Nucleosynthesis of light elements inside thermally pulsing AGB stars

Abstract

Abstract. The structural and nucleosynthetic evolution of 3, 4, 5, 6 and 7 M stars with two metallicities (Z = 0.005 and 0.02) has been computed in detail, from the early pre-main sequence phase up to the thermally pulsing (TP) AGB phase or the onset of off-center carbon burning. Typically 10 to 20 thermal pulses have been followed for each TP-AGB object. This homogeneous and quite large set of models allows us to present an overview of the thermal pulse properties as well as of the nucleosynthesis accompanying the TP-AGB phase of intermediate-mass stars.More specifically, after a brief description of the previous evolutionary stages, predictions are given for the isotopic ratios involving C, N, O, Ne, Mg, Al and Si. Also the surface abundances of 7 Li, 19 F and 23Na are reported. As the asymptotic phase of the thermal pulses has been reached for each star, we also indicate how these abundances will probably evolve until the stars completely loose their envelope, by including the evolution of the nucleosynthesis itself.

Thermohaline instability and rotation-induced mixing

“…It is based on a classical mixing-length formalism with α MLT = 1.6, recovered from solar-calibrated models that include neither atomic diffusion nor rotation and were computed using Geneva models (see Ekström et al 2012). We assume instantaneous convective mixing, except when hot-bottom burning occurs on the TP-AGB, which requires a time-dependent convective diffusion algorithm as developed in Forestini & Charbonnel (1997). The boundary between convective and radiative layers is defined with the Schwarzschild criterion.…”

“…They have been available for a long time as part of standard stellar models (e.g. Schaller et al 1992;Forestini & Charbonnel 1997;Yi et al 2003;Cassisi et al 2006), and have more recently appeared in the literature for rotation-induced models (Brott et al 2011;Ekström et al 2012). However, these latest studies focus more on the evolution of massive stars, and neitherinclude thermohaline mixing nor study the TP-AGB phase for low-and intermediate-mass stars as we do here.…”

Evolution of long-lived globular cluster stars

“…In the case of the AGB scenario, the quantity of helium released by the polluters is set during the second dredgeup episode on the early AGB; it amounts to a maximum of ∼0.36−0.38 in mass fraction, independently of the initial mass of the AGB progenitor (e.g., Doherty et al 2014). On the other hand, the sodium content of the ejecta of the polluter results from the competition between hot-bottom-burning and third dredgeup episodes that may occur during the later thermal-pulse AGB (TP-AGB) phase; therefore, it strongly depends on the initial mass of the polluter (Forestini & Charbonnel 1997;Siess 2007;D'Ercole et al 2010;Ventura et al 2013). Consequently, 2P stars spanning a large range of Na abundances are all expected to be born with very similar helium contents (maximum of 0.36−0.38 in mass fraction if no dilution with the ISM matter is taken into account compared with ∼0.248 for 1P stars; see Sect.…”

Evolution of long-lived globular cluster stars

“…Since the lifetime of the TP-AGB is much shorter than the total duration of the AGB sequence (i.e., between the end of central He-burning and the end of the last TP; e.g. Forestini & Charbonnel 1997;Herwig 2005;Karakas 2005;Marigo et al 2013), we are aware that this second criterion might be too strict from the point of view of the star counts for low-luminosity AGB stars. However, we introduce it in order …”