Three-dimensional non-LTE radiative transfer effects in Fe I lines II. Line formation in 3D radiation hydrodynamic simulations

Here we investigate the effects of horizontal radiative transfer (RT) in combination with non-local thermodynamic equilibrum (NLTE) on important diagnostic iron lines in a realistic atmosphere. Using a snapshot of a 3D radiation-hydrodynamic (HD) simulation and a multilevel iron atom, we computed widely used Fe I line profiles at three different levels of approximation of the RT (3D NLTE, 1D NLTE, LTE). By comparing the resulting line profiles and the circumstances of their formation, we gain new insight into the importance of horizontal RT. We find that the influence of horizontal RT is of the same order of magnitude as that of NLTE, although spatially more localized. Also, depending on the temperature of the surroundings, horizontal RT is found to either weaken or strengthen spectral lines. Line depths and equivalent width may differ by up to 20% from the corresponding LTE value if 3D RT is applied. Residual intensity contrasts in LTE are found to be larger than those in 3D NLTE by up to a factor of two. When compared to 1D NLTE, we find that horizontal RT weakens the contrast by up to 30% almost independently of the angle of line of sight. While the center-to-limb variation (CLV) of the 1D and 3D NLTE contrasts have a similar form, the LTE contrast CLV shows a different run. Determination of temperatures by 1D NLTE inversions of spatially resolved observations may produce errors of up to 200 K if one neglects 3D RT. We find a linear correlation between the intensity difference of 1D and 3D NLTE and a simple estimate of the temperature in the horizontal environment of the line formation region. This correlation could be used to coarsely correct inversions done in 1D NLTE for some of the effects of horizontal RT. Horizontal RT is less important if one considers spatially averaged line profiles because local line strengthening and weakening occur with similar frequency in our HD atmosphere. Thus, the iron abundance is underestimated by 0.012 dex if calculated using 1D NLTE RT. Since effects of horizontal RT are greatest for spatially resolved quantities, the use of 3D RT is particularly important for interpreting high spatial resolution observations.