The Search for the Earliest Stages of Massive Star Formation
(Thompson)
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). Historically these regions of the electromagnetic spectrum have been the most poorly exploited, due to the technological challenges of building the necessary instrumentation. However the recent development of novel sub-mm cameras and the imminent launch of the Herschel Space Observatory means that we can finally explore the Galaxy and locate the birthplaces of massive stars.
The Star Formation Group at Hertfordshire is playing a leading role in a number of large international survey programmes using the James Clerk Maxwell Telescope (JCMT) and the Herschel Space Observatory. These programmes are designed to locate and study the properties of the birth-places of massive stellar clusters (known as preclusters). There are a number of PhD and MSc opportunities to become involved in this research, with tasks ranging from survey design and planning, observational sup-port, advanced databasing and pipeline development, science exploitation and computational modelling.
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SCAMPS is an ongoing project to search for the earliest phases of massive star formation using the SCUBA sub-millimetre bolometer camera on the JCMT. During some of our earlier observations we noticed that a number of well-known ultra-compact HII regions had sub-millimetre companions that were not seen in the mid-infrared. These companions are strong candidates for the cold, dark precluster phase of massive star formation. From follow-up wide-area mapping of these regions we have identified ~20 candidate preclusters and work is now underway to investigate their nature using a wide variety of telescopes. | |
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The JCMT Legacy Surveys are large international projects that will use the new wide-field SCUBA-2 sub-mm camera on the JCMT to leave a lasting legacy to astronomy. At Hertfordshire we are playing a leading role in two Legacy Surveys that are specifically designed to reveal the earliest stages of massive star formation: SASSy (the SCUBA-2 "All"-Sky Survey) and JPS (The JCMT Galactic Plane Survey). Both surveys will revolution-ise our understanding of the massive star formation in our Galaxy by ex-tending the work that was begun with SCAMPS to the entire plane of our Galaxy (~2500 square degrees in the case of SASSy). The surveys will be-gin in January 2007. | |
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Hi-GAL is a proposed survey of the Galactic Plane to be carried out with the Herschel Space Observatory. Herschel will be the biggest far-infrared space observatory ever flown (with a 3.5 m mirror) and is set to launch in late 2007. The Herschel science community is currently choosing a series of Key Observing Projects that will be carried out in the first year of Herschel operations. Hertfordshire is playing a key role in the design of the Hi-GAL survey, which will observe the entire Galactic Plane in 5 infrared band from 70 to 500 µm. Hi-GAL will be a perfect complement to the JCMT Legacy Surveys by providing the multi-wavelength data that will enable us to determine the temperature and hopefully evolutionary state of the massive preclusters. | |