[论文解读] 3D mixing in hot Jupiter atmospheres. I. application to the day/night cold trap in HD 209458b

Vivien Parmentier, Adam P. Showman+1 more|arXiv (Cornell University)|Jan 18, 2013

Astro and Planetary Science参考文献 2被引用 135

一句话总结

本研究对HD 209458b的大气三维环流进行建模，以评估垂直混合如何抵消昼夜冷阱效应，即易凝结物质（如TiO）在较冷的夜晚侧凝结并沉降。研究发现，大尺度环流而非湍流驱动有效的垂直混合，其有效涡旋扩散系数为 $ K_{zz} = 5 \times 10^4 / \sqrt{P_{\text{bar}}} \, \text{m}^2\text{s}^{-1} $，若颗粒小于几微米，TiO可保持在高空；否则将被耗尽，从而阻止平流层加热。

ABSTRACT

Hot Jupiters exhibit atmospheric temperatures ranging from hundreds to thousands of Kelvin. Because of their large day-night temperature differences, condensable species that are stable in the gas phase on the dayside, such as TiO and silicates, may condense and gravitationally settle on the nightside. Atmospheric circulation may counterbalance this tendency to gravitationally settle. This three dimensional (3D) mixing of chemical species has not previously been studied for hot Jupiters, yet it is crucial to assess the existence and distribution of TiO and silicates in the atmospheres of these planets. We perform 3D global circulation models of HD209458b including passive tracers that advect with the 3D flow, including a source/sink on the nightside to represent condensation and gravitational settling of haze particles. We show that global advection patterns produce strong vertical mixing that can keep condensable species lofted as long as they are trapped in particles of sizes of a few microns or less on the night side. We show that vertical mixing results not from small-scale convection but from the large-scale circulation driven by the day-night heating contrast. Although this vertical mixing is not diffusive in any rigorous sense, a comparison of our results with idealized diffusion models allows a rough estimate of the vertical diffusion coefficient. Kzz=5x10**4/Sqrt(Pbar) m2/s can be used in 1D models of HD 209458b. Moreover, our models exhibit strong spatial and temporal variability in the tracer concentration that could result in observable variations during transit or secondary eclipse measurements. Finally, we apply our model to the case of TiO in HD209458b and show that the day-night cold trap would deplete TiO if it condenses into particles bigger than a few microns on the planet's night side, making it unable to create the observed stratosphere of the planet.

研究动机与目标

• 理解三维大气环流如何影响热木星中易凝结物质（如TiO）的垂直混合。
• 评估昼夜冷阱（即TiO在夜晚侧凝结）是否会导致大气中TiO耗尽并阻止平流层加热。
• 量化稳定层结大气中有效垂直涡旋扩散系数（$K_{zz}$），以供一维模型使用。
• 研究由于环流与颗粒沉降耦合导致的示踪剂丰度的空间和时间变化。
• 评估此类变化在凌日与二次掩食观测中的可观测性。

提出的方法

• 使用三维一般环流模型（GCM）模拟HD 209458b大气中的环流，其中被动示踪剂代表易凝结物质。
• 示踪剂随三维流场输运，并在夜晚侧引入源-汇项以模拟凝结成颗粒及重力沉降。
• 模拟了从亚微米至约5 µm的颗粒尺寸，以评估其对垂直混合效率的影响。
• 通过将三维模型结果与理想化扩散模型对比，估算有效垂直涡旋扩散系数（$K_{zz}$）。
• 将模型应用于HD 209458b中的TiO，以确定其在白天侧保持气相状态的条件。
• 分析示踪剂丰度的空间与时间变化，以评估其在凌日与二次掩食光曲线中的潜在可观测性。

实验结果

研究问题

• RQ1 三维大气环流是否足以实现对热木星上易凝结物质（如TiO）的垂直混合，以防止其在夜晚侧因重力沉降而耗尽？
• RQ2 HD 209458b稳定层结大气中的有效垂直涡旋扩散系数（$K_{zz}$）是多少？是否可参数化用于一维模型？
• RQ3 夜晚侧的颗粒尺寸与凝结效率如何影响白天侧大气中TiO的丰度？
• RQ4 由于大尺度环流与颗粒沉降的耦合，示踪剂丰度可能产生何种空间与时间变化？
• RQ5 此类变化是否可在HD 209458b的凌日或二次掩食观测中被探测到？

主要发现

• HD 209458b中的垂直混合由大尺度环流驱动，而非小尺度对流，且其强度足以在夜晚侧颗粒尺寸小于几微米时维持易凝结物质悬浮。
• 有效垂直涡旋扩散系数被参数化为 $ K_{zz} = 5 \times 10^4 / \sqrt{P_{\text{bar}}} \, \text{m}^2\text{s}^{-1} $，适用于约1 bar至约1 µbar范围，可直接用于一维模型。
• 为维持TiO的平流层热逆增，其在夜晚侧形成的颗粒尺寸不得超过约5 µm；若颗粒更大，则导致TiO耗尽。
• 示踪剂丰度存在显著的空间差异：在约0.1 bar高度，赤道区域的耗竭程度高于两极；在约1 bar高度，出现明显的昼夜差异。
• 对于尺寸约5 µm的颗粒，白天侧丰度的时变幅度可达约50%，沿边缘区域可达约75%，其特征时间尺度为数天至约100天。
• 此类变化可能在高度受 irradiated 行星的Kepler波段光曲线中可观测，特别是通过二次掩食映射或反射光变化。