PRESS RELEASE, January 2009
BROWN DWARFS DON'T HANG OUT WITH STARS
STScI News Release 2009-01
Dr. Todd J. Henry
Brown dwarfs, objects that are less massive than stars but larger than planets, just got more elusive, based on a study of 233 nearby multiple-star systems by NASA's Hubble Space Telescope. Hubble found only two brown dwarfs as companions to normal stars. This means the so-called "brown dwarf desert" (the absence of brown dwarfs around solar-type stars) extends to the smallest stars in the universe.
Team leader Sergio Dieterich and Dr. Todd Henry of Georgia State University (GSU) in Atlanta reported the initial results of the survey at the 213th meeting of the American Astronomical Society (AAS) in Long Beach, CA in January 2009. This work has been carried out in collaboration with Dr. David Golimowski at Johns Hopkins University (currently at the Space Telescope Science Institute).
"We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit. 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," says Dieterich.
The 233 stars surveyed are part of the RECONS (Research Consortium on Nearby Stars) survey meant to understand the nature of the Sun's nearest stellar neighbors, both individually and as a population. The current primary goals are to discover and characterize "missing" members of the sample of stars within 32.6 light-years (10 parsecs) of Earth.
RECONS searches for nearby stars through analyzing existing all-sky surveys, combined with observations by a variety of telescopes in both hemispheres. A total of 12 brown dwarfs are currently known within 32.6 light-years of Earth, as compared to 239 red dwarf stars (stars that are largely 20 percent the mass of our Sun and are roughly half its diameter and temperature).
In fact, the number of known brown dwarfs is close to that of known extrasolar planets. However, the number of exoplanets known in this region so far is very likely only a lower limit as smaller-mass exoplanets are not within our capability of detection at present.
The Hubble survey, taken with Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), provides strong statistics pointing to the fact 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 doing deep infrared searches for the underlying brown dwarf population.
This research has enjoyed the consistent support of the Space Telescope Science Institute and Georgia State University.