Testing the PoWR^HD model predictions against the transition mass-loss rate in the Arches Cluster. Left : predicted mass-loss rates versus stellar mass from PoWR^HD models for the T_* = 35 kK sequence (solid black) and from Vink et al. (2000) (dotted blue), compared to the transition mass-loss rate of log(M) ≈ −5.16 in the Arches (horizontal red line) from Vink & Gräfener (2012). The PoWR^HD models are computed with fixed log(L_*/L_⊙) = 6.0, X = 0.7, and Z = 0.02, roughly matching the observed properties of the Arches transition objects (Martins et al. 2008). The T_* = 35 kK sequence shown has surface temperatures close to the observed T_eff(τ_R = 2/3) ≈ 33.4 kK of the transition objects. The stellar masses of the transition objects span ~55—75 M_⊙ corresponding to their averaged luminosity and temperature. Middle : predicted wind efficiency parameter versus mass compared to the transition value η ≈ 0.6 (dashed red) from Vink & Gräfener (2012). Right : predicted wind optical depth versus mass compared to the transition value τ_F(r_sonic) ≈ 1 (dashed red) from Vink & Gräfener (2012).