Birth Environment Affects Star Formation

May 12, 2012
A Hubble Space Telescope image of the massive star cluster R 136 in the Large Magellanic Cloud. R136 is a potential birth site of much more massive stars, although unconfirmed, than observed in "normal" sites of present-day star formation. Credit: NASA, J. Trauger (JPL), J. Westphal (Caltech)

Researchers working at the University of Bonn in Germany have used computer simulations to discover the first evidence that the way in which stars form depends upon the conditions of their birth environments, claims a new study published in the journal Monthly Notices of the Royal Astronomical Society.

In a Friday statement, the Society said that the properties of stars, which are believed to form in interstellar space from gas and dust clouds, are dependent upon the surrounding conditions when they are formed, in much the same way that temperature and cloud constitution determine the type and amount of precipitation we experience on Earth.

“Sites of star formation are the bad weather regions in a galaxy and the forming stars are, in a very rough analogy, like the raindrops condensing out of this material”, Professor Dr. Pavel Kroupa said.

The proof supporting this theory “does not come to us from young regions of ongoing star formation, but from a very old class of objects, so called globular star clusters”, added Dr. Michael Marks, lead author of the new paper. “The number of observed stars less massive than our Sun in globular clusters is at odds with their structure… Nevertheless, using our simulations we found that the connection between star formation and birth environment can be understood when invoking a process that occurs very early in the life of any cluster, called residual-gas expulsion.”

These globular star clusters are enormous congregations filled with the thousands of stars that surround the Milky Way, they said, and the stellar formation within these clusters came to an end billions of years ago. Once the process is completed and a star has been formed, it begins to shine, and ultimately the radiation coming from the cluster forces out the gas from which they had been formed.

“This process leads to expansion of the whole aggregate of stars with the accompanying stripping of some of the stars from the cluster by the gravitational attraction of the young Milky Way. The faster the gas is blown out the stronger is the expansion and the more stars are removed”, Kroupa explained. “The imprint of this process is still visible in the present-day mass distribution”. This means that careful observations of present-day stellar populations in globular clusters allow their initial star content to be reconstructed.

Once the gas is gone, stars of many different masses are left behind, and the researchers believe that at one point, globular clusters must have formed to a far greater amount of massive stars than those that can be observed in individual star-forming locations today.

“Otherwise the star birth region a globular cluster formed from is not destroyed quickly enough and the subsequent expansion is too weak to remove enough stars from the cluster”, Marks said. “If this had happened the distribution of masses of stars we see today would be quite different“¦ We do not observe these extreme environments in the present day, but these may have well been frequent when globular clusters were born around 12 billion years ago.”

The researchers, who Kroupa said “might have uncovered the long expected systematic differences in the star formation process,” will now turn their attention to conducting additional simulations in order to study the impact of these differences on the long-term evolution of these globular clusters, according to the Royal Astronomical Society statement.

Source: RedOrbit Staff & Wire Reports

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