  Recent computations of the interior composition of ultra-massive white dwarfs
(WD) have suggested that some white dwarfs could be composed of neon
(Ne)-dominated cores. This result is at variance with our previous
understanding of the chemical structure of massive white dwarfs, where oxygen
is the predominant element. In addition, it is not clear whether some hybrid
carbon (C) oxygen (O)-Ne white dwarfs might form when convective boundary
mixing is accounted for during the propagation of the C-flame, in the C-burning
stage. Both Ne-dominated and hybrid CO-Ne core would have measurable
consequences for asteroseismological studies based on evolutionary models. In
this work we explore in detail to which extent differences in the adopted
micro- and macro-physics can explain the different final white dwarf
compositions that have been found by different authors. Additionally, we
explored the impact of such differences in the cooling times, crystallization
and pulsational properties of pulsating WDs. We explore the impact of the
intensity of convective boundary mixing during the C-flash, extreme mass-loss
rates, and the size of the adopted nuclear networks on the final composition,
age, crystallization and pulsational properties of white dwarfs. Based on the
insight coming from 3D hydro-dynamical simulations, we expect that the very
slow propagation of the carbon flame will be altered by turbulent entrainment
affecting the inward propagation of the flame. Also, we find that Ne-dominated
chemical profiles of massive WDs recently reported appear in their modeling due
to the overlooking of a key nuclear reaction. We find that the inaccuracies in
the chemical composition of ultra-massive white dwarfs recently reported lead
to differences of 10% in the cooling times and degree of crystallization and
about 8% in the period spacing of the models once they reach the ZZ Ceti
instability strip.
