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: 2013
• 资助国家: 加拿大
• 起止时间: 2013-01-01 至 2014-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=525241
• 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.

白矮星代表着包括太阳在内的银河系97%以上恒星的终点，它们的全球性质的确定因此为我们银河系的恒星形成历史和恒星演化提供了各种有用的信息。在耗尽了中心的核燃料后，白矮星沿着数十亿年的进化序列逐渐冷却。由于这些恒星表面存在着强烈的引力场(是太阳的一万倍)，氢和氦等轻元素往往会漂浮到表面，而较重的元素则会沉没在视线之外。因此，大约80%的白矮星的大气(恒星的外层)以氢为主，即所谓的DA恒星，而其余20%的大气以氦为主。后者包括光谱以中性氦谱线为主的DB星(或DO星的电离氦)，具有强分子碳带的DQ星，具有钙、铁、钠等较重元素吸收特征的DZ星，以及光谱完全没有特征的DC星。然而，这些不同类型的白矮星只在某些特定的温度范围内被发现，这表明存在各种与引力场竞争的物理机制，以改变白矮星外层的化学成分，因为它们沿着冷却序列演化。这些物理机制包括对流混合、核心的对流疏浚、星际介质或星际物质的吸积、辐射加速和恒星风。该计划的目的是通过分析斯隆数字巡天中发现的大约25,000颗白矮星，利用最先进的大气模型技术和蒙特雷亚尔大学正在开发的演化模型，更好地了解白矮星的光谱演化，以确定产生白矮星的各种演化通道，从而更好地了解它们与恒星演化早期阶段的联系。