The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun Edited by G. A. Feiden TiO Band Analysis of Cool Stars Using Model Atmospheres H. V. Şenavcı1, D. O’Neal2, D. Montes3, B. Plez4, F. Allard5, E. Bahar1 1 Ankara University, Faculty of Science, Department of Astronomy and Space Sciences, TR-06100 Tandoğan-Ankara, Turkey 2 Keystone College, School of Arts and Sciences, La Plume, PA, 18440, USA 3 Dpto. Astroﬁsica, Facultad de CC. Fisicas, Universidad Complutense de Madrid, Spain 4 LUPM, Université de Montpellier, CNRS, Montpellier, France 5 Univ Lyon, ENS de Lyon, Univ Lyon 1, Centre de Recherche Astrophysique de Lyon UMR5574, F-69007, Lyon, France Abstract We present the preliminary results of Titanium Oxide (TiO) band analysis using model atmospheres, in order to accurately determine the spot ﬁlling factor and the spot temperature of magnetically active stars. This study includes the basic steps of TiO band analysis code together with the robustness tests using MARCS and PHOENIX Models, which also allows us to perform a comparison between model atmospheres and search for the most convenient one for TiO band analysis of active stars. 1 Introduction Investigating the intensity variation of molecular lines such as Titanium Oxide (TiO) allows us to obtain spotted regions that cannot be dimensionally resolved by existing techniques (Berdyugina, 2002) and provides more sensitive way to determine the spot temperatures (O’Neal et al., 1996). The TiO band analysis technique, which has been extensively used by O’Neal and collaborators, is simply based on the de- termination of spot ﬁlling factors (fs) and spot temperatures (Ts) by ﬁtting the depths of the TiO bands near 7055 Å and 8860 Å with the help of the observed non-active standards called "proxy" stars (see O’Neal et al. 1996 & 2004 for details). Taking the advantage of "cool" atmosphere models, as well as to prevent instrumental and reduction related side-eﬀects of proxies, here we present the TiO band analysis for dwarfs un- der model atmospheres approximation to generate synthetic spectra. 2 Selection of Models TiO bands are visible in the stellar spectra, of which the eﬀective temperatures are below 4000 K. Taking into ac- count that sensitivity together with the main principle of the technique based on two-temperature models, we choose our model temperatures in order to represent the "quiet" and "spotted" photosphere as 4000 K < Teff < 6000 K (G-K dwarf spectra) and 2700 K < Teff < 3900 K (M dwarf spectra), re- spectively. Though we only present here the TiO band anal- ysis using dwarf synthetic spectra, it is critical to accurately determine the surface gravity (logg) values especially in the case of giants, for the selection of convenient model atmo- spheres. In this context, we use the PASTEL Catalogue data by Soubiran et al. (2010), for the determination of logg values as can be seen in Fig. 1. We previously performed TiO band analysis of SB1 type RS CVn binary II Peg using ATLAS9, in which the sub-giant models converge for temperatures between 3000 K - 4000 K (see Senavci et al. 2015 for details). However, we couldn’t ob- tain converged dwarf models as well as the giants for the same temperature range with ATLAS9. We therefore use MARCS (Gustafsson et al., 2008) and latest PHOENIX (Allard et al. 2012a & 2012b) Models that can able to generate cool enough synthetic spectra convenient for TiO band analysis purposes. Those models also enable us to extend the temper- ature range down to 2700 K. 3000 4000 5000 6000 7000 Teff 0 1 2 3 4 5 6 logg dwarfs subgiants giants Figure 1: Teff - logg diagram generated using the PASTEL Catalogue data. As mentioned before, we use dwarf models of Solar com- position with the convenient selection of Teff and logg val- ues described above. In this context, the synthetic spectra using MARCS Models are obtained via the Turbospectrum code (Alvarez & Plez, 1998), together with the help of the linelist from VALD database (Kupka