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arXiv 1301.4522astro-ph.EP
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热木星大气中的三维混合。I.应用于HD 209458 b中的日/夜冷阱

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

Vivien Parmentier, Adam P. Showman, Yuan Lian

摘要:

热蒸发器的大气温度范围从几百到几千开尔文。由于它们的昼夜温差很大,可冷凝的物质,如TiO和硅酸盐,在白天的气相中是稳定的,可能会冷凝并在重力作用下沉降在夜晚。大气环流可以抵消这种重力沉降的趋势。这种化学物质的三维(3D)混合以前没有被研究过,但对于评估这些行星大气中TiO和硅酸盐的存在和分布至关重要。我们执行3D全球环流模型的HD209458 b,包括被动示踪剂,平流与3D流,包括一个源/汇的夜面代表凝结和重力沉降的雾霾颗粒。我们表明,全球平流模式产生强烈的垂直混合,可以保持可冷凝的物种阁楼,只要他们被困在几微米或更小的颗粒大小的夜晚一侧。我们发现,垂直混合的结果,而不是从小尺度对流,但从大尺度环流驱动的昼夜加热对比。虽然这种垂直混合在任何严格意义上是不扩散的,我们的结果与理想化的扩散模型的比较允许一个粗略的估计垂直扩散系数。Kzz= 5x 10 **4/Sqrt(Pbar)m2/s可用于HD 209458 b的1D模型。此外,我们的模型表现出强烈的空间和时间的变化,示踪剂浓度,可能会导致在过境或二次日食测量可观察到的变化。最后,我们将我们的模型应用于HD209458 b中TiO的情况,并表明如果TiO在行星的夜晚凝结成大于几微米的颗粒,则昼夜冷阱将耗尽TiO,使其无法创建观测到的行星平流层。

英文摘要:

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.

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