Asymmetries in asymptotic giant branch stars and their winds. I. From 3D RHD models to synthetic observables

Abstract

[Abridged] AGB stars are significant contributors to the metal enrichment of the interstellar medium. In this paper, we adapted models from advanced RHD simulations as input for radiative transfer software to create synthetic observables. A major goal is to describe an AGB star's non-sphericity and to simulate its effects on the surrounding dusty envelope. We developed tools to translate models of an AGB star and its dust-driven wind from simulations with CO5BOLD into the format used by RADMC-3D. We preserved the asymmetric shape of the star by including it as a `dust species' and by using temperature data computed in CO5BOLD. Circumstellar dust is included using Mg2SiO4 opacity data with spatially dependent grain sizes. We compared images and SEDs created with RADMC-3D of a model with similar output made with a spherically symmetric star. Our CO5BOLD model features substantial and clumpy dust formation just above 3.4 au from the grid centre, and large-scale structures due to giant convection cells are visible on the stellar surface. With the properties of VLTI as a basis, we have created simple synthetic observables. Such optical interferometers should be able to detect these dust clouds. We find that it is important to include asymmetric stellar models since they even affect the SEDs. Effects on flux levels can be linked to the clumpiness of the circumstellar dust and the angle-dependent illumination resulting from temperature variations on the stellar surface causes shifts in the wavelengths of the flux maximum. The methods presented here are an important step towards producing realistic synthetic observables and testing predictions of advanced 3D RHD models. Taking the angle-dependence of SEDs as a proxy for temporal variations in unresolved data, we conclude that not all variability observed in AGB stars should be interpreted as global changes in the sense of spherical models.

17 pages, 14 figures, and 5 tables, accepted for publication in A&A in July 2024

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