Evolution and Nucleosynthesis in Low‐Mass Asymptotic Giant Branch Stars. I. Formation of Population I Carbon Stars

Abstract

New models of thermally pulsing asymptotic giant branch (TP-AGB) stars of low mass and solar chemical composition are presented, namely, Z \ 0.02, and Y \ 0.28. The inÑuence of 1 ¹ M/M _ ¹ 3, various parameters (such as the initial core mass, the envelope mass, the mass-loss rate, the opacity, and the mixing length) on the properties of the models is discussed in detail. Our main Ðndings are the following :1. The third dredge-up (TDU) operates self-consistently (using the Schwarzschild criterion for convection and without invoking any extra-mixing) for masses as low as 1.5The minimum core mass for M _ . which TDU is found is This value is attained after about 10 thermal pulses, almost M H D 0.61 M _ . independently of the initial mass.2. During the early TP-AGB evolution, the relation between the pulse strength (i.e., the luminosity peak of the 3a burning during the pulse) and the core mass is in good agreement with previous Ðndings. However, when TDU is settled on, the strength of the pulse increases more rapidly as the penetration of the convective envelope into the He intershell increases. No asymptotic limit is found.3. Furthermore, the 3a luminosity peak is independent of the previous history : the strength of the pulse in a model with mass loss is the same as in a model without mass loss but having the same core and envelope masses.4. Unless extreme mass-loss rates are assumed, carbon stars are obtained in all the sequences of models with initial mass M º 1.5 after about 24-26 thermal pulses and 15-17 TDU episodes. At M _ C-star formation, the core mass is less than 0.7 and the luminosity is of the order of 104 The M _ , L _ . dredged-up mass increases up to a maximum and then decreases as mass loss and/or the advancement of the H-burning shell consume the envelope. When the envelope mass is reduced below approximately 0.5 TDU eventually vanishes. M _ , 5. If some amount of protons is di †used below the base of the H-rich envelope during TDU, in the interpulse a 13C-pocket is formed and then burnt radiatively via the 13C(a, n)16O reaction, before the onset of a new pulse. Thus, s-process nucleosynthesis occurs in a radiative environment characterized by a fairly low neutron density. In advanced thermal pulses, when the temperature at the bottom of the convective shell approaches 3 ] 108 K, a secondary source of neutrons comes from the marginal activation of the 22Ne(a, n)25Mg reaction.

Quantifying the uncertainties of chemical evolution studies

“…This was because the synthetic AGB models required an assumption about the efficiency of the TDU for these last pulses. Full calculations have shown that the efficiency approaches zero as the envelope mass is decreased (Vassiliadis & Wood 1993;Straniero et al 1997;Karakas et al 2002). The new intermediate-mass Z = 0.0001 AGB models presented in Karakas (2010) were evolved to low envelope masses (∼0.1 M in some cases).…”

AGB stars of the intermediate-age LMC cluster NGC 1846

“…Current stellar evolution models (e.g. Straniero et al 1997;Herwig 2000;Busso et al 2001;Stancliffe et al 2004;Straniero et al 2006;Karakas & Lattanzio 2007) provide quite detailed predictions for the surface abundance changes during the AGB phase. Different models agree qualitatively.…”

Magnetic Bubbles and Extramixing in Stars

“…We shall consider, as a reference, the case of a star of initial mass 1.5 M@ with a metallicity half of the Solar value and an internal structure at the RGB and AGB phases as computed previously [18,19], including mass loss with the parameterization presented in [20]. …”