Maps of the spectral index between the continuum emission at 610 MHz and 1400 MHz (left panel) and its uncertainty (right panel) of W33 Main and G12.81−0.22, overlaid with contours of the radio emission at 1400 MHz at full resolution. The contour levels are as described in Fig. 1 (left panel). The blue-filled ellipse shows the corresponding beam size. The blue cross corresponds to peak emission toward W33 Main and G12.81−0.22 at 1400 MHz.