2024ApJ...975..180L

M-type stars are the most abundant stars in our Galaxy. Their visible spectra are characterized by strong TiO and VO absorption bands with occasional ScO bands near 6000-6100 Å. This region corresponds to the A²Π-X²Σ⁺ band system and can be used to measure the abundance of ScO. In this work, a new Doppler-limited resolution spectrum of ScO obtained in a hollow-cathode experiment is presented. The A²Π-X²Σ⁺ electronic transition is analyzed with special care dedicated to fitting the ${{\rm{A}}}^{{\prime} 2}{\rm{\Delta }}(v+2)$ ∼ A²Π(v) perturbation. Spectroscopic constants of the A²Π(v) states are reported for v ≤ 9, and a partial fit of spectroscopic constants is made for the ${{\rm{A}}}^{{\prime} 2}{\rm{\Delta }}(v)$ states 2 ≤ v ≤ 6. The modeled interaction of the two states reproduces the experimental data remarkably well. New equilibrium constants are extracted from the fit. Band strengths are calculated with the help of a scaled transition dipole moment function, previously calculated with ab initio methods. Finally, a line list of the A²Π-X²Σ⁺ band system is provided, including Einstein A coefficients and oscillator strengths. Using this line list, the spectrum of ScO can be simulated for different temperature and pressure conditions in stellar atmospheres.