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A sleeping giant
Posted: June 9, 2009

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Astronomers using the Submillimeter Array atop Mauna Kea in Hawaii have found a massive, quiescent object in a dark cloud that is likely to be the direct progenitor of a massive star or stars. Dr. Jonathan Swift of the Institute for Astronomy at the University of Hawaii at Manoa is presenting these results today at a press conference at the American Astronomical Society meeting in Pasadena, California. This may be the first time that scientists have been able to see such a region before massive stars form.

A dark lane is seen stretching across this false-color, mid-infrared image of a small piece of the Milky Way. These infrared dark clouds can potentially form young stellar clusters like the one seen in the upper right of the figure. Credit: NASA/JPL-Caltech/E. Churchwell (Univ. of Wisconsin)
Located near the Aquila rift in the Galactic plane at a distance of 23,000 light-years, this cloud condensation has a mass 120 times that of the Sun contained within a volume smaller than the Oort cloud of comets orbiting at the edge of our solar system, and its temperature is less than 18 degrees above absolute zero. Such a large amount of cold dense gas is likely to evolve into one or more massive stars.

Massive stars -- those with a mass of more than 8 times that of the Sun -- are much rarer than Sun-like stars. However, they produce disproportionately more radiation, causing them to lead short, spectacular lives. The extreme radiation from massive stars allows astronomers to identify the farthest structures in the Universe, and the general knowledge of massive stars has played a critical role in understanding the evolution of the cosmos. Massive stars die violently in supernova events so luminous that for short times they can outshine entire galaxies. It is within these death throes that elements heavier than iron are formed, including gold and silver.

The rarity of massive stars and their propensity to quickly destroy the environments from which they form has posed a serious challenge to understanding their formation. But this is changing. New catalogs of cold dense gas in the Galaxy identified through extinction at mid-infrared wavelengths published by Robert Simon and Jill Rathborne and collaborators in 2006 now allow astronomers to select for study regions that have a high potential for forming massive stars before these stars have formed. Dr. Swift, the SMA postdoctoral fellow at the Institute for Astronomy, chose these infrared dark clouds as the prime locations to study the initial conditions of massive star formation. "The SMA is a unique instrument in a superb location that facilitates our ability to map the conditions preceding the formation of massive stars with high resolution."

We know from studies of nearby star-forming regions that Sun-like stars form inside dense cores of molecular gas, but whether or not massive stars form in the same manner is a hotly debated topic. It has been postulated by observers and theorists alike that if massive stars were to form from the collapse of massive cores in a manner similar to Sun-like stars, these cores would need to have 100 solar masses or more contained within about 20,000 AU. The properties of the dense core recently discovered using the Submillimeter Array make it a good candidate for being the direct progenitor of a massive star or stars. "The mass and density of this object along with the lack of evidence for star formation activity is unique, and this fits very well with our expectations for massive pre-stellar cores," notes Dr. Swift.

The Submillimeter Array, a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, is the ideal instrument to survey these regions. The SMA detects light with wavelengths longer than the far-infrared, where the coldest objects in the cosmos glow brightest. Also, by employing a Nobel Prize-winning technology called interferometry, in which signals are combined from two or more small antennas, the SMA produces images of unparalleled resolution at these wavelengths.

The core discovered with the SMA is detected only at these long wavelengths. Not even the Spitzer Space Telescope was able to see this core, which means that no significant amount of star formation has yet taken place. But recent theoretical work and computer simulations suggest that a core with this mass and size can form massive stars in as soon as 50,000 years.

"Perhaps the most exciting thing is that we now know that massive and dense cores with no sign of star formation activity do exist," says Dr. Swift, noting that further study is necessary. In addition to upcoming observations of this core that will utilize the highest resolution capabilities of the SMA, collaborators Thushara Pillai and Steven Longmore, both at Harvard University's Center for Astrophysics, are currently leading surveys with the SMA that will help us better understand the uniqueness of this core and its relevance to high-mass star formation.