closer to the excitation sources than the  emission, while the strong  emission is located slightly to the south of the  emission. The  and the radio continuum emissions are closely coincident. The emission distribution at different wavelengths indicates the existence of an excitation gradient, with the high excitation regions closer to the massive stars in Pis24 than the low excitation ones. The emission at 8 $\mu$m reveals a filament lying $\approx$12 south of the radio emission, indicating that a PDR has developed at the surface of Shell A. Emission at 3.6 and 4.5 $\mu$m is also present in these regions (Fig. 9), with the emission at 4.5 $\mu$m almost coincident with the  emission. The location of the  emission farther away from the excitation sources than the  emission is typical in PDRs. A similar stratified ionization structure is present at the interface between the ionized gas and the molecular material in the pillars of M16, where PDRs have developed [@hester96].

East of 1725, the shell displays strong  emission encircling the H$\alpha$ emitting region. No OB stars (see @massey01) are known to be projected onto this section of the cavity. Inspection of table 5 by @wang07 resulted in the identification of a dozen 