  Carbon fusion is important to understand the late stages in the evolution of
a massive star. Astronomically interesting energy ranges for the 12C+12C
reactions have been, however, poorly constrained by experiments. Theoretical
studies on stellar evolution have relied on reaction rates that are
extrapolated from those measured in higher energies. In this work, we update
the carbon fusion reaction rates by fitting the astrophysical S-factor data
obtained from direct measurements based on the Fowler, Caughlan, & Zimmerman
(1975) formula. We examine the evolution of a 20 M_sun star with the updated
12C+12C reaction rates performing simulations with the MESA (Modules for
Experiments for Stellar Astrophysics) code. Between 0.5 and 1 GK, the updated
reaction rates are 0.35 to 0.5 times less than the rates suggested by Caughlan
and Fowler (1988). The updated rates result in the increase of core temperature
by about 7% and of the neutrino cooling by about a factor of three. Moreover,
the carbon-burning lifetime is reduced by a factor of 2.7. The updated carbon
fusion reaction rates lead to some changes in the details of the stellar
evolution model, their impact seems relatively minor compared to other
uncertain physical factors like convection, overshooting, rotation, and
mass-loss history. The astrophysical S-factor measurements in lower energies
have large errors below the Coulomb barrier. More precise measurements in lower
energies for the carbon burning would be useful to improve our study and to
understand the evolution of a massive star.
