The Hidden Life of Massive Stars
Beneath the familiar Orion Nebula lies an active stellar nursery. The outflow pictured here accompanied the birth of a massive young star. This false-colour image was taken by the NICMOS infrared camera aboard the Hubble Space Telescope. The blue represents light from shocked hydrogen molecules, while the gold shows continuum emission from stars and starlight reflected by dust.

Stars form from the gravitational collapse of hydrogen gas, buried deep inside molecular clouds spread through the interstellar medium of the Galaxy. Their formation is hidden from our direct view. Dust within the clouds absorbs visible starlight, and to detect the presence of young stars requires observations at infrared wavelengths, whose radiation can escape from the cloud’s interior.

Massive stars – stars with at least ten times the mass of the Sun – are responsible for the most spectacular events in the Galaxy. Their luminosities are thousands to millions of times greater than the Sun. The events of their formation produce a spectacular show, as the stand-out performers in clusters of stars containing hundreds to thousands of members. Paradoxically, while their birth can be seen right across the Galaxy, it is poorly understood in comparison with low-mass star formation, which can only be seen within a few hundred light years of the Sun. The short timescale for massive star formation, together with the many competing phenomena at work, and the crowded regions in which it occurs, makes it hard to disentangle cause and effect, and thus to determine the stages the stars pass through as they are born.

The star formation group within the School has been engaged in a project to define an evolutionary sequence of phases through which massive stars pass as they form. Our work started with examination of compact regions of ionized gas that form around the young stars, soon after they switch on. Signposted by the emission from methanol masers, it appears that massive protostars may mark their onset inside a ‘hot molecular core’–a particularly dense and compact molecular cloud rich in organic molecules. These have been created on dust grains, and released into the gas phase through infrared heating as a protostar first begins to collapse in the core of a cloud. Our research has shown that ‘isolated’ methanol masers invariably are associated with cold sources, emitting at sub-millimetre wavelengths (ie. 400 to 800µm). The question we are now asking ourselves is whether there is a chemical signature indicating how far star formation has proceeded, as the mantles of dust evaporate and gas-phase chemistry begins? If so, it may be uncovered through determination of the range of chemical species present, and their excitation state – two experiments we are now equipped to conduct through millimetre-wave astronomy with Mopra and with the Australia Telescope.

Michael Burton



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