EFFECTS OF TIO IN STELLAR ATMOSPHERES

We have computed the frequencies, the gf values, and the excitation en ergy of the lower level for 12 million lines from 5 isotopes of titani um oxide. The data are collected onto a tape which is offered to the c ommunity. By treating the lines in the opacity sampling (OS) approxima tion, we have computed model atmospheres for M type giant stars. We an alyze the effects, on the structure of these models, of TiO in general as well as of various parts of the computed TiO opacity. Our computat ion is an extension of, and based on, the thesis work by Collins (1975 a). The extension includes evaluation of the opacity due to all releva nt isotopes of TiO and due to the epsilon system. Line frequencies and excitation energies of lines in the epsilon-system as well as the f-v alue were computed based on recently determined values from laboratory experiments. We find that the epsilon-system contributes a significan t fraction of the heating caused by TiO in stellar atmospheres, wherea s a detailed treatment of all the isotopic systems were found to be of less importance for the stellar structure. On the other hand, the iso topes affect the synthetic spectra, and their main importance is for s uch calculations. We find that TiO heats the surface layers of a T(eff ) = 3500 K model by almost-equal-to 500 K, a 3100 K model with almost- equal-to 300 K, and a 2800 K model with almost-equal-to 150 K. The amo unt of heating is relatively insensitive to the gravity. The decreasin g effect of TiO for the coolest stars reflects the increase of the par tial pressure of water vapor relative to TiO.