v_sys+wind and v_sys of the Fe II lines detected in atmosphere of KELT-9b. Top panel: fitted vsys+wind for each of the detected lines. The red dots represent the results of D’Arpa et al. (2024). Middle panel: fitted vsys for each of the detected lines. The red dashed horizontal lines indicate the mean value of the v_sys = −21.61 ± 0.77 km s⁻¹, while the grey horizontal dashed lines indicate literature v_sys values: −17.74 ± 0.11 km s⁻¹ (Hoeijmakers et al. 2019), −19.819 ± 0.024 km s⁻¹ (Borsa et al. 2019), and −20.22 ± 0.49 km s⁻¹ (Gaia). Bottom panel: v_wind versus amplitude plot, where green points represent the values calculated by correcting the fitted v_sys+wind by the mean v_sys and the black points represent the values corrected by v_sys fitted for each of the lines separately.