Accretion onto white dwarfs

3.1 Accretion from the ISM and winds 3.1.1 Introductory remarks on white dwarfs The spectra of white dwarfs (WD) are classified according to the scheme devised by Sion et al. (1983), of which we need here to use only the types DA (with strong H lines), DB (with He I lines and no H), and DZ (metallic lines, e.g., Ca, but excluding C, subdivided into DAZ and DBZ). In addition, magnetic fields in WDs play important roles in accretion processes. Their occurrence in isolated form (or as members of noninteracting binaries) is observed by Zeeman splitting or polarization, and the distribution of field strengths appears bimodal: Wickramasinghe and Ferrario (2000, 2005) conclude that ~16% of WDs have strong fields (≥0.5 MG); a much smaller fraction have lower fields, but there are indications of a rise of up to 25% at the kG level. 3.1.2 Accretion from the ISM Most isolated WDs are of type DA or DB, but a small fraction at the cool end of the WD sequence are of type DZ (Fig. 3.1). The reason for ignoring carbon in this spectral type is because it can be dredged up from the interior, whereas the other metals must have a different origin. Levitation by radiation pressure is not strong enough to keep metals in the atmospheres of such stars (for T < 40,000 K), and gravitational settling time scales are short compared with the cooling time scale, so the metals must have been delivered from outside the star – such as from the interstellar medium (ISM).