Results of the Whitworth et al. (1994) model for the conditions in Sh 2-217, using the equations given in the text. We assumed N_Lyc = 1.20 × 10⁴⁸ s^-1, that 70% of the ionizing photons are absorbed by dust, and that the present radius of the H ii region is 5 pc. The different panels show the variation of various physical quantities as a function of the initial density of the ambient neutral medium, n₀. a) Time. The curve labeled t_dyn shows the expansion age of Sh 2-217, needed to reach its actual radius. The other curves show t_frag, the time at which fragmentation starts. Three values of the sound speed are indicated. b) As a), but for the radius of Sh 2-217 at which fragmentation starts. The dashed line indicates the actual radius of Sh 2-217. c) As a), but for the masses of the fragments. An additional curve, for sound speed a_s = 0.5 km s^-1, is drawn in. The three dashed lines indicate the masses (derived from ¹³CO data) of the three “Regions” mapped in more detail (see Fig. 3b).