2025ApJ...994...60B

The zirconium oxide (ZrO) b³Π-a³Δ system is a prominent source of opacity in the atmospheres of S-type stars in the near-infrared region. The emission spectra of ZrO, obtained at the National Solar Observatory located at Kitt Peak in Arizona, have been utilized to analyze this band system. A reanalysis of the 0-0 vibrational band of the b³Π-a³Δ transition has been carried out. For the first time, the vibrational bands with $1\,\leqslant \,v^{\prime} \,\leqslant \,3$ and v″ ≤ 4 for the same electronic transition have been rotationally analyzed using PGOPHER. Additionally, the 0-0 and 1-1 bands of the spin-forbidden triplet-singlet transition (b³Π-X¹Σ⁺) have also been identified and fitted. This new analysis revealed that the “e” parity levels are higher in energy than the “f” parity levels of the b³Π state, in contrast to previous reports. Furthermore, this study allowed for the precise determination of T₀ for the a³Δ state, which is found to be 1383.49144(98) cm^–1. The observed R-heads for the Δv = 0, ±1 vibrational bands have been reported. Spectroscopic constants for the b³Π state for v$^{\prime} \,\leqslant \,3$ have been determined and utilized to derive equilibrium constants. These parameters, in conjunction with ab initio calculations of the transition dipole moment function, have been used to compute band strengths. Furthermore, a line list consisting of line positions, Einstein A values, and oscillator strengths has been generated, which can be used for the spectral modeling of S-type stars.