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

在围绕其他恒星确认的近800颗太阳系外行星中，只有一小部分被认为是固体行星，因此在理论上类似于水星、金星、地球和火星。这些行星几乎完全是通过NASA的开普勒任务确定的，该任务探测行星经过主恒星前面时的凌日。凌日为科学家提供了行星的大小，在某些情况下，从地面上进行的径向速度观测可以提供质量，从而得出密度并指示可能的固体成分。然而，这些行星的实际组成尚不清楚，目前或未来也没有进行这种测量的手段。然而，生命末期的恒星--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生命，轻轻地剥离它们的外层，然后收缩成地球大小的慢慢冷却的余烬。由于高重力，重元素迅速下沉到内部，在这些恒星中留下纯氢或氦大气。那些拥有岩石行星系统的白矮星可能会因少量但可检测到的重元素，如硅、镁和铁而变得化学丰富。在过去的几年里，天文学家已经发现了被小行星岩石碎片“污染”的白矮星，并能够从污染恒星大气的元素中获得小行星的基本组成。在太阳系中，小行星是类地行星遗留下来的积木，因此位于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。银河系经历了三个主要的恒星(和行星)形成时期。第一批恒星形成于110-150亿年前，现在形成了一个围绕银河系圆盘的球形光晕。下一批恒星形成于70-120亿年前，看起来像一个围绕银河系平面的厚圆盘，而像我们的太阳这样的恒星形成于20-80亿年前，占据着我们所知的夜空中的一个薄圆盘，我们称之为“银河系”。这三个恒星群中的每一个在化学上都是不同的，因为可用的物质一开始缺乏重元素，然后在恒星内部经过一个或多个阶段的核处理后变得丰富。这些种群的个体化学成分意味着，他们建造的行星可能与我们太阳系中的行星有很大不同，通过研究它们的小行星，我们可以了解到由较老的光晕和厚盘恒星建立的固体行星的组成，与我们的太阳等较年轻、较薄的盘状恒星相比。这样的岩石行星系统将是宇宙中最古老的行星系统之一，代表着银河系行星形成(如果存在的话，还有生命)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根据恒星在天空中的位置和运动来区分薄盘、厚盘和带有残留行星系统的晕状白矮星。然后，我们可以评估残存的固体行星系统的频率作为银河系年龄的函数，并研究这些恒星群体中的小行星组成。重要的是，通过关注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的小行星的水分含量。这反过来将使科学家很好地掌握有多少水可用于在其他恒星周围建造水资源丰富的宜居行星。

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.