The Ultramassive White Dwarf EUVE J1746−706

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Abstract
We have obtained new optical and extreme ultraviolet (EUV) spectroscopy of the ultramassive white dwarf EUVE J1746-706. We revise Vennes et al.'s original estimates of the atmospheric parameters, and we measure an effective temperature of 46,500 ± 700 K and a surface gravity log g = 9.05 ± 0.15 (~1.2 M☉), in agreement with Balmer line profiles and the EUV continuum. We derive an upper limit on the atmospheric abundance of helium of He/H = 1.3 × 10-4 and a neutral hydrogen column density in the local interstellar medium NH I = 1.8 ± 0.4 × 1019 cm-2 from the EUV spectrum. Our upper limit corresponds to half the helium abundance observed in the atmosphere of the ultramassive white dwarf GD 50. We discuss the possibility that EUVE J1746-706 represents an earlier phase of evolution relative to GD 50 and may, therefore, help us understand the origin and evolution of massive white dwarfs.