4.5 Line depth ratios

Spots on slowly rotating stars can be studied by photometric and spectroscopic molecular line techniques. To discriminate between spot temperature and spot area, these techniques use at least two independent measurements, brightness and colour in photometry and different molecular bands in spectroscopy. The Line Depth Ratio (LDR) technique offers an opportunity to resolve temperature variations with unprecedented accuracy of less than 10 K (Gray, 1996). A technique for studying starspots with LDR was developed by Catalano et al. (2002Jump To The Next Citation Point). It is based on the analysis of depth ratios of a pair of lines, one insensitive and one very sensitive to temperature. Due to presence of a dark spot that produces a decrease of say 10% in the continuum, the depth of both lines would be equally affected and reduced by 10%, while their depth ratio would remain unchanged. Due to the lower temperature of the spot, however, the temperature sensitive line will change its intrinsic depth and, thus, the depth ratio of the two lines will change as well. The amount of depth ratio variation depends on the temperature variation and the fraction of surface covered by spots. Catalano et al. (2002Jump To The Next Citation Point) proposed 10 pairs of lines in the 6100 – 6200 Å wavelength range and calibrated them against effective temperatures of main-sequence stars and giants. Their results for active stars are discussed in Section 5.1. Frasca et al. (2005) combined the LDR technique with the analysis of simultaneous light curves. This technique enables disentangling the spot temperature and area.
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