The Fate of Merging White Dwarfs

We present a smoothed particle hydrodynamics simulation of the coalescence of two carbon-oxygen white dwarfs, and we discuss the subsequent evolution of the resulting object. We consider in particular the case of three hot and massive white dwarfs, PG 0136+251, PG 1658+441, and GD 50, not located in any cluster or association, which have been suggested to result from a coalescence process (Bergeron et al.). We show that the merged object must lose about 90% of its original angular momentum to become sufficiently compact and reach the observed surface gravities log g ≈ 9. Since the three candidates are DA white dwarfs, we examine the efficiency of the accretion process from the interstellar medium to build the observed hydrogen envelope. Finally, the fact that the three merger candidates are located within 80 pc (two within 40 pc) imposes severe constraints on the evolution process. We connect this statistical problem to the evolutionary timescale of the merged object. We show that in order to explain the observed three white dwarfs, the coalescence scenario implies a large timescale.