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Global 3D radiation-hydrodynamics models of AGB stars: Effects of convection and radial pulsations on atmospheric structures
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A137Article in journal (Refereed) Published
Abstract [en]

Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales.

Aims. To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations.

Methods. We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters.

Results. The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features.

Conclusions. Our models self-consistently describe convection, convectively generated acoustic noise, fundamental-mode radial pulsations, and atmospheric shocks of various scales, which give rise to complex changing structures in the atmospheres of AGB stars.

Place, publisher, year, edition, pages
EDP SCIENCES S A , 2017. Vol. 600, article id A137
Keywords [en]
convection, shock waves, methods: numerical, stars: AGB and post-AGB, stars: atmospheres, stars: oscillations
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-324338DOI: 10.1051/0004-6361/201629594ISI: 000400754000072OAI: oai:DiVA.org:uu-324338DiVA, id: diva2:1110356
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC), p2013234Available from: 2017-06-15 Created: 2017-06-15 Last updated: 2018-04-10Bibliographically approved
In thesis
1. Stellar Winds of Cool Giants: Investigating the Mass-Loss Mechanism of AGB Stars
Open this publication in new window or tab >>Stellar Winds of Cool Giants: Investigating the Mass-Loss Mechanism of AGB Stars
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Asymptotic giant branch (AGB) stars are luminous cool giants of low to intermediate mass that are strongly pulsating and non-spherical, with heavy mass loss through a stellar wind. The mass loss makes these stars important for galactic chemistry, as the wind enriches the interstellar medium with new elements and dust, and it determines the final fate of these stars.

The winds of AGB stars are believed to be driven by a combination of pulsation-induced shocks and radiation pressure on dust grains, which form in the atmospheres. The two processes, pulsation and mass loss, are usually simulated using different computational codes, as the physical environment of the atmosphere, where the wind is driven, is vastly different from the interior, where the pulsations originate. In this work we try to bridge this gap.

The dynamical atmosphere and wind code DARWIN is used to study dust driven winds. An extensive grid of DARWIN models is constructed to investigate how the mass-loss rates depend on different stellar parameters. The models reproduce observed dynamical properties and we find a strong correlation between mass-loss rates and luminosities.

The simplified description of stellar pulsation in standard DARWIN models, however, introduces free parameters that need to be constrained. The atmosphere models are highly non-linear and even moderate changes to the pulsation properties may have significant impact on the mass-loss rate and wind velocity.

To self-consistently model the pulsation process, and to study atmospheric structures caused by the convection, the radiation hydrodynamical code CO5BOLD is used to produce an exploratory grid of 3D star-in-a-box models. The resulting models have realistic radii and periods, and give important insights into the complex non-spherical structure of AGB stars. Pulsation properties are derived from the CO5BOLD models and used as input in the DARWIN models. Average wind properties from models with CO5BOLD input agree with the standard DARWIN models, however the winds show large density variations with time, which may affect comparisons with observations.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 74
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1664
Keywords
late-type stars, AGB stars, stellar winds, stellar atmospheres, dust, stellar pulsation, hydrodynamics, radiative transfer
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-348125 (URN)978-91-513-0319-2 (ISBN)
Public defence
2018-05-30, Häggsalen, Ångströmslaboratoriet, Läggerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2018-05-07 Created: 2018-04-10 Last updated: 2018-05-07

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Freytag, BerndLiljegren, SofieHöfner, Susanne

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