A search for helium in ultramassive white dwarfs

We have obtained extreme ultraviolet (EUV) spectra of eight hot, ultramassive white dwarfs (M = 0.9-1.3 M_⊙) with the Extreme Ultraviolet Explorer. The stars are, on average, twice as massive as normal white dwarfs. The relatively high turnout of ultramassive white dwarfs in EUV surveys is difficult to explain in the context of single-star evolution, and it has been suggested that ultramassive white dwarfs may result from the merger of two normal white dwarfs. The final configuration of merged white dwarfs is uncertain, but the resulting stars may have peculiar atmospheric compositions similar to the ultramassive DAO white dwarf GD 50. Our analysis of the EUV spectra of massive white dwarfs shows that all objects, with the possible exception of the magnetic white dwarf PG 1658+441, have EUV continuum emission characteristic of pure hydrogen atmospheres and that the peculiar atmospheric composition of the white dwarf GD 50 remains a unique phenomenon.