THE ISOLATED MASSIVE DA WHITE-DWARF GD-50

Analysis of accurate hydrogen line profiles from optical and ultraviolet spectrophotometry shows that the hot DA white dwarf GD 50 (WD 0346-011) has an exceptionally high surface gravity of log g = 9.0 +/- 0.15; the derived parameters imply a mass of 1.2 M. and a radius of 0.0057 R., if an interior composed of carbon and oxygen is assumed. As such, it is the first well-studied, isolated DA white dwarf with a likely mass larger than that of Sirius B. Moreover, the derived mass is large enough to consider the possibility that the interior is composed of oxygen, neon, and magnesium. If GD 50 has evolved as a single object, it should be quite young. Alternatively, the star could have formed as the result of a merger of a close pair of white dwarfs.