Understanding the spectral evolution of hot hybrid white dwarfs

绝大多数恒星都以白矮星的形式结束生命。根据进化史的不同，我们发现了两个不同的类别，一个富氢，一个富氦。它们出生时温度非常高(有效温度Tef~200,0000 K)，然后在数十亿年内冷却到几千K。沿着这个冷却顺序，白矮星改变了它们的光谱外观，反映了它们大气的化学成分。这种光谱演化是由几种效应的相互作用决定的，例如，元素的辐射悬浮和重力沉降，来自外部来源的物质的质量损失和吸积。我们的项目旨在了解富氢白矮星的一小部分的光谱演化，即所谓的混合型白矮星，它们的光谱中既有氢又有氦。虽然数量不多，但它们代表了大多数(约75%)白矮星所经历的进化阶段。我们将对精心挑选的36颗热白矮星的紫外线和光学光谱进行定量分析，这将使我们能够结合盖亚精确测量的距离，在赫茨斯普龙格-罗素图中定位它们，并得出它们的恒星参数(质量、半径、光度)。我们的目标是测量轻金属和重金属的丰度，以阐明混合白矮星何时以及如何由于元素的引力沉淀而转变为无氦物体的问题。这些结果将有助于阐明不同物理过程对氦和金属丰度的相对重要性。再加上未来的元素扩散计算，这也将使热白矮星能够作为“探测器”来确定太阳系外行星物质的组成，这些物质可以附着在年轻的白矮星上。

The vast majority of all stars end their life as white dwarfs. Depending on the evolutionary history, we find two different classes, one with hydrogen-rich and one with helium-rich atmospheres. They are born very hot (effective temperature Teff~200,0000 K) and cool down to a few 1000 K within billions of years. Along this cooling sequence, the white dwarfs change their spectral appearance, reflecting the chemical composition of their atmosphere. This spectral evolution is determined by the interplay of several effects, for instance, radiative levitation and gravitational settling of elements, mass-loss and accretion of matter from external sources. Our project aims to understand the spectral evolution of a small subgroup of the hydrogen-rich white dwarfs, the so-called hybrid white dwarfs which exhibit both, hydrogen and helium in their spectra. Though small in number, they represent an evolutionary phase run through by the majority (~75%) of all white dwarfs. We will perform a quantitative analysis of ultraviolet and optical spectra of a carefully selected sample of 36 hot (Teff>60,000 K) white dwarfs which allows, together with distances precisely measured by Gaia, to locate them in the Hertzsprung-Russell diagram and to derive their stellar parameters (mass, radius, luminosity). We aim at the abundance measurement of light and heavy metals in order to shed light on the question, when and how the hybrid white dwarfs transform into helium-free objects because of gravitational settling of elements. The results will help to clarify the relative importance of the different physical processes acting on helium and metal abundances. Together with future element diffusion calculations this will also enable to use hot white dwarfs as "detectors" to determine the composition of extrasolar planetary material that can be accreted onto the young white dwarfs.