Calibrating the Mixing‐Length Parameter for a Red Giant Envelope

Asida, S. M.

doi:10.1086/308211

Cited by 16 publications

(22 citation statements)

References 74 publications

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“…Simulations have been extended to patches or slices of red subgiants and giants (Asida & Tuchman 1997;Asida 2000;Freytag & Salaris 1999;Robinson et al 2004;Collet et al 2006), and even to entire AGB stars or red supergiants (see Jacobs et al 1998;Freytag et al 2002). Woodward et al (2003) presented models with extremely high numerical resolution but simple microphysics.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional simulations of the atmosphere of an AGB star

Freytag

Höfner

2008

A&A

158

218

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Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary ("piston") simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres -convection and pulsations -are actually three-dimensional.Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and nongrey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models.Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.

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Section: Introductionmentioning

confidence: 99%

Three-dimensional simulations of the atmosphere of an AGB star

Freytag

Höfner

2008

A&A

158

218

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“…In fact, one of the most spectacular astrophysical success stories ever came precisely from an argument of this type: Fred Hoyle and coworkers ( [131]) realized that the observed (i.e., fairly high) amount of carbon in the Universe would be completely inconsistent with stellar nucleosynthesis arguments, unless the 8 Be(α, γ) 12 C reaction proceeded through a (then unknown) resonance located very near the 8 Be + α threshold, i.e., at an energy around 7.68 MeV. This reaction forms the second "leg" of the triple-α process, the first being 4 He + 4 He ⇄ 8 Be ( [183,226]). The problem with this process, it was argued, is that 8 Be very rapidly decays back into two α particles, thus making it exceedingly unlikely that substantial amounts of 12 C can be formed without a resonance being present close to the 8 Be + α threshold.…”

Section: Thermonuclear Reaction Ratesmentioning

confidence: 92%

“…A recent attempt at calibrating the free parameters of both the mixing length and full spectrum of turbulence theories on the basis of 2D hydrodynamical simulations has been provided by [170] (see also [4]). According to their results, the parameter α ℓ in the former theory presents a variation from ∼ 1.3 for F-type dwarfs to ∼ 1.75 for K-type subgiants, there existing a plateau in the neighborhood of the Sun where α ℓ remains nearly constant.…”

Section: Convection Beyond the Mixing Length Theorymentioning

confidence: 99%

“…In the case of low-mass stars, the most important reactions that must be included in such networks are obviously the H-and He-burning reactions, most important among these being the so-called proton-proton (PP) chain for main-sequence stars, the CNO cycle for RGB, HB, and AGB stars, and the triple-α process for the HB and AGB phases. In addition, the 12 C(α, γ) 16 O reaction becomes increasingly important as a He-burning mechanism towards the end of the HB phase, when He is approaching exhaustion and the triple-α process becomes less frequent (involving, as it does, the almost simultaneous combination of three 4 He nuclei). Proton-capture nucleosynthesis may also be of considerable importance, particularly in view of observed abundance anomalies (especially among globular cluster stars) in such elements as Na and Al ( [112] and references therein).…”

Section: Thermonuclear Reaction Ratesmentioning

confidence: 99%

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Structure and Evolution of Low-Mass Stars: An Overview and Some Open Problems

Catelan¹

2007

AIP Conference Proceedings

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Abstract.A review is presented of some of the ingredients, assumptions and techniques that are used in the computation of the structure and evolution of low-mass stars. Emphasis is placed on several ingredients which are still subject to considerable uncertainty. An overview of the evolution of low-mass stars is also presented, from the cloud collapse phase all the way to the white dwarf cooling curve. Keywords DEFINITION OF "LOW-MASS STARS"Low-mass stars are self-gravitating gaseous (or, rather, plasmatic) bodies that develop electron-degenerate cores (meaning that all low-lying energy states are filled and electron pressure is accordingly dominated by the Pauli exclusion principle) soon after leaving the main sequence (MS) phase, and hence undergo the so-called "helium flash" (i.e., ignition of the triple-α process, whereby three alpha particles are converted into a 12 C nucleus, under degenerate conditions) at the end of their evolution on the red giant branch (RGB). Evolutionary calculations indicate that this corresponds to an upper mass limit M ≈ 2 − 2.5 M ⊙ (e.g., [258,259,109]). Exceptionally, and as a consequence of extreme mass loss on the RGB, some such stars may directly become helium white dwarfs (WD's) before they are ever able to ignite helium in their degenerate cores (e.g., [16] and references therein). At the low-mass end, on the other hand, one finds that objects below M ≃ 0.08 M ⊙ are incapable of quiescent hydrogen burning. This corresponds to the commonly adopted "dividing line" between low-mass stars and the so-called brown dwarfs (see, e.g., [58], and references therein). Empirically, the latter have recently been associated to the new spectral classes L ([156]) and T ([31]). ASTROPHYSICAL IMPORTANCELow-mass stars are of great astrophysical importance for a variety of reasons, which include the following:• Due to the shape of the so-called "Initial Mass Function" ([227]; see also [163] for a recent review), which gives the number of stars that are born in a given

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“…In particular, estimating the growth rate of a turbulent mixing zone which develops from random multi-mode perturbations under Rayleigh-Taylor instability -known as the α parameter -has been proven to be problematic. For that reason it is a good test for code validation (Calder et al, 2002). We used this test with a three-dimansional (3D) version of the hydro-code VULCAN.…”

Section: Problemmentioning

confidence: 99%

3D Simulations of Rayleigh–Taylor Instability Using “VULCAN/3D”

Asida

Livne

Stein

et al. 2005

Astrophys Space Sci

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The growth rate of the turbulent mixing zone, which develops from random perturbations under Rayleigh-Taylor instability, has been studied using the 3D version of the hydrodynamical code VULCAN. Previous studies show large differences between the α parameter of different codes. In its Eulerian mode VULCAN/3D employs Van-Leer scheme for the advection of all variables, and can also use interface tracking for multi-phase flows. Simulations using parallel version of VULCAN/3D give α of about 0.06, a value which agrees very well with experiments and some other simulations.

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Calibrating the Mixing‐Length Parameter for a Red Giant Envelope

Cited by 16 publications

References 74 publications

Three-dimensional simulations of the atmosphere of an AGB star

Three-dimensional simulations of the atmosphere of an AGB star

Structure and Evolution of Low-Mass Stars: An Overview and Some Open Problems

3D Simulations of Rayleigh–Taylor Instability Using “VULCAN/3D”

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