Vol. 606
In section 8. Stellar atmospheres

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. VI. First chromosphere model of a late-type giant

This paper presents the first exploratory 3D hydrodynamical model atmosphere of the chromosphere of a cool red giant star to study the dynamical and thermodynamic properties, as well as the influence on the observable properties of such a star. Three-dimensional radiation hydrodynamics simulations were carried out with the CO^5BOLD model atmosphere code for a star with atmospheric parameters that are similar to those of the K-type red giant star Aldebaran. The authors compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2 nm, and H alpha, as well as the spectral energy distribution of the emergent radiative flux. The initial model is found to quickly develop a dynamical chromosphere that is characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values up to 5000K although the shock fronts remain quite narrow. As for the Sun,the gas temperature distribution in the upper layers is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to the shock waves. These simulations show that the atmospheres of red giant stars are dynamic. This means that many observable properties cannot be reproduced with a one-dimensional static model but require advanced 3D hydrodynamical modelling. Furthermore, including a chromosphere in the models can produce a significant contribution to the emergent UV flux.