Astron. Astrophys. 352, 619-622 (1999)

5. Conclusions

Our observations show that the magnetic white dwarf GD 229 which was considered for many years as the canonical magnetic white dwarf with non-variable polarization, actually is a long-term polarization variable. Since the most probable explanation of this phenomenon is a very slow rotation, we consider our results as an observational evidence of magnetic braking effect in highly magnetic single white dwarfs.

However, the details of how this mechanism works are not clear yet. A steady mass loss at the time of formation in presence of a strong magnetic field could bring the degenerate star to a nearly complete standstill (see Pacini 1970; Brecher & Chanmugam, 1978). It assumes that rotating and nonrotating (or very slow rotating) magnetic white dwarfs are formed under very different conditions of mass loss. Alternativelly, rotators could have been spun up by mass transfer from the undetected companions. An extremely fast rotating and highly massive RE J0317-853 can be even the end product of a double-degenerate merger (Barstow et al. 1995). The apparent absence of rotators with the periods in a range of 100 days - 100 years among the magnetic white dwarfs (see Schmidt & Norsworthy, 1990) is another unexplained phenomenon. Whether it is the effect of the observational selection or natural outcome from the braking process is to be determined.

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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