A technique for determining spot filling factors and temperatures from molecular band modelling (MBM) was suggested by Huenemoerder and Ramsey (1987) and further developed by Neff et al. (1995) and O’Neal et al. (1996). They modelled the observed spectrum by combining spectra of suitable standard stars of different effective temperatures weighted by spot filling factor and continuum surface flux ratio. For instance, observations in two TiO bands of different temperature sensitivity, at 7055 Å and 8860 Å, were combined to estimate the area and the temperature of spots.
Since TiO lines are only formed in starspots on the surfaces of G–K giants and subgiants, polarisation observations in these lines can provide measurements of magnetic fields directly in spatially unresolved spots. As shown in Figure 5, the TiO lines at 7055 Å are rather strongly magnetically sensitive, having effective Landé factors up to 1 (Berdyugina and Solanki, 2002; Berdyugina et al., 2003). The wavelength separation between rotational lines in the band is small and lines of low rotational numbers (larger splitting) almost coincide with those of high numbers (smaller splitting) in the band head. Nonetheless, a clear Stokes signal should be measured from starspots Berdyugina (2002).
An excess of the absorption in the infrared OH lines at 1.5 µmdue to starspots has been detected by O’Neal and Neff (1997) and O’Neal et al. (2001). Since OH lines can be observed at higher temperature than TiO, starting from 5000 K and below, they can be used for studying starspots on hotter stars. Otherwise, they are contaminated by the photospheric contribution, which should be carefully taken into account. On the other hand, OH lines provide a greater temperature range over which starspots can be detected through molecular absorption features.
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