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An international team of astronomers has discovered over two-dozen new free-floating brown dwarfs that reside in two young star clusters. One brown dwarf is a lightweight youngster only about six times heftier than Jupiter. What's more, one cluster contains a surprising surplus of brown dwarfs; it harbors half as many of these astronomical oddballs as normal stars. These findings come from deep surveys and extensive follow-up observations using the Subaru Telescope in Hawaii and the Very Large Telescope (VLT) in Chile, two of the world's largest optical-infrared telescopes.
Sometimes described as failed stars, brown dwarfs are unusual celestial objects that straddle the boundary between stars and planets. When young, they glow brightly from the heat of formation, but they eventually cool down and end up with atmospheres that exhibit planet-like characteristics.
During the course of the SONYC (Substellar Objects in Nearby Young Clusters) Survey, astronomers used Subaru Telescope to take extremely deep images of the NGC 1333 (Figure 1) and rho Ophiuchi star clusters at both optical and infrared wavelengths. Once they identified candidate brown dwarfs from their very red colours, the research team verified their nature with spectra taken at Subaru and the VLT. The team's findings are reported in two upcoming papers in the Astrophysical Journal and will be presented this week at a scientific conference in Garching, Germany.
The six-Jupiter-mass brown dwarf found in the NGC 1333 cluster is one of the puniest free-floating objects known. "Its mass is comparable to those of giant planets, yet it doesn't circle a star. How it formed is a mystery," said Aleks Scholz of the Dublin Institute for Advanced Studies in Ireland, lead author of the first paper.
Several other newly identified brown dwarfs in both NGC 1333 (Figure 2) and rho Ophiuchi clusters have masses that are less than 20 times the mass of Jupiter-placing them at the low end of the mass range for known brown dwarfs. "Brown dwarfs seem to be more common in NGC 1333 than in other young star clusters. That difference may be hinting at how different environmental conditions affect their formation," said Koraljka Muzic of the University of Toronto in Canada, lead author of the second paper.
"Our findings suggest once again that objects not much bigger than Jupiter could form the same way as stars do. In other words, nature appears to have more than one trick up its sleeve for producing planetary mass objects," said Ray Jayawardhana of the University of Toronto, who is the principal investigator of the SONYC Survey.
"We could not have made these exciting discoveries if not for the remarkable capabilities of Subaru and the VLT. Instruments that can image large patches of the sky and take hundreds of spectra at once are key to our success," said Motohide Tamura of the National Astronomical Observatory of Japan.
Other co-authors of this work are Vincent Geers of ETH Zurich in Switzerland and Mariangela Bonavita of the University of Toronto.
The papers are to appear in the Astrophysical Journal and can be accessible from the following links:
- Alexander Scholz, Koraljka Muzic, Vincent Geers, Mariangela Bonavita, Ray Jayawardhana, and Motohide Tamura "Substellar Objects in Nearby Young Clusters (SONYC) IV: A Census of Very Low Mass Objects in NGC1333" (link)
- Koraljka Muzic, Alexander Scholz, Vincent Geers, Ray Jayawardhana, and Motohide Tamura "Substellar Objects in Nearby Young Clusters (SONYC) V: New Brown Dwarfs in ρ Ophiuchi" (link)