NLTE effects of Ti~I in M dwarfs and giants

Abstract

We present detailed NLTE Ti~I calculations in model atmospheres of cool dwarf and giant stars. A fully self-consistent NLTE treatment for a Ti~I model atom with 395 levels and 5279 primary bound-bound transitions is included, and we discuss the implication of departures from LTE in this atom for the strengths of Ti~I lines and TiO molecular bands in cool star spectra. We show that in the atmospheric parameter range investigated, LTE is a poor approximation to Ti~I line formation, as expected from the low collisional rates in cool stars. The secondary effects of Ti~I overionization on the TiO number density and the TiO molecular opacities, however, are found to be negligible in the molecular line forming region for the relatively small parameter range studied in this paper.