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

我提出的研究将独特地限制其他恒星周围类地行星的频率和大量化学成分。至关重要的是，我的研究计划有可能探测到岩石行星体内水的特征，从而限制海洋和生命栖息地的潜力。这是因为恒星在生命的尽头被称为白色矮星。银河系中超过95%的恒星，包括我们的太阳，将以白色矮星的形式结束它们的生命，轻轻地脱落它们的外层，然后缩小到地球大小，慢慢冷却的灰烬。通过研究白色矮星上的岩石行星系统，人们可以瞥见地球的未来。在寻找其他恒星周围的类地行星系统时，白色矮星提供了独特的优势。由于高重力，重元素迅速下沉到内部，留下纯氢或氦的大气。那些拥有岩石行星系统的白色矮星可能会被少量但可检测到的重元素如硅、镁和铁（天文学家简单地称为“金属”）所污染。直到最近，才发现了一些被岩石行星碎片污染的白色矮星，我在识别和了解这些恒星及其环境方面发挥了主导作用。这些系统的形成类似于土星环;一颗大的小行星或卫星太靠近星星，就会被引力撕裂，现在作为一个圆盘围绕着白色矮星运行。这个碎片环逐渐福尔斯落到星星上，金属污染了原本纯净的大气层。我将使用被金属污染的白色矮星来推断它们幸存的岩石行星系统的质量、基本化学成分和水含量。拟议的项目有两个部分：1）观察围绕星星运行的岩石碎片本身，2）测量这些碎片污染星星时的化学含量，并将其分解为组成元素。第一部分需要红外和亚毫米观测，以探测这些金属污染恒星的温暖轨道尘埃。这项工作的基本目标是明确地将轨道上的岩石碎片与受污染的恒星大气层联系起来，并牢固地确立这些物质是行星。来自温暖尘埃的长波长光提供了对轨道粒子的空间分布、温度、大小和组成的测量，从而限制了现在被摧毁的岩石小行星、月球或行星的性质。我将显著增加已知的带有碎片环的白色矮星的数量，以获得对其他恒星周围的类地行星系统频率的上级统计理解。第二步是利用星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大量化学成分。我将使用光学和紫外线观测来测量每颗恒星中几到二十几种重元素的相对元素丰度。通过这样做，将有可能建立围绕其他恒星的类地行星的基本构成，并对它们的多样性进行编目。水可以通过寻找超过岩石单独携带的氧气来识别;简单地计算每种检测到的金属作为氧化物可以揭示额外的氧气作为H2O。通过这种方式，我将确定岩石小行星和卫星中的水质量，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