Abstract: Atmospheric metal features in the spectra of white dwarfs are regarded as anomalies, because any primordial metal in WDs would sink rapidly in short timescale as long as the stars are not hot enough. It is believed that this atmospheric pollution is mainly due to the accretion of the circumstellar matters, such as a debris disk inferred from their infrared excess in the spectra. To date, many samples of polluted WDs have been reported, and theories are put forward to explaining the detected results. This review summarises the developments both in theory and observation of this topic. A comparison of models and comments are also included as the last part of this article.
使用教材：An Introduction to Modern Astrophysics