Cold brown dwarfs are excellent analogs of widely separated, gas giant exoplanets, and provide insight into the potential atmospheric chemistry and physics we may encounter in objects discovered by future direct imaging surveys. We present a low resolution R $\sim$ 300 $M$-band spectroscopic sequence of seven brown dwarfs with effective temperatures between 750 K and 250 K along with Jupiter. These spectra reveal disequilibrium abundances of carbon monoxide (CO) produced by atmospheric quenching. We use the eddy diffusion coefficient (K$_{zz}$) to estimate the strength of vertical mixing in each object. The K$_{zz}$ values of cooler gaseous objects are close to their theoretical maximum and warmer objects show weaker mixing, likely due to less efficient convective mixing in primarily radiative layers. The CO-derived K$_{zz}$ values imply that disequilibrium phosphine (PH$_{3}$) should be easily observable in all of the brown dwarfs, but none as yet show any evidence for PH$_{3}$ absorption. We find that ammonia is relatively insensitive to atmospheric quenching at these effective temperatures. We are able to improve the fit to WISE 0855's $M$-band spectrum by including both CO and water clouds in the atmospheric model.

中文翻译：

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对最冷褐矮星中不平衡 CO 化学的观察

冷棕矮星是广泛分离的气体巨行星系外行星的极好类似物，并提供了对我们可能在未来直接成像调查发现的物体中遇到的潜在大气化学和物理学的洞察。我们展示了有效温度在 750 K 到 250 K 之间的七个褐矮星以及木星的低分辨率 R $\sim$ 300 $M$ 波段光谱序列。这些光谱揭示了大气淬火产生的一氧化碳 (CO) 的不平衡丰度。我们使用涡流扩散系数 (K$_{zz}$) 来估计每个对象中垂直混合的强度。较冷气态物体的 K$_{zz}$ 值接近于它们的理论最大值，而较热的物体表现出较弱的混合，这可能是由于主要辐射层中的对流混合效率较低。CO 衍生的 K$_{zz}$ 值意味着不平衡磷化氢 (PH$_{3}$) 在所有褐矮星中都应该很容易观察到，但目前还没有任何证据表明 PH$_{3} $吸收。我们发现氨在这些有效温度下对大气淬火相对不敏感。通过在大气模型中包含 CO 和水云，我们能够改进对 WISE 0855 的 M$ 波段频谱的拟合。