Two-dimensional hydrodynamical simulation of an expanding  H II region offset from the center of a Bonnor-Ebert sphere. From top to bottom rows: the logarithm of number density n, temperature T, and ionization fraction n_II/n of a slice through the yz-plane at the location of the star are shown. We trace the velocity of the gas in the three-dimensional simulation along the dashed line (see Fig. 4A). Each column corresponds to a snapshot of the simulation at 1–3 Myr. The vectors represent the velocity field in the middle row; the legend indicates the maximum velocity.