The A²Π–X²Σ⁺ Band System of YO

YO bands are conspicuous in the spectra of S stars. The laboratory spectrum of the A 2 Π–X 2 Σ + electronic transition has been recorded with a Fourier transform spectrometer using a composite-wall hollow cathode lamp source. The A 2 Π–X 2 Σ + transition with v ′ ≤ 4 and v ″ ≤ 4 has been rotationally analyzed using the modern spectral fitting program PGOPHER. Vibronic band strengths were calculated using an ab initio transition dipole moment function. A line list for the A 2 Π–X 2 Σ + transition is provided and can be utilized in the modeling of S stars.