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Spitzer observes youngest brown dwarfs
Posted: November 24, 2009

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NASA's Spitzer Space Telescope may have uncovered the youngest pair of brown dwarfs ever seen, a discovery that could help solve the mystery of how these cosmic misfits are formed.

Artist impression of a star being born from a dark, swirling cloud of dust and gas. The new results suggest that brown dwarfs form in a similar way to stars, rather than planets. Image: NASA/JPL-Caltech.

Brown dwarfs fall between planets and stars in terms of their temperature and mass; they are cooler and more lightweight than stars but more massive and warmer than planets. Answering the question of 'do brown dwarfs form like planets or like stars?' is one that is much debated by astronomers. Often called failed stars because they are unable to trigger nuclear fusion in their cores, they are difficult to detect because they are much less luminous then their fully fledged stellar counterparts. To complicate matters further, they also evolve relatively rapidly, making it difficult to capture them in the first throes of youth when they heat up the nearby gas and dust.

By using Spitzer’s heat-sensitive infrared vision, an international team of astronomers found a so-called “proto-brown dwarf” while it was still cocooned in its natal star-forming region in the dark cloud Barnard 213. “We decided to go several steps back in the process when brown dwarfs are really hidden,” says David Barrado of the Centro de Astrobiologia in Madrid. “During this step they would have an opaque envelope, a cocoon, and they would be easier to identify due to their strong infrared excesses. We have used this property to identify them. This is where Spitzer plays an important role because Spitzer can have a look inside these clouds. Without it this wouldn’t have been possible.”

The closely-spaced purple-blue (A) and orange-white (B) objects in this image are likely the two youngest proto brown dwarfs ever seen. The surrounding envelope of cool dust surrounding this nursery can be seen in purple. This image combines near-infrared observations from the Calar Alto Observatory (wavelengths of 1.3 and 2.2 microns, rendered as blue), Spitzer's infrared array camera (4.5-micron (green) and 8.0-micron (yellow) wavelengths), and its multiband imaging photometer (24-micron (red) wavelength), and far-infrared observations from the Caltech Submillimeter Observatory (350 microns (purple)). Image: NASA/JPL-Caltech/Calar Alto Obsv./Caltech Sub. Obsv.

Spitzer’s longer-wavelength infrared camera penetrated the dusty star-forming cloud to observe a baby brown dwarf named SSTB213 J041757. The observation was confirmed with near-infrared imaging from the Calar Alto Observatory in Spain, which revealed not one, but two faint, cool brown dwarfs.

Ploughing through archived data and following up with telescopic measurements from around the world yielded more information about the dusty cocoon in which the proto-brown dwarfs are embedded, enabling the astronomers to build up a picture of the conditions prevailing inside the cloud.

“We were able to estimate that these two objects are the faintest and coolest discovered so far,” says Barrado. The findings potentially solve the mystery about whether brown dwarfs form more like stars or planets. The authors conclude that they form like low-mass stars, the evidence based on their observations of the change in brightness of the objects at various wavelengths that matches that of other very young, low-mass stars.

While further study will confirm whether these two celestial objects are in fact proto brown dwarfs, they are the best candidates so far. “It is a story that has been unfolding piece by piece,” says Barrado. ”Sometimes nature takes its time to give up its secrets.”

The observations were made before Spitzer ran out of its liquid coolant in May 2009, and were backed up with data from the Caltech Submillimeter Observatory in Hawaii, the Calar Alto Observatory in Spain, the Very Large Telescope in Chile and the Very Large Array in New Mexico. Decade-old data was also pulled from the Canadian Astronomy Data Centre archives to provide measurements of how the two objects are moving in the sky. The full details of the study are presented in a paper by Barrado et al in the Astronomy & Astrophysics journal.

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