Two examples of local S curves – effective temperature vs. surface density – in a He disc when irradiation is included. The upper blue set of curves shows the case T_irr = 25 000 K and r = 5 × 10⁸ cm, the lower red set of curves corresponds to T_irr = 5000 K and r = 10¹⁰ cm. In both cases, the solid curve corresponds to α given by Eq. (4), while the dotted curves are for α = 0.05 and α = 0.2 (rightmost and leftmost curves respectively). The primary mass is 1.4 M_⊙.