Archaeology of Exo-Terrestrial Planetary Systems and a Search for Water

我提出的这项研究，将独特地限制围绕其他恒星的类地行星的频率和总体化学成分。至关重要的是，我的研究项目有可能探测到岩石行星体内的水的特征，从而限制海洋和生命栖息地的可能性。这是由生命结束时被称为白矮星的恒星实现的。银河系中超过95%的恒星，包括我们的太阳，将以白矮星的形式结束它们的生命，慢慢地剥离它们的外层，然后收缩成地球大小，慢慢冷却的余烬。通过研究白矮星上的岩石行星系统，人们可以窥见地球的未来。在寻找其他恒星周围的类地行星系统时，白矮星提供了一个独特的优势。由于高重力，重元素迅速下沉到内部，留下纯氢或纯氦的大气层。那些拥有岩石行星系统的白矮星可能会被少量但可检测到的重元素污染，如硅、镁和铁（天文学家简称为“金属”）。直到最近，人们才发现了一些被岩石行星碎片污染的白矮星，我在识别和理解这些恒星及其环境方面发挥了主导作用。这些星系的形成类似于土星环；一颗离恒星太近的大型小行星或卫星被引力撕裂，现在以物质盘的形式围绕白矮星运行。这个碎片环逐渐落在恒星上，用金属污染了原本纯净的大气。我将使用被金属污染的白矮星来推断它们幸存的岩石行星系统的质量、基本化学成分和含水量。这个计划有两个部分：1)观察恒星轨道上的岩石碎片本身，2)测量这些碎片在污染恒星时的化学成分，分解成它的组成元素。第一部分需要红外线和亚毫米观测来探测这些被金属污染的恒星上的温暖的轨道尘埃。这项工作的基本目标是明确地将轨道上的岩石碎片与被污染的恒星大气联系起来，并牢固地将这些物质确定为行星物质。来自温暖尘埃的长波光提供了轨道粒子的空间分布、温度、大小和组成的测量，从而限制了现在被破坏的岩石小行星、月球或行星的性质。我将显著增加已知的带有碎片环的白矮星的数量，以获得对其他恒星周围类地行星系统频率的更好的统计理解。第二步是利用恒星本身来确定落在其大气层上并污染其大气层的行星碎片的总体化学成分。我将利用光学和紫外线观测来测量每颗恒星中数到24种重元素的相对元素丰度。这样，就有可能确定其他恒星周围类地行星的基本组成，并对它们的多样性进行分类。水可以通过寻找岩石携带的氧以外的氧来识别；对每一种被检测到的金属氧化物进行简单的计算，就能发现以水的形式输送的额外氧气。通过这种方式，我将确定岩石小行星和卫星中水的质量，并将这些发现与富含水的太阳系天体进行比较

The research I propose will, uniquely, constrain the frequency and bulk chemical composition of terrestrial planets around other stars. Critically, my research program has the potential to detect the signature of water within rocky planetary bodies and thereby constrain the potential for oceans and habitats for life. This is made possible by stars at the end of their lives called white dwarfs. Over 95% of all stars in the Milky Way, including our Sun, will end their lives as white dwarfs, gently shedding their outer layers, then shrinking to Earth-sized, slowly cooling embers. By studying rocky planetary systems at white dwarfs, one sees a glimpse into the future of the Earth. In the search for terrestrial planetary systems around other stars, white dwarfs offer a unique advantage. Owing to high gravities, heavy elements sink rapidly to the interior, leaving behind pure hydrogen or helium atmospheres. Those white dwarfs with rocky planetary systems can become contaminated by the infall of small, but detectable amounts of heavy elements such as silicon, magnesium, and iron (termed simply 'metals' by astronomers). Only recently, a number of white dwarfs polluted by rocky planetary debris have been discovered, and I have played a lead role in identifying and understanding these stars and their environments. These systems arose analogously to the rings of Saturn; a large asteroid or moon passing too close to the star was torn apart by gravity and now resides as a disk of material orbiting the white dwarf. This ring of debris gradually falls onto the star, contaminating its otherwise-pristine atmosphere with metals. I will use white dwarfs polluted by metals to infer the masses, basic chemical compositions, and water content of their surviving rocky planetary systems. The proposed project has two parts: 1) to observe the rocky debris itself in orbit about the star, and 2) to measure the chemical content of this debris as it pollutes the star, broken down into its constituent elements. The first part requires infrared and submillimeter observations to detect warm orbiting dust at these metal-polluted stars. The basic goal of this work is to unambiguously correlate orbiting rocky debris with the polluted stellar atmospheres, and firmly establish the material as planetary. The long-wavelength light from the warm dust provides a measure of the spatial distribution, temperature, size, and composition of the orbiting particles, thus constraining the nature of the now destroyed, rocky asteroid, moon, or planet. I will significantly increase the number of known white dwarfs with debris rings to gain a superior statistical understanding of the frequency of terrestrial planetary systems around other stars. The second step is to use the star itself to determine the bulk chemical composition of the planetary debris falling onto and polluting its atmosphere. I will measure the relative elemental abundances of several to two dozen heavy elements in each of these stars using optical and ultraviolet observations. In doing so, it will be possible to establish the basic makeup of terrestrial planets around other stars, as well as catalog their diversity. Water can be identified by searching for oxygen in excess of that carried by rocks alone; a simple accounting of each detected metal as an oxide can reveal extra oxygen delivered as H2O. In this way I will determine the mass of water in rocky asteroids and moons, and compare these findings with water-rich Solar System bodies