THE TEMPERATURE SCALE OF METAL-RICH M GIANTS BASED ON TiO BANDS: POPULATION SYNTHESIS IN THE NEAR INFRARED

We have computed a grid of high resolution synthetic spectra for cool stars (2500 < Teff < 6000 K) in the wavelength range λλ 6000 – 10200 Å, by employing an updated line list of atomic and molecular lines, together with state-of-the-art model atmospheres. As a by-product, by fitting TiO bandheads in spectra of well-known M giants, we have derived the electronic oscillator strengths of the TiO γ’, δ, ǫ and φ systems. The derived oscillator strenghts for the γ, ǫ and φ systems differ from the laboratory and ab initio values found in the literature, but are consistent with the model atmospheres and line lists employed, resulting in a good match to the observed spectra of M giants of known parameters. The behavior of TiO bands as a function of the stellar parameters Teff , log g and [Fe/H] is presented and the use of TiO spectral indices in stellar population studies is discussed. Subject headings: Atomic and molecular data, Stars: atmospheres, M giants, globular clusters