Impact of Current Uncertainties in the 12 C+ 12 C Nuclear Reaction Rate on Intermediate-mass Stars and Massive White Dwarfs

Abstract

Recent determinations of the total rate of the 12 C+12 C nuclear reaction show non-negligible differences with thereference reaction rate commonly used in previous stellar simulations. In addition, the current uncertainties indetermining each exit channel constitute one of the main uncertainties in shaping the inner structure of superasymptotic giant branch stars that could have a measurable impact on the properties of pulsating ultramassive whitedwarfs (WDs). We explore how new determinations of the nuclear reaction rate and its branching ratios affect theevolution of WD progenitors. We show that the current uncertainties in the branching ratios constitute the mainuncertainty factor in determining the inner composition of ultramassive WDs and their progenitors. We found thatthe use of extreme branching ratios leads to differences in the central abundances of 20Ne of at most 17%, whichare translated into differences of at most 1.3% and 0.8% in the cooling times and size of the crystallized core,respectively. However, the impact on the pulsation properties is small, less than 1 s for the asymptotic periodspacing. We found that the carbon burns partially in the interior of ultramassive WD progenitors within a particularrange of masses, leaving a hybrid CONe-core composition in their cores. The evolution of these new kinds ofpredicted objects differs substantially from the evolution of objects with pure CO cores. Differences in the size ofthe crystallized core and cooling times of up to 15% and 6%, respectively, lead to distinct patterns in the periodspacing distribution.