  Stars with initial masses 7 Msun . MZAMS . 9 Msun reach temperatures high
enough to ignite C under degenerate conditions after the end of He-core burning
(Garcia-Berro & Iben 1994). These isolated stars are expected to evolve into
the so-called super AGB (SAGB) phase and may end their lives as ultra-massive
ONe WDs (see Siess 2006,2007, 2010; Camisassa et al. 2019, and references
therein). The exact proportions of O and Ne found in the core at the end of the
SAGB phase will determine the cooling times and pulsational properties of these
WDs. Uncertainties affecting the rates of nuclear reactions occurring during
the C burning phase should have a measurable impact on the distribution of 16O,
20Ne, 23Na and 24Mg and, consequently, on the evolution of the WD. Here we
present a study of the impact of uncertainties in the 12C(12C, {\alpha})20Ne
and 12C(12C, p)23Na nuclear reaction rates (and their branching ratios) on the
chemical structure of intermediate- to high-mass progenitors at the end of the
C-burning phase. Using the stellar evolution code Modules for Experiments in
Stellar Astrophysics (MESA) we computed evolutionary sequences for stars with
initial masses 7.25<= MZAMS /Msun <=8.25, from the ZAMS to the SAGB phase,
adopting different prescriptions for the 12C+12C burning rates. We found that
adopting lower reaction rates for the 12C+12C burning delays C-ignition by at
most 2700 yrs, and the ignition takes place in a position further from the
center. Our results shows that differences in the 20Ne central abundances
remain modest, below 14%.
