Far Ultraviolet Spectroscopic Explorer observation of the ultramassive white dwarf PG 1658+441

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the ultramassive (M = 1.31 M(.)), magnetic (B(s) = 2.3 MG) white dwarf PG 1658+ 441. The far-ultraviolet (FUV) spectrum exhibits very broad Lyman lines and quasi-molecular Lybeta satellites, but weak Lygamma satellites may also be present. PG 1658+ 441 is the hottest white dwarf known to show these satellite features. We fit the Lyman lines with stellar models and obtain atmospheric parameters consistent with a published analysis of the Balmer lines. By averaging results obtained for the different FUSE segments, we determine T(eff) = 29,620 +/- 500 K and log g = 9.31 +/- 0.07. The models match the data over large portions of the spectrum, but discrepancies remain near the satellite features. Finally, no trace elements have been identified in the FUV spectrum, and we provide abundance upper limits for C, N, Si, and P.