Kinematics, Galactic Age, Chemistry and Water Fraction of Asteroid-Polluted White Dwarfs from the Sloan Digital Sky Survey

在近800颗被证实围绕其他恒星运行的太阳系外行星中，只有一小部分被认为是固体行星，因此在理论上与水星、金星、地球和火星相似。这些行星几乎完全是通过美国宇航局的开普勒任务发现的，该任务探测到行星在主星前面经过时的凌日现象。凌日为科学家们提供了行星的大小，在某些情况下，地面上的径向速度观测可以提供质量，从而给出密度，并表明可能的固体成分。然而，这些行星的实际组成是未知的，目前或未来都没有办法进行这样的测量。然而，生命结束的恒星——被称为白矮星——在研究类地系外行星系统方面提供了独特的优势。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。由于高引力，重元素迅速下沉到内部，在这些恒星中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以通过少量但可检测的重元素（如硅、镁和铁）来丰富化学物质。在过去的几年里，天文学家发现了被小行星的岩石碎片“污染”的白矮星，并且能够从污染恒星大气的元素中获得小行星的基本成分。在太阳系中，小行星是类地行星的剩余基石，因此白矮星的太阳系外小行星表明了它们的岩石系外行星的基本组成。银河系中有三个主要的恒星（和行星）形成时期。第一批恒星形成于110 - 150亿年前，现在位于环绕银河系圆盘的球形晕中。接下来的恒星形成于70 - 120亿年前，在银河平面周围形成一个厚盘，而像我们的太阳这样的恒星形成于20 - 80亿年前，占据一个我们在夜空中称为“银河系”的薄盘。这三种恒星群中的每一种在化学上都是不同的，因为可用的物质最初缺乏重元素，后来在恒星内部经过一个或多个阶段的核处理后富集。这些群体的个体化学成分表明，它们形成的行星可能与我们太阳系中的行星有很大的不同，通过研究它们的小行星，我们可以了解由较老的光晕和厚的盘状恒星形成的固体行星的组成，与年轻的、薄的盘状恒星（如我们的太阳）相比。这样的岩石行星系统将是宇宙中最古老的系统之一，代表了银河系中行星形成（和生命，如果存在的话）的第一波和第二波。该项目将通过利用现有的大型数据库来识别数百颗被小行星污染的白矮星。收集到的数据将使我们能够通过恒星在天空中的位置和运动来区分薄盘、厚盘和带有残余行星系统的晕白矮星。然后，我们可以评估作为银河系年龄函数的残余固体行星系统的频率，并研究这些恒星群中的小行星组成。重要的是，通过关注大气层以氦为主的白矮星和残留的行星系统，我们可以通过研究恒星大气中的微量氢来评估它们小行星的含水量。反过来，这将使科学家们很好地掌握有多少水可以用于在其他恒星周围建造富含水的、可居住的行星。

Of the nearly 800 extrasolar planets confirmed around other stars, only a small number of these are thought to be solid planets and thus similar in theory to Mercury, Venus, Earth, and Mars. These planets have been identified almost exclusively through NASA's Kepler mission, which detects the transit of the planet as it passes in front of the host star. The transit provides scientists with the size of the planet, and radial velocity observations from the ground can provide the mass in some case, thus giving the density and indicating a likely solid composition. However, the actual composition of these planets is unknown and no current or future means exist to make such measurements.However, stars at the end of their lives -- called white dwarfs -- offer a unique advantage in the study of terrestrial exoplanetary systems. Over 95% of all stars in the Milky Way, including our Sun, will end their lives as white dwarf stars, gently shedding their outer layers, then shrinking to Earth-sized, slowly cooling embers. Owing to high gravities, heavy elements sink rapidly to the interior, leaving behind pure hydrogen or helium atmospheres in these stars. Those white dwarfs with rocky planetary systems can become chemically-enriched by small, but detectable amounts of heavy elements such as silicon, magnesium, and iron. In the last several years, astronomers have discovered white dwarfs ''polluted'' by rocky debris from asteroids, and been able to obtain the basic asteroid composition from the elements polluting the stellar atmosphere. In the Solar System, asteroids are the leftover building blocks of the terrestrial planets, and thus extrasolar asteroids at white dwarfs indicate the basic composition of their rocky exoplanets.There have been three major epochs of star (and planet) formation in the Galaxy. The first stars formed 11-15 billion years ago and now lie in a spherical halo that encircles the disk of the Galaxy. The next stars formed 7-12 billion years ago and appear as a thick disk about the Galactic plane, while stars like our Sun formed 2-8 billion years ago and occupy a thin disk we recognize as the 'Milky Way' in the night sky. Each of these three star populations are chemically distinct as the material available was at first deficient in heavy elements, and then later enriched after one or more stages of nuclear processing within the interior of stars. The individual chemistry of these populations implies that the planets they built may differ substantially from those in our Solar System, and by studying their asteroids we can learn about the composition of solid planets built by the older halo and thick disk stars, as compared to younger, thin disk stars like our Sun. Such rocky planetary systems will be among the oldest in the Universe and represent the first and second waves of planet formation (and life, if present) in the Galaxy.This project will identify hundreds of asteroid-polluted white dwarfs via the exploitation of large existing databases. The collected data will allow us to distinguish between thin disk, thick disk, and halo white dwarfs with remnant planetary systems by the stars' positions and motions in the sky. We can then assess the frequency of remnant, solid planetary systems as a function of Galactic age and study the asteroid composition within these stellar populations. Importantly, by focusing on white dwarfs with helium-dominated atmospheres and remnant planetary systems, we can assess the water content of their asteroids by studying the trace hydrogen in the stellar atmosphere. This in turn will give scientists a good handle on how much water is available for the construction of water-rich, habitable planets around other stars.