Atmospheric Circulation and Evolution of Short-Period Extrasolar Giant Planets

AST0307664显示在围绕其他恒星发现的巨行星中，有五分之一的行星的轨道距离它们的恒星不到0.1个天文单位。从地球上看，这些行星的小轨道增加了它们凌日恒星的可能性。一颗这样的行星，HD209458b，已经被观测到每3.5天凌日一次。从这颗行星从其恒星前面经过时恒星光线变暗的情况来看，我们现在知道这颗行星的半径是木星的1.35倍。在接下来的几年里，预计还会有数十次凌日探测，从而可以计算出更多行星的半径，还可能直接测量反照率、昼夜温差和大气成分。此外，计划于2006年发射的开普勒任务应该可以推断出至少100颗凌日热木星的半径和反照率。要理解这些测量，就需要了解可能的大气环流制度。昼夜温差取决于地球上平流(即风)的速度。反照率取决于云层形成的位置，而硅酸盐或碱化合物等可凝聚物质的丰度则取决于环流的几何形状。此外，了解半径将需要了解环流的知识。半径代表了行星冷却和收缩的历史，它通过环流对温度分布、云层丰度和内部能量传输的影响而受到环流的影响。使用真实大气温度的HD209458b的演化模型预测的半径太小，这表明计算中缺少一些热源。最好的假设是，大气热机产生的动能被输送到地球内部，在那里它抵消了导致行星收缩的能量损失。在这个项目中，亚利桑那大学的Adam Showman博士将研究短周期行星(即所谓的“热木星”)的大气环流和演化。Showman博士及其合作者将对HD209458b(和其他热木星)的大气环流进行三维、非线性的数值模拟，以确定(I)环流的性质，包括风速、昼夜温差和垂直结构，从而影响这些行星的红外线光曲线；(Ii)云形成的位置，以及对整个行星的反照率及其空间变化的影响；(Iii)动能产生和耗散的大小和深度，这对于评估大气热机能否将HD209458b的半径“膨胀”到足以满足其测量值是至关重要的。这些模拟将与现有和即将进行的观测紧密联系在一起，是首批调查动力学对这些行星可观测特性影响的模拟之一。该项目的更广泛影响是：(I)将进行太阳系外行星的图形可视化，与模拟一致，以显示这些行星可能是什么样子。这些将用于公开和科学陈述和采访，以提高公众对太阳系外行星的兴趣。(Ii)将通过科学讲座、公开演讲和采访向科学界和公众广泛传播研究成果。(3)一名研究生将完成本文所述的博士论文工作。(Iv)Showman博士将利用这项研究来说明他所教授的研究生和本科生课程中发现的兴奋。

AST 0307664ShowmanOne fifth of the giant planets discovered around other stars are orbiting at distances less than 0.1 Astronomical Unit from their stars. These planets' small orbits increase the likelihood that they will transit their stars as viewed from Earth. One such planet, HD209458b, has already been observed to transit its star every 3.5 days. From the dimming of the stellar light as the planet passes in front of its star, we now know that the planet's radius is 1.35 times that of Jupiter. Dozens more transit detections are expected within the next several years, allowing additional planets' radii to calculated, and direct measurements of albedo, day-night temperature differences, and atmospheric composition are also likely. Furthermore, the Kepler mission, scheduled for launch in 2006, should allow the radii and albedos of at least 100 transiting hot Jupiters to be inferred. An understanding of these measurements will require knowledge of possible atmospheric circulation regimes. The day-night temperature difference depends on the speed of advection (i.e., winds) across the planet. The albedo depends on where clouds form, and the abundance of condensible species such as silicates or alkali compounds depends on the geometry of the circulation. Furthermore, understanding the radius will require knowledge of the circulation. The radius represents the planet's history of cooling and contraction, which is affected by the circulation through the circulation's influence ontemperature profile, cloud abundance, and internal energy transport. Evolution models of HD209458b that use realistic atmospheric temperatures predict a radius that is too small, which suggests that some heat source is missing from the calculations. The best hypothesis is that kinetic energy produced by the atmospheric heat engine is transported into the interior, where it counteracts the loss of energy that causes planetary contraction. In this project Dr. Adam Showman, University of Arizona, will investigate the atmospheric circulation and evolution of short-period planets (the so-called "hot Jupiters"). Dr. Showman and collaborators will conduct three-dimensional, nonlinear numerical simulations of the atmospheric circulation of HD209458b (and other hot Jupiters) to determine (i) the nature of the circulation, including wind speeds, day-night temperature differences, and vertical structure, with implications for the infrared light curve of these planets, (ii) locations of cloud formation, with implications for albedo and its spatial variation across the planet, and (iii) the magnitude and depth of kinetic energy production and dissipation, which is crucial in evaluating whether the atmospheric heat engine can "inflate" the radius of HD209458b enough to satisfy its measured value. The simulations will be tightly linked to existing and upcoming observations and are among the first to investigate the effects of dynamics on the observable properties of these planets. Broader impacts of the project are: (i) Graphic visualization of extrasolar planets will be performed,consistent with the simulations, to show what these planets may look like. These will be used in public and scientific presentations and interviews to increase public interest in extrasolar planets. (ii) The results of the research will be widely disseminated to the scientific community and general public through scientific talks, public presentations, and interviews. (iii) A graduate student will perform the work described here for a Ph.D. thesis. (iv) Dr. Showman will use the research to illustrate the excitement of discovery in the graduate and undergraduate courses that he teaches.***