Understanding the Spectral Evolution of White Dwarf Stars

白矮星代表了银河系中97%以上的恒星的终点，包括太阳，因此确定它们的整体特性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，白矮星在几十亿年的进化过程中逐渐冷却下来。由于这些恒星表面存在强烈的引力场（引力场是太阳的1万倍），像氢和氦这样的轻元素倾向于漂浮到表面，而较重的元素则沉入视线之外。因此，大约80%的白矮星有以氢为主的大气层（恒星的外层），即所谓的DA星，而剩下的20%有以氦为主的大气层。后者包括DB星，其光谱以中性氦谱线（或DO星的电离氦谱线）为主，DQ星具有较强的分子碳谱带，DZ星具有钙、铁、钠等较重元素的吸收特征，DC星的光谱完全没有特征。然而，这些不同类型的白矮星只在一些特定的温度范围内被发现，这表明在白矮星沿着冷却顺序进化的过程中，存在着各种物理机制，这些机制与引力场竞争，改变了白矮星外层的化学成分。这些物理机制包括对流混合、核心的对流吸积、星际介质或星周物质的吸积、辐射加速和恒星风。这个项目的目的是为了更好地理解白矮星的光谱演变，通过分析斯隆数字巡天项目中发现的25000颗左右的白矮星，利用最先进的模型大气技术和蒙特雷姆塔尔大学正在开发的进化模型，以确定产生白矮星的各种进化渠道。这样就可以更好地理解与恒星演化早期阶段的联系。

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.