scholarly journals APPLICATION OF A THEORY AND SIMULATION-BASED CONVECTIVE BOUNDARY MIXING MODEL FOR AGB STAR EVOLUTION AND NUCLEOSYNTHESIS

2016 ◽  
Vol 827 (1) ◽  
pp. 30 ◽  
Author(s):  
U. Battino ◽  
M. Pignatari ◽  
C. Ritter ◽  
F. Herwig ◽  
P. Denisenkov ◽  
...  
Keyword(s):  
Mixing Model ◽  
Convective Boundary ◽  
Boundary Mixing ◽  
Star Evolution ◽  
Simulation Based

scholarly journals NuGrid stellar data set – III. Updated low-mass AGB models and s-process nucleosynthesis with metallicities Z= 0.01, Z = 0.02, and Z = 0.03

2019 ◽  
Vol 489 (1) ◽  
pp. 1082-1098 ◽  
Author(s):  
U Battino ◽  
A Tattersall ◽  
C Lederer-Woods ◽  
F Herwig ◽  
P Denissenkov ◽  
...  
Keyword(s):  
Data Access ◽  
Mixing Model ◽  
Parametric Models ◽  
Asymptotic Giant Branch ◽  
Convective Boundary ◽  
Yield Data ◽  
Data Set ◽  
Boundary Mixing ◽  
New Models ◽  
Low Mass

ABSTRACT The production of the neutron-capture isotopes beyond iron that we observe today in the Solar system is the result of the combined contribution of the r-process, the s-process, and possibly the i-process. Low-mass asymptotic giant branch (AGB) (1.5 < M/M⊙ < 3) and massive (M > 10 M⊙) stars have been identified as the main site of the s-process. In this work we consider the evolution and nucleosynthesis of low-mass AGB stars. We provide an update of the NuGrid Set models, adopting the same general physics assumptions but using an updated convective-boundary-mixing model accounting for the contribution from internal gravity waves. The combined data set includes the initial masses MZAMS/M⊙ = 2, 3 for Z = 0.03, 0.02, 0.01. These new models are computed with the mesa stellar code and the evolution is followed up to the end of the AGB phase. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid mppnp code. The convective-boundary-mixing model leads to the formation of a 13C-pocket three times wider compared to the one obtained in the previous set of models, bringing the simulation results now in closer agreement with observations. Using these new models, we discuss the potential impact of other processes inducing mixing, like rotation, adopting parametric models compatible with theory and observations. Complete yield data tables, derived data products, and online analytic data access are provided.

scholarly journals Relative importance of convective uncertainties in massive stars

2020 ◽  
Vol 496 (2) ◽  
pp. 1967-1989 ◽  
Author(s):  
Etienne A Kaiser ◽  
Raphael Hirschi ◽  
W David Arnett ◽  
Cyril Georgy ◽  
Laura J A Scott ◽  
...  
Keyword(s):  
Convective Zone ◽  
Stellar Structure ◽  
Convective Boundary ◽  
Hydrogen Burning ◽  
Surface Evolution ◽  
Boundary Mixing ◽  
The Core ◽  
Boundary Location ◽  
Parameter Values ◽  
The Impact

ABSTRACT In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called ‘overshoot’, namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses of 15, 20, and $25\, \rm {M}_\odot$. We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main sequence with more CBM, which is more in agreement with observations. Furthermore, during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves redwards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the 12C to 16O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduces and we find differences of up to $70{{\ \rm per\ cent}}$ for the core masses and the total mass of the star.

scholarly journals Convective boundary mixing in low- and intermediate-mass stars – I. Core properties from pressure-mode asteroseismology

2020 ◽  
Vol 493 (4) ◽  
pp. 4987-5004 ◽  
Author(s):  
George C Angelou ◽  
Earl P Bellinger ◽  
Saskia Hekker ◽  
Alexey Mints ◽  
Yvonne Elsworth ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Measurement Precision ◽  
Observational Constraints ◽  
Intermediate Mass ◽  
Convective Boundary ◽  
Mode Identification ◽  
Boundary Mixing ◽  
Intermediate Mass Stars ◽  
F Stars

ABSTRACT Convective boundary mixing (CBM) is ubiquitous in stellar evolution. It is a necessary ingredient in the models in order to match observational constraints from clusters, binaries, and single stars alike. We compute ‘effective overshoot’ measures that reflect the extent of mixing and which can differ significantly from the input overshoot values set in the stellar evolution codes. We use constraints from pressure modes to infer the CBM properties of Kepler and CoRoT main-sequence and subgiant oscillators, as well as in two radial velocity targets (Procyon A and α Cen A). Collectively, these targets allow us to identify how measurement precision, stellar spectral type, and overshoot implementation impact the asteroseismic solution. With these new measures, we find that the ‘effective overshoot’ for most stars is in line with physical expectations and calibrations from binaries and clusters. However, two F-stars in the CoRoT field (HD 49933 and HD 181906) still necessitate high overshoot in the models. Due to short mode lifetimes, mode identification can be difficult in these stars. We demonstrate that an incongruence between the radial and non-radial modes drives the asteroseismic solution to extreme structures with highly efficient CBM as an inevitable outcome. Understanding the cause of seemingly anomalous physics for such stars is vital for inferring accurate stellar parameters from TESS data with comparable timeseries length.

scholarly journals Impact of convective boundary mixing on the TP-AGB

2020 ◽  
Vol 493 (4) ◽  
pp. 4748-4762
Author(s):  
G Wagstaff ◽  
M M Miller Bertolami ◽  
A Weiss
Keyword(s):  
Surface Composition ◽  
Mass Range ◽  
Asymptotic Giant Branch ◽  
Observational Constraints ◽  
Theoretical Argument ◽  
Convective Boundary ◽  
Convective Envelope ◽  
Boundary Mixing ◽  
Turbulent Entrainment ◽  
Convective Boundaries

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.

scholarly journals The First 3D Simulations of Carbon Burning in a Massive Star

2016 ◽  
Vol 12 (S329) ◽  
pp. 237-241 ◽  
Author(s):  
A. Cristini ◽  
C. Meakin ◽  
R. Hirschi ◽  
D. Arnett ◽  
C. Georgy ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Radial Profile ◽  
Three Dimensional ◽  
Stable Region ◽  
Entrainment Rate ◽  
Convective Boundary ◽  
Boundary Mixing ◽  
Turbulent Entrainment ◽  
Large Eddy ◽  
Carbon Burning

AbstractWe present the first detailed three-dimensional hydrodynamic implicit large eddy simulations of turbulent convection for carbon burning. The simulations start with an initial radial profile mapped from a carbon burning shell within a 15 M⊙stellar evolution model. We considered 4 resolutions from 1283to 10243zones. These simulations confirm that convective boundary mixing (CBM) occurs via turbulent entrainment as in the case of oxygen burning. The expansion of the boundary into the surrounding stable region and the entrainment rate are smaller at the bottom boundary because it is stiffer than the upper boundary. The results of this and similar studies call for improved CBM prescriptions in 1D stellar evolution models.

scholarly journals On the recent parametric determination of an asteroseismological model for the DBV star KIC 08626021

2019 ◽  
Vol 630 ◽  
pp. A100 ◽  
Author(s):  
Francisco C. De Gerónimo ◽  
Tiara Battich ◽  
Marcelo M. Miller Bertolami ◽  
Leandro G. Althaus ◽  
Alejandro H. Córsico
Keyword(s):  
White Dwarf ◽  
Chemical Structure ◽  
Reaction Rate ◽  
Current Knowledge ◽  
Convective Boundary ◽  
Dwarf Stars ◽  
Boundary Mixing ◽  
Chemical Structures ◽  
White Dwarf Stars ◽  
Formation And Evolution

Context. Asteroseismology of white dwarf stars is a powerful tool that allows us to reveal the hidden chemical structure of white dwarfs and infer details about their present and past evolution by comparing the observed periods with those obtained from appropriate stellar models. A recent asteroseismological study has reproduced the period spectrum of the helium-rich pulsating white dwarf KIC 08626021 with the unprecedented precision of (Pobs − Pmodel)/Pmodel <  10−8. The chemical structure derived from that asteroseismological analysis is notably different from that expected for a white dwarf according to currently accepted formation channels. It therefore poses a challenge to the theory of stellar evolution. Aims. We explore the relevant micro- and macrophysics processes that act during the formation and evolution of KIC 08626021 and might lead to a chemical structure similar to that found through asteroseismology. We quantify to which extent it is necessary to modify the physical processes that shape the chemical structure in order to reproduce the most important features of the asteroseismic model. Methods. We modeled the previous evolution of KIC 08626021 by exploring specific changes in the 12C(α, γ)16O reaction rate, screening processes, microscopic diffusion, and convective boundary mixing during core-He burning. Results. We find that in order to reproduce the core chemical profile derived for KIC 0862602, the 12C+α nuclear reaction rate has to be increased by a factor of ∼10 during the helium-core burning, and reduced by a factor of ∼1000 during the following helium-shell burning as compared with the standard predictions for this rate. In addition, the main chemical structures derived for KIC 0862602, such as the very thin helium-pure envelope, the mass of the carbon-oxygen core, and the pure C buffer, cannot be reconciled with our current knowledge of white dwarf formation. Conclusion. We find that within our current understanding of white dwarf formation and evolution, it is difficult to reproduce the most important asteroseismologically derived features of the chemical structure of KIC 08626021.

scholarly journals Dependence of convective boundary mixing on boundary properties and turbulence strength

2019 ◽  
Vol 484 (4) ◽  
pp. 4645-4664 ◽  
Author(s):  
A Cristini ◽  
R Hirschi ◽  
C Meakin ◽  
D Arnett ◽  
C Georgy ◽  
...  
Keyword(s):  
Convective Boundary ◽  
Boundary Mixing ◽  
Boundary Properties
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