得了双星的高分辨率CRIRES+K波段光谱，这些光谱使用大气检索框架进行分析。我们检测并检索了$^{12}$CO、H$_2$O、CH$_4$、NH$_3$、H$_2$S、HF以及$^{13}$CO同位素的丰度。两个大气层都处于化学不平衡状态，Luhman 16A的垂直混合比B更强（$K_\mathrm{zz,A}\sim10^{8.7}$，$K_\mathrm{zz,B}\sim10^{8.2}\ \mathrm{cm^2\ s^{-1}}$）。测试的化学模型，自由和非平衡化学，产生一致的混合比，并与在较短波长下的先前工作一致。气体C/O比显示出硅酸盐氧化物云中氧的捕获证据。虽然C/O比与太阳一致，但金属度有所增强，$\mathrm{[C/H]}\sim0.15$。碳同位素比测量为$\mathrm{^{12}C/^{13}C_A}=74^{+2}_{-2}$和$\mathrm{^{12}C/^{13}C_B}=74^{+3}_{-3}$。双星的金属度和同位素的共同约束加强了它们可能共享形成的可能性。$\mathrm{^{12}C/^{13}C}$比与现今星际介质一致，但低于太阳系值。这表明了近期的继承，并证实了Luhman 16AB作为Oceanus移动团成员的相对较年轻年龄（$\sim500$ Myr）
英文摘要：
The distinct formation pathways of directly-imaged exoplanets and isolated brown dwarfs might leave imprints in the inherited elemental and isotopic abundances, but such measurements require careful characterisation of the atmospheres. In particular, L-T transition objects exhibit signs of dynamics that drive their atmospheres out of chemical equilibrium. In this work, we studied the nearest L-T brown dwarfs, Luhman 16A and B, to assess the chemical dis-equilibrium in their atmospheres and to investigate their elemental and isotopic composition. As part of the ESO SupJup Survey, we obtained high-resolution CRIRES$^+$ K-band spectra of the binary, which were analysed using an atmospheric retrieval framework. We detect and retrieve the abundances of $^{12}$CO, H$_2$O, CH$_4$, NH$_3$, H$_2$S, HF, and the $^{13}$CO isotopologue. Both atmospheres are in chemical dis-equilibrium with somewhat stronger vertical mixing in Luhman 16A compared to B ($K_\mathrm{zz,A}\sim10^{8.7}$, $K_\mathrm{zz,B}\sim10^{8.2}\ \mathrm{cm^2\ s^{-1}}$). The tested chemical models, free- and dis-equilibrium chemistry, yield consistent mixing ratios and agree with earlier work at shorter wavelengths. The gaseous C/O ratios show evidence of oxygen trapping in silicate-oxide clouds. While the C/O ratios are consistent with solar, the metallicities are modestly enhanced with $\mathrm{[C/H]}\sim0.15$. The carbon isotope ratios are measured at $\mathrm{^{12}C/^{13}C_A}=74^{+2}_{-2}$ and $\mathrm{^{12}C/^{13}C_B}=74^{+3}_{-3}$. The coincident constraints of metallicities and isotopes across the binary reinforce their likely shared formation. The $\mathrm{^{12}C/^{13}C}$ ratios are aligned with the present-day interstellar medium, but lower than the solar-system value. This suggests a recent inheritance and corroborates the relatively young age ($\sim500$ Myr) of Luhman 16AB as members of the Oceanus moving group.
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