First detection of the TiO i1Pi-a1Delta system in stellar spectra and its laboratory characterization

TiO plays an important role in determining the atmospheric structure of M-type stars and in shaping the visual part of their spectra. We compute synthetic spectra for late-type M giants and identify systematic discrepancies in the wavelength range 5810-5850 A. To investigate the origin of these discrepancies, we analyse experimental TiO absorption cross-sections. We report the detection of a molecular band of the singlet system $^{1}Π$-a$^{1}Δ$ of the TiO, with an R-head located at 5814.8 A, overlapping the 1-3 band of the C$^3Δ$-X$^3Δ$ system. The lower state of the band is identified as the a$^1Δ$ v"=0 state, while the upper state is most likely the $^1Π$ in its ground vibrational state. The empirical band intensity is derived by comparing the relative strengths of neighbouring bands from the C$^3Δ$-X$^3Δ$ and B$^3Π$-X$^3Δ$ systems in the experimental cross-section. The band intensity is further validated by synthetic spectrum calculations for the late-type giant 30 Her (M6 III) and comparison with its observed spectrum from the MELCHIORS library. The newly identified band is sufficiently strong to affect the flux distribution in the spectra of cool stars.