Exploring masses and CNO surface abundances of red giant stars

Abstract

A grid of evolutionary sequences of stars in the mass range 1.2-7M ⊙ , 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 2.2M ⊙ 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, γ) 15 O reaction is important for predicting the 14 N/ 15 N ratio in red giants.

Stellar Yields From Metal-Rich Asymptotic Giant Branch Models

“…Extra mixing also on the AGB has been invoked to explain the composition of roughly 10% of meteoritic stardust oxide grains (the Group 2 grains), which show depletions in 18 O (Nollett et al 2003;Palmerini et al 2011). Halabi & Eid (2015) perform a comprehensive analysis of model predictions against observations of C, N and O isotopic ratios of Galactic RGB stars. In their study they found agreement between their solar-like composition models spanning a range from 1.2-7M and the predictions in Karakas & Lattanzio (2014) for the 16 O/ 17 O ratio after first and second dredge.…”

“…As discussed by Halabi & Eid (2015) there are few observational constraints on the 14 N/ 15 N ratio, especially for RGB stars. Hedrosa et al (2013) measured the 14 N/ 15 N ratio in a sample of AGB stars and found evidence that some 15 N production occurs in C-rich AGB stars.…”

The contribution of chemical abundances in nova ejecta to the interstellar medium

“…The top region represents novae with CO WD accretors, and the bottom region represents novae with ONeMg WD accretors, respectively. Using a grid of evolutionary sequences of stars, Halabi & Eid (2015) calculated the isotopic abundances ratios of log 14 N/ 15 N vs. log 12 C/ 13 C of some red giants, which are plotted by the triangles in Figure 4. It is evident that 12 C/ 13 C in nova ejects in AGB in ElEid (1994) and in Smith & Lambert (1990), respectively.…”

The impact of the revised17O(p,α)14N reaction rate on17O stellar abundances and yields

“…4 and 5 in Lebzelter et al 2015). More details on the theoretical estimation of the O isotopic ratios can be found in several papers, including Dearborn (1992), Boothroyd et al (1994), El Eid (1994, Stoesz & Herwig (2003), Karakas & Lattanzio (2014), Halabi & Eid (2015). We used the same stellar evolution code (FUNS) as in Cristallo et al (2015).…”

“…The various uncertainties affecting stellar models and the related nucleosynthesis results, including those due to the treatment of mixing, are discussed in some detail in Section 2 of Straniero et al (2014) (see also Sections 4 and 5 in Lebzelter et al (2015)). More details on the theoretical estimation of the O isotopic ratios can be found in several papers, including Dearborn (1992); Boothroyd et al (1994);El Eid (1994); Stoesz & Herwig (2003); Karakas & Lattanzio (2014); Halabi & Eid (2015). We used the same stellar evolution code (FUNS) as in Cristallo et al (2015).…”