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Fig. 3

image

Ratio of chromospheric Ca II H and K flux to bolometric flux, , vs. BV 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 BV for stars in the catalogue.

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