stars projected onto the cavity. Based on 2MASS colour-colour and colour-magnitude diagrams, these stars are A-type or later.

In summary, the southern, eastern, and western edges of G353.12+0.86 show a clear stratification, with the high excitation ionized gas closer to the massive stars in Pis24 that the low excitation ionized gas and the PAHs, pinpointing the last ones the interface between the ionized and molecular gas.

The emission distribution in all bands displayed in Figs. 1, 5 and 7 reveal that the shell is an interstellar bubble blown by the massive stars in the cluster Pis24. The interstellar gas has been swept-up and compressed onto the molecular wall by the stellar winds of the massive stars in the stellar cluster. The emission distribution resembles some of the IR bubbles described by @watson08 [@watson09].

The physical parameters of the ionized gas were estimated from the image at 1.46 GHz. These values were derived from the expressions by @mezger67. The ionized mass was multiplied by 1.27 to take into account the contribution of He singly ionized (10% He abundance). We adopted a distance of 2.5 kpc and an electron temperature of 8000 $\pm$ 1000 K, in agreement with estimates from radio recombination 