Model atmosphere analysis of white dwarfs in the Sloan Digital Sky Survey footprint of Gaia: the definitive work

白矮星代表了我们银河系97%以上恒星的终点，包括我们自己的太阳。这些地球大小的天体有简并的核心(量子力学描述的一种独特的物质状态)，在这种状态下不再发生核聚变。因此，白矮星只是在数十亿年的时间里慢慢冷却下来，它们是我们银河系中迄今为止最古老的成员。它们可以提供对银河系人口和恒星形成历史的洞察。因此，白矮星对于天体物理研究的所有主要领域都至关重要，因此，对它们的性质(质量、冷却年龄、大气和内部组成)的确定提供了关于行星系统、恒星、银河系和其他星系的各种有用信息。此外，作为密度极高的天体，白矮星提供了精致的物理环境，可以在其中测试和改进高密度物理模型。这项研究建议的主要科学目标是利用目前可用的所有光谱和光度学数据，但最重要的是，GAIA SPACE任务(全球天体物理天体测量干涉仪)提供的天体测量数据，全面了解白矮星的性质和演化，该任务最近发布了对超过10亿颗恒星的距离测量，其中包括约26万颗白矮星。多年来，我们蒙特利尔大学的团队一直在开发最先进的模型大气，以使用所谓的光度和光谱技术来测量白矮星的基本恒星参数(温度、质量、半径、化学成分、年龄)。光谱技术依赖于将具有足够强吸收线的恒星的光谱(目前从斯隆数字天空勘测获得的超过33,000条吸收线)与模型大气的预测相匹配，而光度学方法依赖于用类似的模型来拟合观测到的能量分布，这是一项需要了解恒星距离的技术。直到最近，只能对300颗左右的白矮星进行直接距离测量，这严重限制了光度学方法在大样本上的适用性。盖亚任务现在已经测量了几乎每一颗光谱确认的白矮星以及许多白矮星候选者的距离，这在白矮星社区是一场真正的革命。这些大型数据集将使我们不仅能够改进我们对白矮星参数的测定，而且最重要的是，通过首次比较从两种方法获得的参数来研究我们的模型大气计算的有效性。这些结果将反过来为研究银河系的初始到最终质量关系、恒星形成速率、初始质量函数和化学演化提供关键信息，这些领域的科学研究都将对天体物理学的其他领域产生深刻影响。

White dwarf stars represent the endpoint of more than 97% of the stars in our galaxy, including our own Sun. These Earth-sized objects have degenerate cores (a unique state of matter described by quantum mechanics) in which nuclear fusion no longer occurs. As such, white dwarfs simply cool off slowly over billions of years and they are by far the oldest members of our Galaxy. They can provide insights into Galactic populations and the history of star formation. Hence white dwarfs are fundamentally important for all major areas of astrophysical research, and the determination of their properties (mass, cooling age, atmospheric and internal composition) thus provides a variety of useful information on planetary systems, the stars, the Galaxy and other galaxies as well. Also, being extremely dense objects, white dwarfs provide exquisite physical environments where high-density physics models can be tested and improved. The main scientific objective of this research proposal is to provide a full understanding of the nature and evolution of white dwarf stars using all the currently available spectroscopic and photometric data, but most of all, astrometric data from Gaia space mission (Global Astrometric Interferometer for Astrophysics), which recently released distance measurements of over one billion stars, including around 260,000 white dwarfs. Over the years, our group at University of Montreal has been developing state-of-the-art model atmospheres to measure the fundamental stellar parameters of white dwarfs (temperature, mass, radius, chemical composition, age) using the so-called photometric and spectroscopic techniques. The spectroscopic technique relies on fitting optical spectra of stars with strong enough absorption lines (over 33,000 of which are currently available from the Sloan Digital Sky Survey) with the predictions of model atmospheres, while the photometric method relies on fitting the observed energy distribution with similar models, a technique that requires the knowledge of stellar distances. Until recently, direct distance measurements were only available for 300 white dwarfs or so, severely restricting the applicability of the photometric method to large samples. The Gaia mission has now measured the distance to almost every single spectroscopically confirmed white dwarf, and many white dwarf candidates as well, a true revolution in the white dwarf community. These large data sets will allow us not only to improve our determinations of white dwarf parameters, but most importantly, to study the validity of our model atmosphere calculations by comparing for the first time the parameters obtained from both methods. These results will in turn provide crucial information for studying the initial-to-final mass relation, the stellar formation rate, the initial mass function, and the chemical evolution of the Galaxy, all areas of scientific research that will have a deep impact on other areas of astrophysics.