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Young stars in chaos

April 04, 2002

It is not only teenagers who like to congregate in intimate groups and disturb their neighbours and surroundings.

As Matthew Bate (University of Exeter), will be explaining to the UK National Astronomy Meeting in Bristol on Friday 12 April, young stars also like to hang around in crowds and undergo chaotic close encounters with each other during their formative years.

After performing one of the largest and most complex simulations of star formation to date, Matthew Bate, Ian Bonnell (University of St Andrews) and Volker Bromm (Harvard-Smithsonian Center for Astrophysics) have found that these cosmic furnaces form in a much more chaotic manner than is generally believed.

To perform the calculation, the astronomers used the supercomputer at the United Kingdom Astrophysical Fluid Facility (UKAFF), a national computing facility for astronomy sited at the University of Leicester. The calculation was so enormous that it required 100,000 CPU hours,
roughly 10% of the time available on the 128-processor supercomputer during 2001.

The simulation followed the collapse of an interstellar gas cloud which was over one light year across and 50 times the mass of the Sun, eventually resulting in the formation of a cluster of 50 stars and brown dwarfs.

One of the big surprises found by the astronomers was how chaotic and dynamic the process of star formation is. The results showed that stars form so close together that they often interact with each other well before they have grown to full size.

In the small, new-born stellar groups, the stars compete with each other for the remaining gas. This process is inherently unfair, with the more massive stars tending to gather more gas than the lower mass stars, while the lowest mass stars are kicked out of the group.

About half of the objects are ejected so quickly that they don`t manage to gather enough gas to become stars at all. Rather, they become brown dwarfs, objects with less than 1/13 the mass of the Sun. Unable to generate energy by fusing hydrogen into helium, they cannot continue to shine like the Sun and quickly fade away.

The new calculation supports recent astronomical surveys suggesting that there may be as many brown dwarfs as stars in our Galaxy, and indicates that the high frequency of brown dwarfs is a natural consequence of the competition between stars during their formation.

Another surprise is that many of the encounters between the stars and brown dwarfs in such clusters are close enough to strip off the outer parts of the dusty discs surrounding the young stars. Although many of the discs are initially very large, by the end of the calculation the majority of them have been truncated to less than the size of our Solar System.

Since most stars are believed to form in large star clusters, this suggests that planetary systems like our own may be the exception rather than the rule.

Royal Astronomical Society (RAS)