Fig. 4.
pEW′ of chromospheric lines as a function of spectral type. 
is the sum of the Na D1 and Na D2 lines. The dashed line in the left middle panel marks our activity threshold of 
Å. Ca IRT-b and -c are similar to Ca IRT-a and therefore not shown. The reference star for each spectral subtype up to M6.0 has pEW′ = 0 Å for all lines by definition.
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