Example of optically thin thermal jet emission: H₂O maser site G090.21+2.32 (Sect. 4.2). Labels and symbols as in Fig. 1. Panelb: we divided the C and K bands in twosub-bands to show that the spectral index within each band is consistent with the spectral index interpolated to the Ku-band frequency. The SED shows a region of optically thin (resolved) emission below 15 GHz and a region of optically thick (compact) emission above 15 GHz. The spectral index map of panelc was computed within the (linear) range of optically thin emission (C and Ku bands only).