On the mass of the white dwarf in UZ Fornacis

Abstract

We present phase-resolved spectroscopy of the eclipsing AM Herculis star UZ For obtained when the system was in its low state of accretion. Faint residual Halpha-emission and NaI absorption were used to trace the secondary star and infer its orbital velocity K_2. The measured radial velocity amplitude of NaI K_2 * sin(i) = 285 \pm 50 kmps suggests a low-mass white dwarf with M_wd = 0.44 \pm 0.15 Msun (1 sigma-errors). The Halpha emission line on the other hand, visible only for part of the orbital cycle and supposed to originate only on the illuminated hemisphere facing the white dwarf, displays a similar radial velocity amplitude, K'_2 sin(i) = 308 \pm 27 kmps. The standard K_2-correction applied by us then suggests a white dwarf mass of up to 1 Msun. Compared with earlier results the new ones enlarge the window in which the white dwarf mass may lie and resolves the conflict between mass estimates based on photometry and spectroscopy. They leave some ambiguity in the location of emission and absorption components in these and former observations.