Inward Propagation of Nuclear-Burning Shells in Merging C-O and He White   Dwarfs
  We have investigated the consequences of merging double white dwarf systems by calculating evolutionary models of accreting white dwarfs. We have considered two cases; a massive C-O white dwarf of ~1M_sun accreting C-O mixture, and a low mass white dwarf with an initial mass of 0.4M_sun accreting matter composed mostly of helium.   The accretion rate of the C-O white dwarf is assumed to be 1x10^{-5}M_sun/y. After carbon burning is ignited at $M_r\sim1.04M_\odot$, the flame propagates inward due to heat conduction. By inserting enough grid points to resolve the structure of the flame, we have obtained almost steady burning in most phase of evolution, but we have found a new phenomenon that the strength of flame sometimes oscillates due to a thermal instability in early phase of the evolution. The flame propagates to reach the center and the C-O mixture has been burned into an O-Ne-Mg mixture without causing an explosive phenomenon.   For helium accreting low mass white dwarfs, we have considered accretion rates of 1x10^{-7} and 1x10^{-6}M_sun/y. After a fraction of M_sun is accreted to the white dwarf, helium is ignited in the outer part and a shell flash occurs. Such a shell flash diminishes when about 10% of helium in the convective shell is burned into carbon and oxygen. The next shell flash occurs at a shell interior to the previously flashed shell. After less than 30 shell flashes, the helium ignition occurs at the center and steady burning begins. Thus, the merging produces a helium star which burns helium at the center. 
H. Saio and K. Nomoto