  The core-degenerate (CD) scenario for type Ia supernovae (SN Ia) involves the
merger of the hot core of an asymptotic giant branch (AGB) star and a white
dwarf, and might contribute a non-negligible fraction of all thermonuclear
supernovae. Despite its potential interest, very few studies, and based on only
crude simplifications, have been devoted to investigate this possible scenario,
compared with the large efforts invested to study some other scenarios. Here we
perform the first three-dimensional simulations of the merger phase, and find
that this process can lead to the formation of a massive white dwarf, as
required by this scenario. We consider two situations, according to the mass of
the circumbinary disk formed around the system during the final stages of the
common envelope phase. If the disk is massive enough, the stars merge on a
highly eccentric orbit. Otherwise, the merger occurs after the circumbinary
disk has been ejected and gravitational wave radiation has brought the stars
close to the Roche lobe radius on a nearly circular orbit. Not surprisingly,
the overall characteristics of the merger remnants are similar to those found
for the double-degenerate (DD) scenario, independently of the very different
core temperature and of the orbits of the merging stars. They consist of a
central massive white dwarf, surrounded by a hot, rapidly rotating corona and a
thick debris region.
