The Orion Nebula, the closest massive star forming region to Earth (1,500 light years). (photo Jason Ware)	The 1.2mm continuum map of a massive star formation region - G15.03-0.67. The map, taken using the SIMBA instrument on the SEST, traces the earliest stages in the formation of massive stars. The “box” in the image represents the position of the UC HII region, and the “plus” symbol the methanol maser.

Because of the nature of massive star formation (i.e., rare, rapid, clustered, and at large distances) the study of these regions – in particular the earliest stages in their evolution, which are not easily distinguished – remains a difficult task.

Much work has been done recently in trying to pin-point the evolutionary sequence of massive stars. It is now thought that they undergo a process of evolution similar to low mass stars, producing distinct indicators of their existence at periodic intervals throughout their evolution. Two of these tracers of massive star formation include the methanol maser and the Ultra Compact HII region (aka UC HII).

The methanol maser, detectable at 6.7 & 12.2 GHz, is thought to be the first detectable tracer of massive star formation. The radio-bright UC HII region is thought to evolve from the methanol maser. They are produced as the high-mass star increases in mass and luminosity, producing a large number of high-energy UV photons. These photons ionise the surrounding molecular cloud, producing the UC HII region.

By searching for these identifiers of massive star formation it is possible to trace the evolutionary stages of massive stars, provided of course that we know the order in which these tracers appear. This order can be determined by comparing the temperature of these objects, and from this inferring an age.

Complementary to this, we have detected objects in millimetre-wave continuum emission, devoid of these traditional tracers – possibly suggesting a younger age, or that methanol maser emission is directional.

The purpose of my PhD research is to improve our understanding of massive stars, in particular the earliest stages of their evolution. An understanding of massive star formation will allow an insight into the mechanisms involved in the formation of the Galaxy, which is shaped by the presence of these massive stars (e.g. jets, outflows, supernovae winds). Studying these objects may also aid understanding of the first generation of stars formed after the Big Bang, which are thought to have been very massive.

Tracey Hill