Global 3D radiation-hydrodynamics models of AGB stars

Freytag, B.; Liljegren, S.; Höfner, Susanne

doi:10.1051/0004-6361/201629594

Cited by 141 publications

(220 citation statements)

References 52 publications

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“…At the outflow velocities measured, material would have to be coherently ejected from above convective cells for a period of more than two years to produce the observed distribution. This is longer than typical convective timescales (Freytag et al 2017). …”

Section: Resultsmentioning

confidence: 85%

Rotation of the asymptotic giant branch star R Doradus

Vlemmings

Khouri

Beck

et al. 2018

A&A

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High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 × 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of ω R sin i = (3.5 ± 0.3) × 10 −9 rad s −1 , which indicates a rotational velocity of |υ rot sin i| = 1.0 ± 0.1 km s −1 at the stellar surface (R * = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Φ = 7 ± 6• . We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.

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Section: Resultsmentioning

confidence: 85%

Rotation of the asymptotic giant branch star R Doradus

Vlemmings

Khouri

Beck

et al. 2018

A&A

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“…12). An exploratory grid of such simulations for AGB stars by Freytag et al (2017) gives first indications of how convection patterns and pulsation properties depend on stellar parameters. The 3D models demonstrate how long-lasting giant convection cells, consisting of wide upflow regions that are surrounded by turbulent downdrafts, together with short-lived smaller surface granules and radial fundamental-mode pulsations, give rise to variable surface structures and changes in luminosity.…”

Section: Pulsation Convection and Atmospheric Levitation Due To Shomentioning

confidence: 99%

Mass loss of stars on the asymptotic giant branch

Höfner

Olofsson

2018

Astron Astrophys Rev

471

376

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As low-and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newlyformed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angularresolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their windforming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angularresolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building B Susanne Höfner

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“…Barthes 1998, for a detailed discussion). Three-dimensional interior pulsation models with realistic hydro-dynamical treatment should provide a more realistic description of convection, and therefore the pulsation properties; however, these models are still in their infancy, and only a small part of the relevant stellar parameter space explored so far (see Freytag & Höfner 2008;Freytag et al 2017). Furthermore, possible 3D effects on mass loss have not yet been investigated.…”

Section: Comparison To 1d Pulsation Modelsmentioning

confidence: 99%

Pulsation-induced atmospheric dynamics in M-type AGB stars

Liljegren

Höfner

Eriksson

et al. 2017

A&A

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Context. Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to a two-step process. First, matter in the stellar atmosphere is levitated by shock waves, induced by stellar pulsation, and second, this matter is accelerated by radiation pressure on dust, resulting in a wind. In dynamical atmosphere and wind models the effects of the stellar pulsation are often simulated by a simplistic prescription at the inner boundary. Aims. We test a sample of dynamical models for M-type AGB stars, for which we kept the stellar parameters fixed to values characteristic of a typical Mira variable but varied the inner boundary condition. The aim was to evaluate the effect on the resulting atmosphere structure and wind properties. The results of the models are compared to observed mass-loss rates and wind velocities, photometry, and radial velocity curves, and to results from 1D radial pulsation models. The goal is to find boundary conditions which give realistic atmosphere and wind properties. Methods. Dynamical atmosphere models are calculated, using the DARWIN code for different combinations of photospheric velocities and luminosity variations. The inner boundary is changed by introducing an offset between maximum expansion of the stellar surface and the luminosity and/or by using an asymmetric shape for the luminosity variation. Ninety-nine different combinations of theses two changes are tested. Results. The model atmospheres are very sensitive to the inner boundary. Models that resulted in realistic wind velocities and massloss rates, when compared to observations, also produced realistic photometric variations. For the models to also reproduce the characteristic radial velocity curve present in Mira stars (derived from CO ∆v = 3 lines), an overall phase shift of 0.2 between the maxima of the luminosity and radial variation had to be introduced. This is a larger phase shift than is found by 1D radial pulsation models. Conclusions. We find that a group of models with different boundary conditions (29 models, including the model with standard boundary conditions) results in realistic velocities and mass-loss rates, and in photometric variations. To achieve the correct line splitting time variation a phase shift is needed.

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Global 3D radiation-hydrodynamics models of AGB stars

Cited by 141 publications

References 52 publications

Rotation of the asymptotic giant branch star R Doradus

Rotation of the asymptotic giant branch star R Doradus

Mass loss of stars on the asymptotic giant branch

Pulsation-induced atmospheric dynamics in M-type AGB stars

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