Unraveling the Enigma of Monster Stars

A team of astronomers at the University of Bonn in Germany has suggested that the four ultramassive stars, discovered two years ago and located in the giant star cluster R136 in the nearby galaxy the Large Magellanic Cloud, were created from the merger of lighter stars in tight binary systems.

A red dwarf, the Sun, a blue dwarf, and R136a1 (ESO / M. Kornmesser)

Until the discovery of these objects in 2010, with the heaviest more than 300 times as massive as our Sun, observations of the Milky Way and other galaxies suggested that the upper limit for stars formed in the present day Universe was about 150 times the mass of the Sun. This value represented a universal limit and appeared to apply wherever stars formed.

“Not only the upper mass limit but the whole mass ingredient of any newborn assembly of stars appears identical irrespective of the stellar birthplace,” said Prof Dr Pavel Kroupa of the University of Bonn, a co-author of the new paper in the journal Monthly Notices of the Royal Astronomical Society. “The star birth process seems to be universal.”

The Large Magellanic Cloud (LMC) has many star forming regions, with by far the most active being the 1000 light year diameter ‘Tarantula Nebula’ where the four supermassive stars are found. This cloud of gas and dust is a highly fertile breeding ground of stars in the LMC also known as the 30 Doradus (30 Dor) complex. Near the center of 30 Dor is R136, by far the brightest stellar nursery not just in the LMC but in the entire ‘Local Group’ of more than 50 galaxies.

The four ultrabright ultramassive stars in R136 – one of them, known as R136a1, has a current mass of 265 times that of the Sun – are quite an exception to this widely accepted limit. Does their discovery mean that the star birth in the 30 Dor region is happening in a very different way from elsewhere in the Universe? If so it would challenge the universal nature of the process of star formation, a fundamental premise of modern astronomy.

The team modeled the interactions between stars in a R136-like cluster. Their computer simulation assembled the model cluster star by star, so as to resemble the real cluster as closely as possible, creating a cluster of more than 170,000 stars packed closely together.

An image of the R136 cluster, obtained at high resolution with ESO’s Very Large Telescope, provides unique details of its stellar content. One of the most massive stars, known as R136a1, has been found to have a current mass of 265 times that of the Sun (located at the centre of the image). It also has the highest luminosity, close to ten million times greater than the Sun (ESO / P. Crowther / C.J. Evans)

To compute how even this relatively basic system changes over time, the model had to solve 510,000 equations many times over. The simulation is complicated by the effect of the nuclear reactions and hence energy released by each star and what happens when two stars happen to collide, a frequent event in such a crowded environment. These highly intensive, star by star calculations are known as ‘direct N-body simulations’ and are the most reliable and accurate way to model clusters of stars.

“With all these ingredients, our R136 models are the most difficult and intensive N-body calculations ever made”, said Prof Kroupa and team member Seungkyung Oh.

“Once these calculations were done, it quickly became clear that the ultramassive stars are no mystery,” added lead investigator Dr Sambaran Banerjee. “They start appearing very early in the life of the cluster. With so many massive stars in tight binary pairs, themselves packed closely together, there are frequent random encounters, some of which result in collisions where two stars coalesce into heavier objects. The resulting stars can then quite easily end up being as ultramassive as those seen in R136.”

“Imagine two bulky stars closely circling each other but where the duo gets pulled apart by the gravitational attraction from their neighboring stars. If their initially circular orbit is stretched enough, then the stars crash into each other as they pass and make a single ultramassive star”, Dr Banerjee said.

“Although extremely complicated physics is involved when two very massive stars collide, we still find it quite convincing that this explains the monster stars seen in the Tarantula”, he added.

“This helps us relax”, Prof Kroupa concluded, “Because the collisions mean that the ultramassive stars are a lot easier to explain. The universality of star formation prevails after all.”

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Bibliographic information: Banerjee et al. 2012. The emergence of super-canonical stars in R136-type star-burst clusters. Accepted for publication in Monthly Notices of the Royal Astronomical Society; arXiv: 1208.0826v1