Chemical composition of planetary debris disks around white dwarfs

很长一段时间以来，人们都知道大部分相对较冷的白色矮星（WD）的大气层被金属污染，这表明星周物质正在不断吸积。直到最近，人们还认为星际物质是吸积的。但主要是基于斯皮策在WD潮汐破裂半径内发现的尘埃盘，现在普遍接受恒星吸积残留行星物质。WD大气中金属丰度的测定为间接研究太阳系外行星物质的组成提供了令人兴奋的可能性。这需要对吸积和扩散速率有很好的了解，因为它们以复杂的方式改变了吸积物质的丰度模式。然而，这种知识可能受到特定不确定性的影响，在这里，我们提出了一种替代和补充的方法。最近发现的一些“尘埃”WD也有气态金属盘，被解释为尘埃盘材料的研磨残留物。这些气体盘的定量光谱分析提供了直接测量吸积材料的组成的机会。为此，我们将开发用于气盘的非LTE模型。初步工作证明了这种方法的可行性。

It is known for a long time that the atmospheres of a large fraction of relatively cool white dwarfs (WD) are polluted by metals, suggesting ongoing accretion of circumstellar material. Until recently it was believed that interstellar matter is accreted. But mainly based on the Spitzer discovery of dust disks within the WD tidal disruption radius, it is now commonly accepted that the stars accrete remnant planetary material. The abundance determination of metals in the WD atmosphere offers the exciting possibility to study, though indirectly, the composition of extrasolar planetary material. This requires a good knowledge about accretion and diffusion rates because they transform the abundance pattern of the accreted material in a complicated way. This knowledge, however, can be afflicted with particular uncertainties.Here we propose an alternative and complementary approach. A few “dusty” WDs were recently discovered to also have gaseous metal disks which are interpreted as the grinded remains of dust disk material. The quantitative spectral analysis of these gas disks offers the opportunity to directly measure the composition of the accreting material. To this end we will develop non-LTE models for the gas disks. Preliminary work proved the viability of this approach.