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14 November 2006
 
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Mass cut-off between stars and brown dwarfs revealed

  • 19:00 17 August 2006
  • NewScientist.com news service
  • David Shiga
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The core of the ancient star cluster NGC 6397, seen in this Hubble image, is an ideal hunting ground for stars only slightly more massive than brown dwarfs (Image: NASA/ESA/H Richer/UBC)
The core of the ancient star cluster NGC 6397, seen in this Hubble image, is an ideal hunting ground for stars only slightly more massive than brown dwarfs (Image: NASA/ESA/H Richer/UBC)
 

The faintest stars ever seen in an ancient star cluster have been imaged by the Hubble Space Telescope.

The observations provide the most accurate measurement ever made of the mass boundary between lightweight stars and "failed" stars called brown dwarfs – the dividing line is at about 80 times the mass of Jupiter, in line with theoretical predictions.

Stars and brown dwarfs are both made of the same materials – mostly hydrogen and helium – but their long-term behaviour is different. Stars – even those with very low mass, called red dwarfs – can burn hydrogen for many billions of years. Brown dwarfs, on the other hand, are not massive enough to sustain hydrogen fusion for long, fizzling out after just 1 billion years or so.

Previously, scientists calculated that the minimum mass needed to sustain long-term hydrogen burning is about 75 Jupiters. But observational confirmation has been hard to come by because young brown dwarfs and young low mass stars look very similar.

Ancient clusters

One way to distinguish the two is to look for the faintest stars in very old star clusters. Stars in clusters are thought to share approximately the same age, so old clusters should contain no observable brown dwarfs – the failed stars would have already cooled and faded from view.

Any faint objects seen in such clusters would be red dwarfs or white dwarfs, dense cores of mostly carbon and oxygen that are the cooling embers of stars like the Sun.

"Globular" star clusters – so named because of their round shape – are ideal for studying these faint stars because they are more than 10 billion years old, and contain hundreds of thousands or millions of stars.

Now, astronomers led by Harvey Richer of the University of British Columbia in Vancouver, Canada, have used Hubble to find the faintest red dwarfs ever seen in a globular cluster. They looked at a relatively nearby cluster called NGC 6397, which is 8500 light years from Earth.

Stark contrast

The cluster is 13.5 billion years old, nearly as old as the universe. "The brown dwarfs have by now faded off into obscurity so there is a very stark contrast between the stars that could burn hydrogen and the ones that couldn't," says team member Jay Anderson of Rice University in Houston, Texas, US.

Theory predicts that the mass cut-off for what constitutes a star is different for objects of different metallicity, which refers to the proportion of elements heavier than hydrogen the object contains.

For objects with a metallicity similar to that of the Sun, theory suggests that anything with less than 7.5% the mass of the Sun – or about 75 Jupiters – will be a brown dwarf.

In the case of NGC 6397, which has a metallicity 100 times lower than that of the Sun, the dividing line is expected to be at 8.3% the mass of the Sun, or about 83 Jupiters.

The members of the dim red dwarf population seen in the Hubble images appear to be heavier than this limit, in agreement with theoretical calculations.

Tightest constraint

The new observations are sensitive to stars 10 to 20 times fainter than in previous work on the same cluster, Anderson told New Scientist.

"This kind of observation is probably the best kind of constraint one can get," says Gibor Basri of the University of California, Berkeley, US. The theoretically predicted hydrogen burning limits "have stood up to what observational tests we have," he told New Scientist. "I don't think anyone thinks they're off by a lot."

The observations also revealed the telltale signs of very old white dwarfs. Although white dwarfs start out with temperatures of about 100,000° Kelvin, some of those seen in the Hubble images are old enough to have cooled below 4000° K.

That is low enough for hydrogen atoms in their atmospheres to join together to form molecules. This makes the white dwarfs bluer than they would otherwise appear, an effect rarely observed before.

Journal reference: Science (vol 313, p 936)

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