Chemical composition of planetary debris disks around white dwarfs

点击翻译按钮获取中文摘要

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.