Formation of chromospheric Fe I lines in the near-ultraviolet in 1D atmospheres

In the near-ultraviolet part of the solar spectrum lie several Fe I lines with very broad profiles that are typical of chromospheric lines. These lines are largely unexplored because high-resolution data in this region were lacking. This changed with the successful flight in 2024, when spectropolarimetric data were recorded with high spatial, spectral, and temporal resolution covering a large variety of solar targets. The aim of this work is to investigate the formation of the lines and lay the groundwork for further studies. We computed the spectrum of the lines at 358.12 nm, 371.99 nm, 406.36 nm, and 407.17 nm emerging from the standard 1D FAL-atmospheres using the non-local thermodynamic equilibrium radiative transfer code RH. The lines are affected by overionization in the wings, but their line cores are strongly affected by scattering. The line cores form well into the chromosphere in the tested atmosphere models, except in the colder FALX model, where the line core forms at the temperature minimum (which lies at traditional chromospheric heights). In the presence of a vertical magnetic field, the Stokes V signal is multilobed. The profile can be decomposed into two broad photospheric lobes and two sharper lobes that form in the flanks of the chromospheric line core. We investigated the properties of four lines of Fe I that form in the lower chromosphere. The results provide a basic understanding of the formation of the lines, which will be useful for later analyses of the formation in 3D magnetohydrodynamic simulations and for an eventual investigation of their diagnostic potential.