Evidence for helium in the magnetic white dwarf GD 229

The nature of the strong absorption features in the white dwarf GD 229 has been a real mystery ever since it was found to be magnetic in 1974. All attempts to explain the spectrum by line components of hydrogen failed. With the first sets of newly calculated line data for He I in a strong magnetic field we could identify most of the absorption structures in the spectrum with stationary line components in a range of magnetic fields between 300 and 700 MG. This is much lower than previously speculated and is comparable to the highest fields found in hydrogen rich magnetic white dwarfs. The reason for the large number of spectral features in GD 229 is the extreme accumulation of stationary components of transitions in He I in a narrow interval of field strengths.