Granulation in K-type dwarf stars: II. Hydrodynamic simulations and 3D spectrum synthesis

Abstract

Aims. To explore the impact of surface inhomogeneities on stellar spectra, granulation models need to be computed. Ideally, the most fundamental characteristics of these models should be carefully tested before applying them to the study of more practical matters, such as the derivation of photospheric abundances. Our goal is to analyze the particular case of a K-dwarf.Methods. We construct a three-dimensional radiative-hydrodynamic model atmosphere of parameters Teff = 4820 K, log g = 4.5, and solar chemical composition. Using this model and 3D spectrum synthesis, we computed a number of Fe iâ and Fe ii line profiles. The observations presented in the first paper of this series were used to test the model predictions. The effects of stellar rotation and instrumental imperfections are carefully taken into account in the synthesis of spectral lines.Results. The theoretical line profiles show the typical signatures of granulation: the lines are asymmetric, with their bisectors having a characteristic C-shape and their core wavelengths shifted with respect to their laboratory values. The line bisectors span from about 10 to 250 m-s, depending on line strength, with the stronger features showing larger span. The corresponding core wavelength shifts range from about-200m s-1 for the weak Fe I lines to almost +100 m s-1 in the strong Fe iâ features. Based on observational results for the Sun, we argue that there should be no core wavelength shift for Fe I lines of EW ≈ mÅ. The cores of the strongest lines show contributions from the uncertain top layers of the model, where non-LTE effects and the presence of the chromosphere, which are important in real stars, are not accounted for. The Fe II lines suffer from stronger granulation effects due to their deeper formation depth which makes them experience stronger temperature and velocity contrasts. For example, the core wavelength shifts of the weakest Fe ii lines are about-600 m s-1. The comparison of model predictions to observed Fe i line bisectors and core wavelength shifts for our reference star, HIP 86 400, shows excellent agreement, with the exception of the core wavelength shifts of the strongest features, for which we suspect inaccurate theoretical values. Since this limitation does not affect the predicted line equivalent widths significantly, we consider our 3D model validated for photospheric abundance work.