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Explaining the winds of cool giant and supergiant stars with global 3D models

Periodic Reporting for period 1 - EXWINGS (Explaining the winds of cool giant and supergiant stars with global 3D models)

Reporting period: 2020-09-01 to 2022-02-28

During late phases of stellar evolution, when the nuclear fuel in the interior is almost used up, stars grow dramatically in size, and become Asymptotic Giant Branch (AGB) or Red Supergiant (RSG) stars, depending on their initial mass. They eject a significant part of their mass through stellar winds, and enrich the surrounding interstellar medium with newly-produced elements and dust grains. Low-to-intermediate mass stars (with initial masses of about 0.8–8 solar masses) turn into white dwarfs after passing through the AGB phase, while the more massive RSG stars eventually explode in core-collapse supernova events.

The overarching aim of project EXWINGS is a breakthrough in understanding the winds of cool giant and supergiant stars. An important objective is to develop a novel theoretical approach, that is, 3D global dynamical models which describe the pulsations, convection and winds of such evolved stars from first principles, and test the resulting dynamical structures against spatially resolved observations. Such models are a critical missing link in the current picture of the cosmic matter cycle and a necessary ingredient for understanding the late phases of stellar evolution. They are essential to answer long-standing questions, in particular:

• How are the winds launched and which physical processes determine their properties?
• How do the mass-loss rate and other wind properties depend on fundamental stellar parameters?
• Which types of dust are produced by AGB and RSG stars, and in which quantities?
During the early phases of the project, we have taken a major step towards the goal of understanding the winds of evolved stars. We have produced the first global 3D RHD “star-and-wind-in-a-box” models for AGB stars. The simulations explore the interplay of interior dynamics (convection, pulsation), atmospheric shocks, dust formation, and wind acceleration in full 3D geometry. They describe the growth of silicate grains, and they account for the effects of radiative pressure on dust. Spanning from the stellar center to the inner wind region, these new models feature a much larger computational domain than our earlier “star-in-a-box” models. This allows us to follow the emerging 3D structures to a distance where the outflow is established, and to compute mass loss rates. Efforts to explore the effects of different stellar parameters (mass, effective temperature, luminosity), and to compare the modelling results to state-of-the-art observations are under way. Pulsation properties of our existing 3D models have been analyzed, finding good agreement with observations.
An ultimate goal of project EXWINGS is a predictive theory of mass loss and dust production in evolved stars, based on first physical principles. Realistic modeling of mass loss is a prerequisite for improving the general understanding of supernova progenitors and their circumstellar environment, as well as models of galactic chemical evolution (origin of chemical elements), since AGB and RSG stars contribute substantially to the cosmic matter cycle. Based on our successful development of the first global 3D “star-and-wind-in-a-box” models for AGB stars, a predictive description of their mass loss is within reach.