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Are heavyweight stars
born like our Sun?

Posted: 29 January 2010

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Obscured by dust, catching the rapid formation of massive stars in the act is nigh on impossible, but new Gemini observations hint that these stellar heavyweights may be born in a similar way to lightweights like our Sun.

“The problem is that when the most massive stars form it happens very quickly compared to stars like our Sun, and by the time they break free of their natal clouds they are already the finished article,” says Ben Davies of the University of Leeds, UK, and the Rochester Institute of Technology. “If you want to see a massive star in the process of forming, you need to be able to see through the obscuring clouds to where the action is.”

Artist’s conception of W33A showing the accretion disc (yellow/orange), torus (dark ring around disc) and bi-polar outflow jets (blue) within the dense clouds of its stellar nursery. Image: Gemini Observatory, artwork by Lynette Cook.

Existing theories for massive star formation include scaled-up versions of low mass star formation, or a completely different physical process altogether. Now, new observations that combine adaptive optics – which removes the blur of the Earth's atmosphere – with sensitive infrared spectroscopic observations afforded by the Near-Infrared Integral Field Spectrograph (NIFS) on the Frederick C. Gillett Gemini North telescope on Hawaii’s Mauna Kea, have allowed astronomers to penetrate the thick dust clouds surrounding massive protostar W33A that lies12,000 light-years away toward the constellation of Sagittarius.

“While the optical light is attenuated by about a factor of 10,000, [meaning it is completely obscured in optical light as seen by the human eye] much of the infrared light can pass through the intervening material. This affords us a glimpse of what is happening deep inside W33A’s natal cloud,” says Davies. “We were not only able to resolve the inner nebula on small spatial scales, but also probe its dynamics by measuring the Doppler-shift of light from the glowing gas to determine its velocity and how it flows around the forming star. This is an amazingly powerful tool for understanding the inner workings of how stars actually form.”

Team member Melvin Hoare, also of the University of Leeds, adds that what they saw was “reassuringly familiar, like nice cup of tea!”

The proto-star is already ten times more massive than our Sun, and is rapidly putting on weight. The observations also hint at an accretion disc embedded within a torus of gas and dust, and the astronomers see material being blasted away from the forming star's poles at speeds of up to 300 kilometres per second. “These features are all common to the formation process found in much smaller stars,” states Davies.

Previous studies of W33A – nicknamed a Massive Young Stellar Object – only hinted at its dynamic nature, and the level of detail offered by combining adaptive optics with spectroscopy has surely set a precedent for the future of massive star formation studies.

Colin Aspin of the Institute for Astronomy at the University of Hawaii adds, “This result provides us with one of the first clues that high-mass stars form in similar ways to their low-mass counterparts and shows the power of integral-field near-infrared spectroscopy as a way of probing the youngest phases of stellar evolution.”