Relation between the ratio of the surface flux of the Ca ii IRT line to the stellar bolometric luminosity, |$R^{\prime }_{\rm IRT}$|⁠, and the Rossby number, N_R. The circle symbols represent PMS stars in molecular clouds, and the square symbols indicate PMS stars in moving groups. The filled symbols represent PMS stars with narrow Ca ii IRT emission lines, and the open symbols show PMS stars with broad Ca ii IRT emission lines. The cross symbols represent low-mass stars in the young open clusters IC 2391 and IC 2602, as studied in Marsden, Carter, and Donati (2009).