NGC 3503 and its molecular environment
  We present a study of the molecular gas and interstellar dust distribution in
the environs of the HII 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.
  We based our study in ^{12}CO(1-0) observations of a region of ~0.6 deg. 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size obtained with the 4-m NANTEN telescope, unpublished radio continuum data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.
  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 +/- 0.4 kpc
the total molecular mass and density yield (7.6 +/- 2.1) x 10^3 Msun and 400
+/- 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 HII 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 HII region that probably has undergone a
champagne phase.
