The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm (∼200–400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate-resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species, like water, methane, and ammonia; species that trace chemical reactions, like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (Guaranteed Time Observation program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855–0714 (using NIRSpec G395M spectra), which has an effective temperature of ∼264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH3D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH3). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.

中文翻译：

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原太阳系 D 到 H 丰度和十亿分之一的 PH3 在最冷的褐矮星中


最冷的 Y 光谱型褐矮星在质量和温度上与冷和暖 （∼200-400 K） 的巨型系外行星相似。因此，我们可以用它们的大气层作为行星大气层的代理，测试我们对这些复杂、凉爽的世界的物理和化学的理解。在这些寒冷的温度下，它们的大气层足够冷，可以形成水云，并且化学时间尺度增加，与较温暖的行星类别相比，增加了化学不平衡的可能性。JWST 观测正在彻底改变这些具有高信噪比、中等分辨率近红外和中红外光谱的世界的表征。这些光谱已被用于测量主要物种的丰度，如水、甲烷和氨;追踪化学反应的物质，如一氧化碳;甚至是一氧化碳和氨的同位素体。在这里，我们使用已发表的固定狭缝（保证时间观测程序 1230）和新的平均时间序列观测（GO 程序 2327）来展示已知最冷的 Y 矮星 WISE 0855-0714（使用 NIRSpec G395M 光谱）的大气反演结果，其有效温度为 ∼264 K。我们通过对氘代甲烷 （CH3D） 和标准甲烷的相对测量，提出了在太阳系外大气中检测氘的方法。由此，我们首次推断出太阳系外的亚恒星天体的 D/H 比。我们还提出了严格限制的十亿分之一磷化氢丰度 （PH3）。我们讨论了我们对这些结果的解释以及对褐矮星和巨型系外行星形成和演化的影响。