Colour-magnitude diagram of NGC 330 with Hα emitters marked in red (left). Isochrone fits with ages 30 and 36 Myr are plotted in purple and green, respectively. Both isochrones have an initial rotation of v_rot/v_crit = 0.6, a distance modulus of 18.8 mag, and a reddening of E(B–V) = 0.1 mag. Be fraction as a function of fractional turn-off mass as measured by the 30 Myr isochrone (in purple) and the 36 Myr isochrone (in green; right). Dashed lines show theoretical upper limit given by Eq. (30) with α = −1.9, ΔM/M_2, i = 0, and κ = −1.0, 0, as given by the legend.