Abstract
ABSTRACT The treatment of convective boundaries remains an important source of uncertainty within stellar evolution, with drastic implications for the thermally pulsing stars on the asymptotic giant branch (AGB). Various sources are taken as motivation for the incorporation of convective boundary mixing (CBM) during this phase, from s-process nucleosynthesis to hydrodynamical models. In spite of the considerable evidence in favour of the existence of CBM on the pre-AGB evolution, this mixing is not universally included in models of TP-AGB stars. The aim of this investigation is to ascertain the extent of CBM, which is compatible with observations when considering full evolutionary models. Additionally, we investigate a theoretical argument that has been made that momentum-driven overshooting at the base of the pulse-driven convection zone should be negligible. We show that, while the argument holds, it would similarly limit mixing from the base of the convective envelope. On the other hand, estimations based on the picture of turbulent entrainment suggest that mixing is possible at both convective boundaries. We demonstrate that additional mixing at convective boundaries during core-burning phases prior to the thermally pulsing AGB has an impact on the later evolution, changing the mass range at which the third dredge-up and hot-bottom burning occur, and thus also the final surface composition. In addition, an effort has been made to constrain the efficiency of CBM at the different convective boundaries, using observational constraints. Our study suggests a strong tension between different constraints that makes it impossible to reproduce all observables simultaneously within the framework of an exponentially decaying overshooting. This result calls for a reassessment of both the models of CBM and the observational constraints.
Highlights
The treatment of mixing at convective boundaries presents a challenge which, despite the advent of 3D hydrodynamical simulations (Hurlburt et al 1994; Freytag et al 1996; Herwig et al 2007; Baraffe et al 2017), has proven to be a persistent source of uncertainties in 1D stellar evolution models
GARSTEC sequences were computed with a mixture of Reimers (1975), Wachter et al (2002), and van Loon et al (2005) prescriptions as described in Weiss & Ferguson (2009), while LPCODE sequences rely on a mixture of Schroder & Cuntz (2005), Groenewegen et al (1998), and Groenewegen et al (2009), as described in Miller Bertolami (2016), where we have explored current constraints on convective boundary mixing (CBM) during core hydrogen burning, core helium burning, mixing from the lower boundary of a convective envelope, and from the convective boundaries of pulse-driven convection zones
We have performed an extensive exploration of the impact of Convective Boundary Mixing (CBM) on the predictions of stellar evolution models for the Thermally Pulsating Asymptotic Giant Branch (AGB) phase (TP-AGB)
Summary
The treatment of mixing at convective boundaries presents a challenge which, despite the advent of 3D hydrodynamical simulations (Hurlburt et al 1994; Freytag et al 1996; Herwig et al 2007; Baraffe et al 2017), has proven to be a persistent source of uncertainties in 1D stellar evolution models. The application of the intensity of CBM calibrated from core hydrogen burning stars to all convective boundaries can lead to results which are in tension with observations when applied to all boundaries during the TPAGB evolution This has been shown to be the case for the stellar evolution codes used here (Weiss & Ferguson 2009; Andrews et al 2015; Miller Bertolami 2016), and provides an additional motivation for examining the influence of additional mixing at the various convective boundaries.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
