Oxygen isotopic ratios in first dredge-up red giant stars and nuclear reaction rate uncertainties revisited

Abstract

We describe a general yet simple method to analyse the propagation of nuclear reaction rate uncertainties in a stellar nucleosynthesis and mixing context. The method combines post-processing nucleosynthesis and mixing calculations with a Monte Carlo scheme. With this approach we reanalyze the dependence of theoretical oxygen isotopic ratio predictions in first dredge-up red giant branch stars in a systematic way. Such predictions are important to the interpretation of pre-solar Al_2 O_3 grains from meteorites. The reaction rates with uncertainties were taken from the NACRE compilation (Angulo etal., 1999). We include seven reaction rates in our systematic analysis of stellar models with initial masses from 1 to 3 M_sun. We find that the uncertainty of reaction rate for reaction O18(p,alpha)N15 typically causes an error in the theoretical O16/O18 ratio of about +20/-5 per cent. The error of the O16/O17 prediction is +-10 to 40 per cent depending on the stellar mass, and is persistently dominated by the comparatively small uncertainty of the O16(p,gamma)F17 reaction. With the new estimates on reaction rate uncertainties by the NACRE compilation, the p-capture reactions O17(p,alpha)N14 and O17(p,gamma)F18 have virtually no impact on theoretical predictions for stellar mass <= 1.5 M_sun. However, this is not the case for masses > 1.5 M_sun, where core mixing and subsequent envelope mixing interact. In these cases where core mixing complicates post-dredge-up surface abundances, uncertainty in other reactions have a secondary but noticeable effect on surface abundances.