The most metal-rich asymptotic giant branch stars

Abstract

We present new stellar evolutionary sequences of very metal-rich stars evolved with the Monash Stellar Structure code and with MESA. The Monash models include masses of $1-8M_{\odot}$ with metallicities $Z=0.04$ to $Z=0.1$ and are evolved from the main sequence to the thermally-pulsing asymptotic giant branch (AGB). These are the first $Z=0.1$ AGB models in the literature. The MESA models include intermediate-mass models with $Z=0.06$ to $Z=0.09$ evolved to the onset of the thermally-pulsing phase. Third dredge-up only occurs in intermediate-mass models $Z \le 0.08$. Hot bottom burning (HBB) shows a weaker dependence on metallicity, with the minimum mass increasing from 4.5$M_{\odot}$ for $Z=0.014$ to $\approx 5.5 M_{\odot}$ for Z = 0.04, $6M_{\odot}$ for $ 0.05 \le Z \le 0.07$ and above 6.5$ M_{\odot}$ for $Z\ge 0.08$. The behaviour of the $Z=0.1$ models is unusual; most do not experience He-shell instabilities owing to rapid mass-loss on the early part of the AGB. Turning off mass-loss produces He-shell instabilities, however thermal pulses are weak and result in no third dredge-up. The minimum mass for carbon ignition is reduced from 8$M_{\odot}$ for $Z=0.04$ to 7$M_{\odot}$ for $Z=0.1$, which implies a reduction in the minimum mass for core-collapse supernovae. MESA models of similarly high metallicity ($Z=0.06 - 0.09$) show the same lowering of the minimum mass for carbon ignition: carbon burning is detected in a $6 M_{\odot}$ model at the highest metallicity ($Z=0.09$) and in all $7 M_{\odot}$ models with $Z \ge 0.06$. This demonstrates robustness of the lowered carbon burning threshold across codes.