Optically thin [Oi]63 μm/[Oi]145 μm flux ratios as a function of temperature are shown in dotted lines for collisions with atomic hydrogen H and in solid lines for collisions with molecular hydrogen H₂ (as in Liseau et al. 2006). The logarithms of the density (in cm^-3) are indicated for each curve. The broken line outlines the ratio of optically thick lines. Observed line ratios are depicted by the shaded areas for the L1448-B2 position and the L1157-R position. The data have been smoothed to a common angular resolution of 12.′′6.