Understanding the Spectral Evolution of White Dwarf Stars

了解白矮星的光谱演化

• 批准号: 261483-2013
• 负责人: Bergeron, Pierre
• 金额: $ 3.79万
• 依托单位: Université de Montréal
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2015
• 资助国家: 加拿大
• 起止时间: 2015-01-01 至 2016-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=571049
• 关键词: Understanding; Spectral; Evolution; White; Dwarf

White dwarf stars represent the endpoint of more than 97% of the stars in the Galaxy, including the Sun, and the determination of their global properties thus provides a variety of useful information on the star formation history in our galaxy, and on stellar evolution in general. Having exhausted the nuclear fuel at their center, white dwarfs gradually cool off along an evolutionary sequence over several billions of years. Because of the intense gravitational field present at the surface of these stars (10,000 times that of the Sun), light elements such as hydrogen and helium tend to float to the surface, while heavier elements sink out of sight. Consequently, about 80% of white dwarfs have atmospheres (the outer layers of a star) dominated by hydrogen, the so-called DA stars, while the remaining 20% have helium-dominated atmospheres. The latter include the DB stars, whose spectra are dominated by neutral helium lines (or ionized helium for the DO stars), the DQ stars with strong molecular carbon bands, the DZ stars with absorption features of heavier elements such as calcium, iron, sodium, etc., and the DC stars, whose spectra are completely featureless. However, these various types of white dwarfs are found only in some specific range of temperatures, indicating that there exists a variety of physical mechanisms that compete with the gravitational field to alter the chemical composition of the outer layers of white dwarfs as they evolve along their cooling sequence. Such physical mechanisms include convective mixing, convective dredge-up from the core, accretion from the interstellar medium or circumstellar material, radiative acceleration, and stellar winds. The objective of this program is to provide a better understanding of the Spectral Evolution of White Dwarf Stars by analyzing the 25,000 or so white dwarfs discovered in the Sloan Digital Sky Survey with state-of-the-art model atmosphere techniques and evolutionary models being developed at the Université de Montréal, in order to identify the various evolutionary channels that produce white dwarfs, thus providing a better understanding of the connection with the earlier phases of stellar evolution.

白色矮星代表了银河系中97%以上恒星（包括太阳）的终点，因此确定它们的全局性质可以提供有关银河系中星星形成历史以及一般恒星演化的各种有用信息。白色矮星耗尽了中心的核燃料后，在数十亿年的进化过程中，沿着一个进化序列逐渐冷却下来。由于这些恒星表面存在强烈的引力场（是太阳的10，000倍），氢和氦等轻元素倾向于漂浮到表面，而较重的元素则下沉看不见。因此，大约80%的白色矮星的大气层（星星的外层）主要由氢组成，即所谓的DA恒星，而剩下的20%则由氦组成。后者包括光谱以中性氦线（或电离氦为主导的DO星）为主的DB星，具有强分子碳带的DQ星，具有钙、铁、钠等重元素吸收特征的DZ星，而DC星的光谱毫无特色然而，这些不同类型的白色矮星只在某些特定的温度范围内被发现，这表明当沿着它们的冷却序列演化时，存在着各种各样的物理机制与引力场竞争，以改变白色矮星外层的化学成分。这些物理机制包括对流混合，对流从核心的疏浚，来自星际介质或星周物质的吸积，辐射加速和恒星风。该计划的目标是通过分析斯隆数字巡天发现的大约25，000颗白色矮星，利用蒙特利尔大学正在开发的最先进的模型大气技术和演化模型，更好地了解白色矮星的光谱演化，以确定产生白色矮星的各种演化通道，从而更好地理解与恒星演化早期阶段的联系。