Exploring masses and CNO surface abundances of red giant stars

Abstract

A grid of evolutionary sequences of stars in the mass range $1.2$-$7$ M$_{\odot}$, with solar-like initial composition is presented. We focus on this mass range in order to estimate the masses and calculate the CNO surface abundances of a sample of observed red giants. The stellar models are calculated from the zero-age main sequence till the early asymptotic giant branch (AGB) phase. Stars of M $\leqslant$ $2.2$M$_{\odot}$ are evolved through the core helium flash. In this work, an approach is adopted that improves the mass determination of an observed sample of 21 RGB and early AGB stars. This approach is based on comparing the observationally derived effective temperatures and absolute magnitudes with the calculated values based on our evolutionary tracks in the Hertzsprung-Russell diagram. A more reliable determination of the stellar masses is achieved by using evolutionary tracks extended to the range of observation. In addition, the predicted CNO surface abundances are compared to the observationally inferred values in order to show how far standard evolutionary calculation can be used to interpret available observations and to illustrate the role of convective mixing. We find that extra mixing beyond the convective boundary determined by the Schwarzschild criterion is needed to explain the observational oxygen isotopic ratios in low mass stars. The effect of recent determinations of proton capture reactions and their uncertainties on the $^{16}$O$/^{17}$O and $^{14} $N$/^{15}$N ratios is also shown. It is found that the $^{14}$N$($ p$,\gamma)^{15}$O reaction is important for predicting the $^{14}$N$/^{15}$N ratio in red giants.