J vs. J − K diagram for the stars located at less than 10′ from the centre of the radio-continuum emission (red symbols). The main sequence for O stars (Martins & Plez 2006) and low-mass stars (Pecaut & Mamajek 2013) are drawn in orange and black curved lines, respectively, for a distance of 2.1 kpc and zeroreddening. The reddening vector corresponding to an O9.5 star is drawn as a slanting line using the extinction law of Rieke & Lebofsky (1985). The green and blue symbols represent stars located at less than 1 pc and 0.5 pc from the 24 μm centre, respectively. Possible exciting stars are numbered as #1, #2, and #3 in the figure. They are identified in Fig. C.4.