Upper three panels: the rotational lines of the three UCHIIRs in emission. The gray dashed line shows v_lsr. Red lines indicate the position of the hfs components of the – parity line of the doublet, while blue labels show the hfs of the + transition. (Labels are shifted for better visibility.) The dashed green line shows the τ = 0.1 hfs fit to the spectra. The lower sideband rest velocity is shifted according to the v_lsr of the individual sources and is indicated on the upper axis of each plot, while the rest velocity scale is the same for all sources. The lowest panel shows the transition toward NGC 7538 IRS1.