Two-dimensional Simulation of Convection in a Red Giant Envelope

Asida, S. M.; Tuchman, Y.

doi:10.1086/311055

Cited by 6 publications

(6 citation statements)

References 21 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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“…Previous studies of thermally driven convection in slab geometry have shown velocity spectra which possess a k −5/3 Kolmogorov-Obukhov-like range [1], [2], [3] and have examined a wide variety of statistical measures [1], [3]- [6]. Axially symmetric 2D numerical models of radially deep sectors have also been used to test mixing-length parameters relevant to red giant envelopes [7], [8].…”

Section: Introductionmentioning

confidence: 99%

Convection in Slab and Spheroidal Geometries

Porter

Woodward

Jacobs

2000

Annals of the New York Academy of Sciences

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Three‐dimensional numerical simulations of compressible turbulent thermally driven convection, in both slab and spheroidal geometries, are reviewed and analyzed in terms of velocity spectra and mixing‐length theory. The same ideal gas model is used in both geometries, and resulting flows are compared. The piecewise‐parabolic method (PPM), with either thermal conductivity or photospheric boundary conditions, is used to solve the fluid equations of motion. Fluid motions in both geometries exhibit a Kolmogorov‐like k−5/3 range in their velocity spectra. The longest wavelength modes are energetically dominant in both geometries, typically leading to one convection cell dominating the flow. In spheroidal geometry, a dipolar flow dominates the largest scale convective motions. Downflows are intensely turbulent and up drafts are relatively laminar in both geometries. In slab geometry, correlations between temperature and velocity fluctuations, which lead to the enthalpy flux, are fairly independent of depth. In spheroidal geometry this same correlation increases linearly with radius over the inner 70 percent by radius, in which the local pressure scale heights are a sizable fraction of the radius. The effects from the impenetrable boundary conditions in the slab geometry models are confused with the effects from non‐local convection. In spheroidal geometry nonlocal effects, due to coherent plumes, are seen as far as several pressure scale heights from the lower boundary and are clearly distinguishable from boundary effects.

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“…Modeling the convection in LPVs (Asida & Tuchman 1997) An accurate calculation of the convective energy transport in envelopes of AGB stars is one of the crucial demands in any attempt for modeling these stars. Its importance is caused by the fact that the convective region in these stars is very efficient and large (in mass and radius).…”

Section: 2mentioning

confidence: 99%

“…This difficulty was partially solved using the following procedure (Asida & Tuchman 1997): two dimensional calculations were followed only for about one year -long enough for the establishment of the convective flow, but still far below the thermal time-scale needed for achieving full energy balance. However, this period of time was long enough to estimate, for each calculated model, its deviation from an accurate energy balance.…”

Section: 2mentioning

confidence: 99%

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Theoretical pulsation models for Long Period Variables

Tuchman

1999

Symp. - Int. Astron. Union

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Three issues, which are to my opinion the main obstacles in modeling the variability of Miras and Long Period Variables (LPVs), are presented and discussed: a.The initial static models - Possible influences of the evolutionary details, the helium shell flashes and the use of the core-mass luminosity relation.b.Modeling the convective energy transport - The mixing-length theory, its limitations and validity. First attempts for two-dimensional calculations of the convective flow in AGB stars.c.The surface conditions - Introduction to the main difficulties and uncertainties. What should be done in the future.Finally, the ancient problem, concerning the mode of pulsation, in Miras, is discussed using the recent radii observations confronted with the results of our full non-linear calculations.

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Two-dimensional Simulation of Convection in a Red Giant Envelope

Cited by 6 publications

References 21 publications

Three-dimensional simulations of the atmosphere of an AGB star

Three-dimensional simulations of the atmosphere of an AGB star

Convection in Slab and Spheroidal Geometries

Theoretical pulsation models for Long Period Variables

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