The Earliest Stages of Massive Star Formation
(Thompson, Chrysostomou)
Mark Thompson is the Principal Investigator of the SCUBA-2 "All-Sky" Survey (SASSy) and Survey Manager of the JCMT Plane Survey (JPS).
Massive stars are a powerful force in our Galaxy and others, acting to disrupt giant molecular clouds and trigger new generations of star formation before exploding as supernovae. Once massive stars have formed they are highly visible but their earliest stages are still shrouded in secrecy as their birthplaces are hidden deep within the densest regions of dark molecular clouds. One of the best signposts of a massive young stellar object (also known as a YSO) is the presence of an ultra-compact HII region, which is a small dense photo-ionised bubble of gas surrounding the newly formed star. Whilst ultra-compact HII regions can easily be seen out to the far edge of our Galaxy (and even nearby galaxies) by radio telescopes such as the VLA, they only tell us about massive stars that are at least 100,000 years old and not about the earliest stages of their formation.
The schematic above illustrates some of the known stages in the evolution of a massive stellar cluster. The earliest stages on the far left are still shrouded in secrecy. The middle panel shows a potential young massive stellar cluster in formation from a young ultra-compact HII region and hot molecular core (Garay et al 1994). The rightmost panel shows the later stages where the strong radiation from the massive stars disrupts and disperses the surrounding molecular cloud to reveal the cluster to visible telescopes.
The difficulty in finding earlier stages than that of the massive YSO is compounded by a number of problems: massive stars and YSOs are rare in the Galaxy; they typically lie at large distances from the Sun and form in clusters; they are cold and dark (emitting no visible radiation); and most crucially they evolve very quickly (in astronomical terms). So in order to find these rare early stages in massive star formation we need large dedicated search programmes. Plus, in order to locate these cold, dark regions we must use the far-infrared and sub-millimetre regions of the spectrum (at which they emit their most energy). At Hertfordshire we play a leading role in the next generation of Milky Way surveys designed to achieve these goals (the SCUBA-2 SASSy and JPS surveys) and the Herschel Hi-GAL survey. By combining SCUBA-2 and Herschel we will identify the cold dark regions within molecular clouds that are the future reservoir for massive star formation.
In addition we also play a leading role within radio wavelength surveys (MeerGAL and EMU) aimed at identifying the immediate precursor of ultracompact HII regions, the so-called hypercompact HII region. These objects represent the stage at which a massive star is first beginning to ionise its environment and allow us to probe the birth of HII regions and the start of the massive star's interaction with its environment. By taking an inherently multi-wavelength approach to massive star formation we aim to reveal the full picture of the process from the development of molecular cloud cores to the time at which the stars begin to shine in the ultraviolet.