Context :

Aims : We present a study of the molecular gas and interstellar dust distribution in the environs of the H ii region NGC 3503 associated with the open cluster Pis 17 with the aim of investigating the spatial distribution of the molecular gas linked to the nebula and achieving a better understanding of the interaction of the nebula and Pis 17 with their molecular environment .

Methods : We based our study in ^ { 12 } CO ( 1-0 ) observations of a region of \sim 0 \aas@@fstack { \circ } 6 in size obtained with the 4-m NANTEN telescope , unpublished radio continuum data at 4800 and 8640 MHz obtained with the ATCA telescope , radio continuum data at 843 MHz obtained from SUMSS , and available IRAS , MSX , IRAC-GLIMPSE , and MIPSGAL images .

Results : We found a molecular cloud ( Component 1 ) having a mean velocity of –24.7 km s ^ { -1 } , compatible with the velocity of the ionized gas , which is associated with the nebula and its surroundings .
Adopting a distance of 2.9 \pm 0.4 kpc the total molecular mass and density yield ( 7.6 \pm 2.1 ) \times 10 ^ { 3 } M _ { \odot } and 400 \pm 240 cm ^ { -3 } , respectively .

The radio continuum data confirm the existence of an electron density gradient in NGC 3503 .
The IR emission shows the presence of a PDR bordering the higher density regions of the nebula .
The spatial distribution of the CO emission shows that the nebula coincides with a molecular clump , with the strongest CO emission peak located close to the higher electron density region .
The more negative velocities of the molecular gas ( about –27 km s ^ { -1 } ) , is coincident with NGC 3503 .
Candidate YSOs were detected towards the H ii region , suggesting that embedded star formation may be occurring in the neighbourhood of the nebula .
The presence of a clear electron density gradient , along with the spatial distribution of the molecular gas and PAHs in the region indicates that NGC 3503 is a blister-type H ii region that probably has undergone a champagne phase .

Conclusions :