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Dust-driven winds of AGB stars: The critical interplay of atmospheric shocks and luminosity variations
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.
Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
2016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 589, article id A130Article in journal (Refereed) Published
Resource type
Text
Abstract [en]

Context. Winds of AGB stars are thought to be driven by a combination of pulsation-induced shock waves and radiation pressure on dust. In dynamic atmosphere and wind models, the stellar pulsation is often simulated by prescribing a simple sinusoidal variation in velocity and luminosity at the inner boundary of the model atmosphere.

Aims. We experiment with different forms of the luminosity variation in order to assess the effects on the wind velocity and mass-loss rate, when progressing from the simple sinusoidal recipe towards more realistic descriptions. This will also give an indication of how robust the wind properties derived from the dynamic atmosphere models are.

Methods. Using state-of-the-art dynamical models of C-rich AGB stars, a range of different asymmetric shapes of the luminosity variation and a range of phase shifts of the luminosity variation relative to the radial variation are tested. These tests are performed on two stellar atmosphere models. The first model has dust condensation and, as a consequence, a stellar wind is triggered, while the second model lacks both dust and wind.

Results. The first model with dust and stellar wind is very sensitive to moderate changes in the luminosity variation. There is a complex relationship between the luminosity minimum, and dust condensation: changing the phase corresponding to minimum luminosity can either increase or decrease mass-loss rate and wind velocity. The luminosity maximum dominates the radiative pressure on the dust, which in turn, is important for driving the wind. An earlier occurrence of the maximum, with respect to the propagation of the pulsation-induced shock wave, then increases the wind velocity, while a later occurrence leads to a decrease. These effects of changed luminosity variation are coupled with the dust formation. In contrast there is very little change to the structure of the model without dust.

Conclusions. Changing the luminosity variation, both by introducing a phase shift and by modifying the shape, influences wind velocity and the mass-loss rate. To improve wind models it would probably be desirable to extract boundary conditions from 3D dynamical interior models or stellar pulsation models.

Place, publisher, year, edition, pages
2016. Vol. 589, article id A130
Keyword [en]
stars: late-type, stars: AGB and post-AGB, stars: atmospheres, stars: winds, outflows, infrared: stars, line: profiles
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-298682DOI: 10.1051/0004-6361/201527885ISI: 000375318300142OAI: oai:DiVA.org:uu-298682DiVA, id: diva2:947148
Funder
Swedish Research Council
Available from: 2016-07-07 Created: 2016-07-06 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
Keyword
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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Liljegren, SoofieHöfner, SusanneEriksson, Kjell

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