Ratio of chromospheric Ca II H and K flux to bolometric flux, , vs. B − V of 4454 main-sequence stars. The symbols and colour palette are the same as in Fig. 2. The Sun at minimum activity is shown by the black ⊙ symbol. The dashed grey lines indicate the Vaughan-Preston gap, adopted from Fig. 2 of Noyes et al. (1984a). The histogram on the right shows the distribution of chromospheric activity, and the histogram at the top shows the distribution of B − V for stars in the catalogue.
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