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

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Without stellar winds, life as we know it would not exist. Critical elements like carbon are produced inside luminous cool giant stars, transported to the surface by turbulent gas flows, and ejected into interstellar space by massive outflows of gas and dust. Direct evidence for this scenario comes in the form of dust grains produced in the stellar winds, and detected in meteorites by their isotopic composition. Nevertheless, the current understanding of stellar winds is far from sufficient to draw a realistic, quantitative picture of their effects on stellar evolution and their contribution to the enrichment of the interstellar medium with newly-produced elements and dust. Project EXWINGS aims at a breakthrough in understanding the winds of cool giant and supergiant stars with a novel modeling approach. We will produce global radiation-hydrodynamical ‘star-and-wind-in-a-box’ simulations, based on our recently-developed prototype. For the first time, it will be possible to follow the flow of matter, in full 3D geometry, all the way from the convective, pulsating interior of a cool giant, through its atmosphere and dust-formation zone, into the dust-driven wind region. Extending our unique approach to the warmer, more luminous red supergiants, we will explore alternative driving scenarios for their still enigmatic winds, involving magneto-hydrodynamic waves and radiation pressure on molecules and dust. The current progress in high-angular-resolution instruments, giving resolved views of the stellar atmospheres where the winds originate, presents an excellent opportunity for testing the new models. An ultimate goal is a predictive theory of mass loss and dust production in evolved stars, based on first physical principles. The results of EXWINGS will have a major impact on understanding stellar and galactic chemical evolution, on tracing the origin of building blocks for terrestrial planets, and on constraining physical properties of supernova progenitors.