标题： 两种分子的故事：晚期T和Y矮星大气模型中CO$_2$的低估和PH$_3$的高估 显示英文标题

标题： A Tale of Two Molecules: The Underprediction of CO$_2$ and Overprediction of PH$_3$ in Late T and Y Dwarf Atmospheric Models

摘要： JWST的灵敏度和光谱覆盖范围使我们能够测试对超冷矮星大气化学的假设，特别是关于磷化氢(PH$_3$)和二氧化碳(CO$_2$)的丰度。 在本文中，我们使用四颗晚期T型和Y型矮星的NIRSpec PRISM光谱（$\sim$0.8$-$5.5$\mu$m，$R\sim$100）来表明标准的亚恒星大气模型在4.1$-$4.4$\mu$m波长范围内难以再现，因为它们预测了过量的磷化氢和二氧化碳不足。 为了帮助量化这一差异，我们基于Elf Owl化学和温度剖面生成了一个模型网格，其中将这两种分子的丰度作为参数。 这些对PICASO模型的拟合显示出对CO$_2$含量高一个数量级的偏好，与标准模型相比，PH$_3$含量有所减少。这种趋势意味着在UNCOVER$-$BD$-$3中声称的磷化氢检测可能实际上是由于CO$_2$含量超过标准大气模型预测值；然而，光谱的信噪比不足以区分这两种情况。 我们讨论可能解释观测到的PH$_3$的缺乏和CO$_2$的过量的大气机制，包括随高度变化的垂直涡旋扩散系数($K_{\mathrm{zz}}$)，错误的化学路径，或元素以NH$_4$H$_2$PO$_4$等形式凝结出来。 然而，我们认为所需的CO$_2$增强的原因是淬火近似不能准确预测CO$_2$的丰度，因为CO$_2$在CO淬火后仍与CO保持化学平衡。 显示英文摘要

摘要： The sensitivity and spectral coverage of JWST is enabling us to test our assumptions of ultracool dwarf atmospheric chemistry, especially with regards to the abundances of phosphine (PH$_3$) and carbon dioxide (CO$_2$). In this paper, we use NIRSpec PRISM spectra ($\sim$0.8$-$5.5 $\mu$m, $R\sim$100) of four late T and Y dwarfs to show that standard substellar atmosphere models have difficulty replicating the 4.1$-$4.4 $\mu$m wavelength range as they predict an overabundance of phosphine and an underabundance of carbon dioxide. To help quantify this discrepancy, we generate a grid of models using PICASO based on the Elf Owl chemical and temperature profiles where we include the abundances of these two molecules as parameters. The fits to these PICASO models show a consistent preference for orders of magnitude higher CO$_2$ abundances and a reduction in PH$_3$ abundance as compared to the nominal models. This tendency means that the claimed phosphine detection in UNCOVER$-$BD$-$3 could instead be explained by a CO$_2$ abundance in excess of standard atmospheric model predictions; however the signal-to-noise of the spectrum is not high enough to discriminate between these cases. We discuss atmospheric mechanisms that could explain the observed underabundance of PH$_3$ and overabundance of CO$_2$, including a vertical eddy diffusion coefficient ($K_{\mathrm{zz}}$) that varies with altitude, incorrect chemical pathways, or elements condensing out in forms such as NH$_4$H$_2$PO$_4$. However, our favored explanation for the required CO$_2$ enhancement is that the quench approximation does not accurately predict the CO$_2$ abundance, as CO$_2$ remains in chemical equilibrium with CO after CO quenches.