2010A&A...524A..36K

We use the parallax measurements of REJ 0317-853 to determine its mass, radius, and cooling age and thereby constrain its evolutionary origins. We observed REJ 0317-853 with the Hubble Space Telescope's Fine Guidance Sensor to measure the parallax of REJ 0317-853 and its binary companion, the non-magnetic white dwarf LB9802. In addition, we acquired spectra of comparison stars with the Boller & Chivens spectrograph of the SMARTS telescope to correct the parallax zero point. For the corrected parallax, we determine the radius, mass, and the cooling age with the help of evolutionary models from the literature. The properties of REJ 0317-853 are constrained using the parallax information. We discuss the different cases of the core composition and the uncertain effective temperature. We confirm that REJ 0317-853 is close to the Chandrasekhar's mass limit in all cases and almost as old as its companion LB9802. The precise evolutionary history of REJ 0317-853 depends on our knowledge of its effective temperature. It is possible that it had a single star progenitor possible if we assume that the effective temperature is at the cooler end of the possible range from 30000 to 50000K; if T_eff is instead at the hotter end, a binary-merger scenario for REJ 0317-853 becomes more plausible.