Abstract
Context
. Titanium monoxide plays an important role in determining the atmospheric structure of M-type stars and in shaping the visible part of their spectra. The precision of synthetic spectra when confronted with high-resolution observations depends on the completeness of the included transitions and on the accuracy of line positions and line intensities.
Aims
. We aim to systematically assess the quality of synthetic spectra computed with up-to-date TiO line lists by comparing them with high-resolution, high signal-to-noise spectra of cool stars calibrated in absolute or relative flux.
Methods
. We computed synthetic spectra for late-type M giants and identified systematic discrepancies in the wavelength range 5810–5850 A. To investigate the origin of these discrepancies, we analyzed experimental TiO absorption cross sections.
Results
. We report the detection of a molecular band of the singlet system
1
Π–a
1
Δ of the TiO, with a R-head located at 5814.8 Å, overlapping the 1–3 band of the α (C
3
Δ–Χ
3
Δ) system. The lower state of the band is identified as the a
1
Δ v″=0 state, while the upper state is most likely the
1
Π in its ground vibrational state. The empirical band intensity is derived by comparing the relative strengths of neighboring bands from the C
3
Δ–Χ
3
Δ and B
3
Π–Χ
3
Δ 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.
Conclusions
. The i
1
Π (optionally 2
1
Π) electronic state has been predicted by theoretical calculations. While energies of two vibrationally excited states were previously inferred from de-perturbation analyses of the
α
system, accurate spectroscopic constants for this electronic state were lacking. The newly identified band is sufficiently strong to affect the flux distribution in the spectra of cool stars.