Brown dwarfs don't hang out with stars

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 07 January, 2009

According to the results of a Hubble Space Telescope survey, brown dwarfs and normal stars don’t like to hang out together.

Based on a study of 233 nearby multiple star systems, only two brown dwarfs – objects that are less massive than stars but larger than planets – were found as companions to normal stars. This means that the apparent absence of brown dwarfs around solar-type stars, the so-called ‘brown dwarf desert’, extends to the smallest stars in the Universe.

“We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit,” says Sergio Dieterich of Georgia State University in Atlanta, who presented the results at the American Astronomical Society meeting earlier this week. “Especially when we consider the fact that brown dwarfs binaries do exist, the fact that there are very few binaries whose components lie on different sides of the hydrogen burning limit is significant.”

This diagram shows the various stellar populations within 32 light-years of Earth as measured by the RECONS survey. The largest population consists of red dwarfs, but the population of slightly lower mass brown dwarfs drops off precipitously. This is known as the "brown dwarf desert." Image: NASA/ESA/A.Feild (STScI)/T. Henry (Georgia State University).

The 233 stars surveyed make up part of the RECONS (Research Consortium on Nearby Stars) survey designed to understand the nature of the Sun's nearest stellar neighbours, both individually and as a population. RECONS makes use of existing all-sky surveys and combines them with observations from northern and southern hemisphere telescopes. The current primary goal of RECONS is to scan the archived data to discover and characterise ‘missing’ members of the sample of stars within 32.6 light years of Earth.

A total of 12 brown dwarfs are currently known to reside within 32.6 light-years of Earth, compared with 239 red dwarf stars –stars that are around 20 percent the mass of our Sun but roughly half its diameter and temperature. The Hubble survey provides strong evidence that brown dwarfs do not exist around even the least massive stars. "If mass ratio was the driving factor we would expect to find more brown dwarfs around small red stars than around solar type stars," says Dieterich.

All-sky surveys planned for the next decade, with advanced telescopes like the Large Synoptic Survey Telescope, promise to ultimately solve the puzzle of the brown dwarf desert by performing deep infrared searches for the underlying brown dwarf population.

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