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J. Dupuis (JHU), S. Vennes (ANU), P. Chayer (JHU/UVIC)
We report on the observation by FUSE of PG1658+440, one of the most massive isolated white dwarf (DA, Teff = 30510 K, Mass = 1.31 solar mass). This star and other ultra-massive white dwarfs may be example of white dwarfs with O/Ne/Mg core descending from more massive progenitors than white dwarfs normally do. It has even been suggested that some of them may be the result of the coalescence of two normal mass white dwarfs. PG1658+440 is also a magnetic white dwarf with a rather low magnetic field of 2.3 MG (Schmidt et al. 1992, ApJ, 394, 603) in agreement with the idea that magnetic white dwarfs tend to be more massive. We present an analysis of the far-UV spectrum of this unique object. The spectrum exhibits very broad Lyman lines as expected from the high surface gravity of this star. There is a hint for the presence of weak quasi-molecular Lyman Beta satellite features. No photospheric features from light elements or metals have been detected in the spectrum indicating a highly pure hydrogen atmosphere.