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 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 Evolution and Nucleosynthesis in Extremely Metal-Poor, Asymptotic Giant Branch Stars

2009 ◽  
Vol 26 (3) ◽  
pp. 145-152 ◽  
Author(s):  
Nobuyuki Iwamoto
Keyword(s):  
Mass Loss ◽  
Surface Composition ◽  
Mass Range ◽  
Big Bang ◽  
Asymptotic Giant Branch ◽  
Light Nuclei ◽  
Efficient Production ◽  
Asymptotic Giant Branch Stars ◽  
Secondary Star ◽  
Low Mass

AbstractWe evolve extremely metal-poor ([Fe/H]≃–3), thermally pulsing Asymptotic Giant Branch (AGB) models with the mass range of 1–8 M⊙. The chemical yields ejected from the models are obtained by considering mass loss. We find that the 1- and 2-M⊙ AGB models are not affected by hot bottom burning (HBB). Nevertheless, they produce large amount of 7Li in an H-flash event. The occurrence of this event is associated with the ingestion of protons from the overlying H-rich envelope into the He convective shell driven by thermal pulse. The resulting 7Li abundances in the ejecta are higher than the primordial one predicted in Big-Bang nucleosynthesis. The efficient production of 7Li by the operation of HBB is also confirmed in the models of 4–8 M⊙. If these AGB stars have a low-mass companion, it is probable that mass loss from the primary AGB star brings the materials enriched in 7Li into the secondary star. This makes the surface composition of the secondary Li-rich. The formation of Li-rich stars, however, is strongly dependent on the mass loss history and binary separation. The nucleosynthesis for the other light nuclei is also calculated up to the end of the AGB phase. We find that the abundance patterns of the metal-poor stars CS 29528–041 and CS 29497–030 are well reproduced by yields from our AGB models.

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 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 The Creation of Lithium in Giant Stars

1995 ◽  
Vol 10 ◽  
pp. 451-452 ◽  
Author(s):  
I.-Juliana Sackmann ◽  
Arnold I. Boothroyd
Keyword(s):  
Mass Range ◽  
Convective Transport ◽  
Asymptotic Giant Branch ◽  
Intermediate Mass ◽  
Convective Envelope ◽  
Giant Stars ◽  
Intermediate Mass Stars ◽  
The Creation ◽  
Maximum Temperatures ◽  
First Time

A time-dependent “convective diffusion” algorithm for convective transport in the mixing-length framework has been coupled for the first time with a self-consistent full evolutionary computation, in order to investigate theoretically the creation of superrich lithium stars on the asymptotic giant branch. For intermediate mass stars in the mass range from 4 to 7 M⊙ with both Population I and II compositions, hot bottom burning in the convective envelope was found, with maximum temperatures Tce at the base of the convective envelope ranging from 20 to 100 million K, depending on stellar mass and mass loss rates. For Tce ≥ 40 million K, lithium-rich giants were produced (with log ε(7Li) ≳ 1, i.e., above the normal observed range in giants). For Tce ≥ 50 million K, superrich lithium giants were created, with log ε(7Li) ≳ 3 (i.e., larger than the present cosmic7Li abundance).

scholarly journals Linking 1D Stellar Evolution to 3D Hydrodynamic Simulations

2014 ◽  
Vol 9 (S307) ◽  
pp. 98-99 ◽  
Author(s):  
A. Cristini ◽  
R. Hirschi ◽  
C. Georgy ◽  
C. Meakin ◽  
D. Arnett ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Richardson Number ◽  
Front Propagation ◽  
Bulk Richardson Number ◽  
Convective Boundary ◽  
Hydrodynamic Simulations ◽  
Boundary Mixing ◽  
The Core ◽  
Convective Boundaries ◽  
Initial Results

AbstractIn this contribution we present initial results of a study on convective boundary mixing (CBM) in massive stellar models using the GENEVA stellar evolution code (Eggenbergeret al.2008). Before undertaking costly 3D hydrodynamic simulations, it is important to study the general properties of convective boundaries, such as the: composition jump; pressure gradient; and “stiffness”. Models for a 15M⊙star were computed. We found that for convective shells above the core, the lower (in radius or mass) boundaries are “stiffer” according to the bulk Richardson number than the relative upper (Schwarzschild) boundaries. Thus, we expect reduced CBM at the lower boundaries in comparison to the upper. This has implications on flame front propagation and the onset of novae.

scholarly journals Convective core entrainment in 1D main sequence stellar models

2021 ◽  
Author(s):  
L J A Scott ◽  
R Hirschi ◽  
C Georgy ◽  
W D Arnett ◽  
C Meakin ◽  
...  
Keyword(s):  
Main Sequence ◽  
Mass Dependence ◽  
Bulk Richardson Number ◽  
Chemical Gradient ◽  
Convective Boundary ◽  
Boundary Mixing ◽  
Turbulent Entrainment ◽  
Stellar Convection ◽  
Convective Core ◽  
Solar Metallicity

Abstract 3D hydrodynamics models of deep stellar convection exhibit turbulent entrainment at the convective-radiative boundary which follows the entrainment law, varying with boundary penetrability. We implement the entrainment law in the 1D Geneva stellar evolution code. We then calculate models between 1.5 and 60 M⊙ at solar metallicity (Z = 0.014) and compare them to previous generations of models and observations on the main sequence. The boundary penetrability, quantified by the bulk Richardson number, RiB, varies with mass and to a smaller extent with time. The variation of RiB with mass is due to the mass dependence of typical convective velocities in the core and hence the luminosity of the star. The chemical gradient above the convective core dominates the variation of RiB with time. An entrainment law method can therefore explain the apparent mass dependence of convective boundary mixing through RiB. New models including entrainment can better reproduce the mass dependence of the main sequence width using entrainment law parameters A ∼ 2 × 10−4 and n = 1. We compare these empirically constrained values to the results of 3D hydrodynamics simulations and discuss implications.

scholarly journals Asymptotic Giant Branch Stars: Thermal Pulses, Carbon Production, and Dredge Up; Neutron Sources and s-Process Nucleosynthesis

1991 ◽  
Vol 145 ◽  
pp. 257-274
Author(s):  
Icko Iben
Keyword(s):  
Neutron Source ◽  
Lower Boundary ◽  
Source Model ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Core Mass ◽  
Convective Envelope ◽  
Thermal Pulses ◽  
Giant Branch Stars

A brief review is given of the structure of asymptotic giant branch (AGB) stars and of the characteristics of the thermal pulses which these stars experience. Following a pulse, model AGB stars with a large core mass easily dredge up fresh carbon, which is the main product of incomplete helium burning, and s-process isotopes, which are made as a consequence of the activation of the 22Ne neutron source. Model AGB stars of small core mass activate the 13C neutron source and produce s-process isotopes in nearly the solar system distribution. They also dredge up fresh carbon and s-process isotopes, but only if overshoot or some other form of “extra” mixing beyond the lower boundary of the convective envelope is invoked.

scholarly journals Fundamental problems and basic tests of stellar evolution theory — The case of carbon stars

1984 ◽  
Vol 105 ◽  
pp. 3-19
Author(s):  
Icko Iben
Keyword(s):  
Solar System ◽  
Theoretical Work ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Carbon Stars ◽  
Core Mass ◽  
Convective Envelope ◽  
Thermal Pulse ◽  
R Process ◽  
Neutron Rich Isotopes

Carbon stars are thought to be in the asymptotic giant branch (AGB) phase of evolution, alternately burning hydrogen and helium in shells above an electron-degenerate carbon-oxygen (CO) core. The excess of carbon relative to oxygen at the surfaces of these stars is thought to be due to convective dredge-up which occurs following a thermal pulse. During a thermal pulse, carbon and neutron-rich isotopes are made in a convective helium-burning zone. In model stars of large CO core mass, the source of neutrons for producing the neutron-rich isotopes is the 22Ne(α, n)25Mg reaction and the isotopes are produced in the solar system s-process distribution. In models of small core mass, the 13C(α, n) 16O reaction is thought to be responsible for the release of neutrons, and the resultant distribution of neutron-rich isotopes is expected to vary considerably from one star to the next, with the distribution in isolated instances possibly resembling the solar system distribution of r-process isotopes. After the dredge-up phase following each pulse, the 13C is made by the reactions 12C(p,γ) 13N(β+ v) 13C in a zone of large 12C abundance and small 1H abundance that has been established by semiconvective mixing during the dredge-up phase. There is qualitative accord between the properties of carbon stars in the Magellanic Clouds and properties of model stars, but considerably more theoretical work is required before a quantitative match is achieved.The observed paucity of AGB stars more luminous than MBOL ∼ −6 is interpreted to mean that the AGB lifetime of a star more luminous than this is at least a factor of ten smaller than the AGB lifetime of stars less luminous than this, or, at most 105 yr. Since, with current estimates of the 22Ne(α, n)25Mg reaction rate R22, only AGB model stars more luminous than MBOL ∼ −6 can produce s-process isotopes in the solar system distribution, it is inferred that either (1) the current estimates of R22 are too small by one to two orders of magnitude, allowing less luminous AGB stars to contribute, (2) the solar system distribution is not equivalent to the average Galactic distribution, being rather the consequence of a unique injection into the protosolar nebula of matter from a massive intermediate-mass AGB star, or (3) the estimates of the temperatures in the convective shell that are given by extant models are too low by, sav, 10 or 15 percent.The absence of carbon stars more luminous than MBOL ∼ −6 is suggested to be due primarily to the fact that ∼ 106 yr of AGB evolution is necessary to produce surface C/O > 1, rather than to be due to the burning of dredged-up carbon into nitrogen at the base of the convective envelope during the interpulse quiescent hydrogen-burning phase. Thus, the positive correlation between the nitrogen and helium abundances in planetary nebulae is perhaps primarily a consequence of the second dredge-up episode rather than a consequence of processes occurring during the thermally pulsing phase.

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