Distribution of the spectral slopes of the flux ratios. The white bars represent the whole sample, red bars represent nominal, broad band detections, blue bars represent targets with excess only in the long wavelength spectral channel, and orange bars represent stars with negative (< − 3σ) excess in one or more channels. White bars can be hidden by colored bars, while colored bars are stacked; i.e., they cannot hide each other. The three targets with significantly positive slopes are HD 172555, HD 182572, and HD 210049. The star with a/σ_a ~ 0 but with excess only in the long wavelength channel is HD 45184. These cases are discussed in Sect. 4.3.2.