Gum 48d: an evolved HII region with ongoing star formation
  High mass star formation and the evolution of HII regions have a substantial
impact on the morphology and star formation history of molecular clouds. The
HII region Gum 48d, located in the Centaurus Arm at a distance of 3.5 kpc, is
an old, well evolved HII region whose ionizing stars have moved off the main
sequence. As such, it represents a phase in the evolution of HII regions that
is less well studied than the earlier, more energetic, main sequence phase. In
this paper we use multi-wavelength archive data from a variety of sources to
perform a detailed study of this interesting region. Morphologically, Gum 48d
displays a ring-like faint HII region associated with diffuse emission from the
associated PDR, and is formed from part of a large, massive molecular cloud
complex. There is extensive ongoing star formation in the region, at scales
ranging from low to high mass, which is consistent with triggered star
formation scenarios. We investigate the dynamical history and evolution of this
region, and conclude that the original HII region was once larger and more
energetic than the faint region currently seen. The proposed history of this
molecular cloud complex is one of multiple, linked generations of star
formation, over a period of 10 Myr. Gum 48d differs significantly in morphology
and star formation that the other HII regions in the molecular cloud; these
differences are likely the result of the advanced age of the region, and its
different evolutionary status.
