Location of the radio continuum emission (gray scale) overlaid on the H₂CO observation lines (black contour lines) and H110α emission lines (dash contour lines) toward a) MON R2, b) S156, c) DR17/L906, and d) M17/M18. H₂CO contour levels are the same as Fig. 1 for four regions. For the DR17/L906 region: H110α contour levels are 0.67, 0.89, 1.11, 1.33, 1.56, and 1.78 K km s^-1; for the M17/M18 region: H110α contour levels are 2.91, 4.36, 5.81, 7.26, 8.72, 10.17, and 11.62 K km s^-1. The gray bars are given in units of K.