Stellar population astrophysics (SPA) with the TNG: Identification of a sulphur line at λ_air = 1063.6 nm in GIANO-B stellar spectra

Context. In the advent of new infrared high-resolution spectrometers, accurate and precise atomic data in the infrared are urgently needed. Identifications, wavelengths, strengths, broadening, and hyper-fine splitting parameters of stellar lines in the near-infrared are in many cases not accurate enough to model observed spectra, and in other cases, these parameters do not even exist. Some stellar features are unidentified. Aims. The aim with this work is to identify a spectral feature at λ_vac = 1063.891 nm or λ_air = 1063.600 nm that is visible in spectra of stars of different spectral types that are observed with the GIANO-B spectrometer. Methods. The search for spectral lines to match the unidentified feature in line lists from standard atomic databases was not successful. However, by investigating the original published laboratory data, we were able to identify the feature and solve the problem. To confirm its identification, we modelled the presumed stellar line in the solar intensity spectrum and found an excellent match. Results. We find that the observed spectral feature is a stellar line originating from the 4s′-4p′ transition in S I, and that the reason for its absence in atomic line databases is a neglected air-to-vacuum correction in the original laboratory measurements from 1967 for this line only. From interpolation we determine the laboratory wavelength of the S I line to be λ_vac = 1063.8908 nm or λ_air = 1063.5993 nm, and the excitation energy of the upper level to be 9.74978 eV.