L_X∕Ṁ for Taurusas a function of age (data from Güdel et al. 2007). Although a least-square fit (dashed line) shows a systematic increase with age, scatter is evident, in part owing to the diversity of stellar masses. Overplotted is the curve (red, continuous) corresponding to one solar mass (Telleschi et al. 2007; see beginning of Sect. 4.2). For reference, the normalizations used in Eq. (22) amount to L_X ∕Ṁ = 10^7.7 J kg⁻¹.