Application of a Theory and Simulation-Based Convective Boundary Mixing Model for Agb Star Evolution and Nucleosynthesis

Abstract

The s-process nucleosynthesis in Asymptotic Giant Branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up where the 13 C pocket for the s process in AGB stars forms. In this work we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 and M = 3M , and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundance of 12 C and 16 O are increased by CBM at the bottom of pulse-driven convection zone. This mixing is affecting the 22 Ne(α,n) 25 Mg activation and the s-process efficiency in the 13 C-pocket. In our model CBM at the bottom of the convective envelope during the third dredgeup represents gravity wave mixing. We take further into account that hydrodynamic simulations indicate a declining mixing efficiency already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the 13 C-pocket with a mass of ≈ 10 −4 M . The final s-process abundances are characterized by 0.36 < [s/Fe] < 0.78 and the heavy-to-light s-process ratio is −0.23 < [hs/ls] < 0.45. Finally, we compare our results with stellar observations, pre-solar grain measurements and previous work.

i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

“…Based on numerous investigations of the effect that CBM at the bottom of the He-shell flash convection zone has on stellar evolution simulations (e.g. Karakas et al 2010;Battino et al 2016), its investigation in 3D simulations (Herwig et al 2007) and the better agreement of observations of novae with models that include CBM at the bottom of the H-flash convection , we conclude that CBM with f bot = 0.008 at the bottom of the He-shell flash convection zone is at present the most realistic assumption.…”

“…The accreted material and the WD both have the solar initial composition (Grevesse & Noels 1993;Lodders 2003). Like in our nova models, we have taken into account convective boundary mixing (CBM) at the bottom of the He-flash convective zone in some of the evolution tracks via the exponential, diffusive model (Freytag et al 1996;Herwig 2000;Battino et al 2016) implemented in mesa. The efficiency parameter f bot specifies the e-folding distance of the exponential decay of the mixing efficiency (Table 1).…”

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

“…Internal gravity waves caused by convective motions beating on the convective/radiative interface are also suggested by Denissenkov & Tout (2003); Battino et al (2016) to produce the PMZ and produce PMZ similar to those resulting from the overshoot models.…”

“…In fact, a higher amount of 12 C in the intershell leads to a higher abundance of 13 C, and subsequently a higher number of free neutrons. Full s-process models including this effect have been developed by Pignatari et al (2016) and Battino et al (2016).…”

Constraints of the Physics of Low-Mass Agb Stars From Ch and Cemp Stars

“…Later, when the temperature rises again, those protons are captured by the abundant 12 C, leading to the formation of the so-called 13 C-pocket. The physics of such a mechanism is a highly debated subject and many hypotheses have been postulated: gravity waves plus Kelvin-Helmholtz instabilities (Battino et al 2016), magnetic fields (Trippella et al 2016), stochastic mixing events (Bisterzo et al 2015) and ballistic plumes penetration (Cristallo et al 2009b). Moreover, the possibility that other mechanisms, such as rotation, stretch the recently formed 13 C profile at a later time cannot be excluded (Piersanti et al 2013).…”