Examples of fitting the spectral index to the fluxes from VLA (8 mm–6.4 cm) and ALMA observations (1 mm). Left: Per-emb-5 has an intra-band Ka-band spectral index in agreement with the spectral index between 1 and 9 mm wavelengths. Center: for Per-emb-19 Ka-band spectral index is too low to be explained only by dust emission, some free–free contamination cannot be excluded. Right: in Per-emb-20, negative spectral index at Ka-band shows that emission is heavily contaminated by other effects, in such cases the Ka-band flux is treated as an upper limit of the dust emission.