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An extensive grid of DARWIN models for M-type AGB stars: I. Mass-loss rates and other properties of dust-driven winds
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics. Univ Padua, Dipartimento Fis & Astron Galileo Galilei, Vicolo Osservatorio 3, I-35122 Padua, Italy.ORCID iD: 0000-0003-2964-7943
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics. Stockholm Univ, AlbaNova Univ Ctr, Oscar Klein Ctr, Dept Astron, S-10691 Stockholm, Sweden.ORCID iD: 0000-0001-7662-4184
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.ORCID iD: 0000-0003-2356-643x
Department of Physics and Astronomy "G. Galilei" University of Padova.
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A100Article in journal (Refereed) Published
Abstract [en]

Context: The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass.

Aims: The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters.

Methods: We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg2SiO4 grains. The grid consists of similar to 4000 models, with luminosities from L-* = 890 L-circle dot to L-* = 40 000 L-circle dot and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M-circle dot to 3 M-circle dot. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si).

Results: We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L-*/M-* : increasing L-*/M-* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than similar to 14 000 L-circle dot, and consequently at pulsation periods longer than similar to 800 days. The final grain radii range from 0.25 to 0.6 mu m. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.

Place, publisher, year, edition, pages
2019. Vol. 626, article id A100
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-348124DOI: 10.1051/0004-6361/201935366ISI: 000472465400001OAI: oai:DiVA.org:uu-348124DiVA, id: diva2:1196668
Funder
Swedish Research CouncilEU, European Research Council, 6 15 604
Note

Title in dissertation list of papers: An extensive grid of DARWIN models for M-type AGB stars I. Mass loss and the properties of wind and dust

Available from: 2018-04-10 Created: 2018-04-10 Last updated: 2019-08-05Bibliographically 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)
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Supervisors
Available from: 2018-05-07 Created: 2018-04-10 Last updated: 2018-10-08

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Bladh, SaraLiljegren, SofieHöfner, Susanne

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