A class of cosmic oddballs exists that doesn't fit in with
either stars or planets, instead occupying a murky middle ground.
Known as brown dwarfs, these misfits fall somewhere between
planets and stars in terms of their temperature and mass. They are
cooler and more lightweight than stars and more massive (and normally
warmer) than planets.
This has generated a debate among astronomers: Do brown
dwarfs form like planets or like stars?
Scientists now have an additional clue – a baby brown dwarf
was discovered by NASA's Spitzer
Space Telescope – that suggests brown dwarfs develop like light-weight
stars.
Brown dwarf quandry
Brown dwarfs are born of the same dense, dusty clouds that
spawn stars and planets. But while they may share the same galactic nursery,
brown dwarfs are often called "failed" stars because they lack the mass of
their hotter, brighter stellar siblings.
Without that mass, the gas at their core does not get hot
enough to trigger the thermonuclear fusion that burns hydrogen — the main
component of these molecular clouds — into helium. Unable to ignite as stars,
brown dwarfs end up as cooler, less luminous objects that are more difficult to
detect – a challenge that was overcome in this case by Spitzer's heat-sensitive
infrared
vision.
To complicate matters, young brown dwarfs evolve rapidly,
making it difficult to catch them when they are first born.
The first brown dwarf was discovered in 1995, and while
hundreds have been found since, astronomers had not been able to unambiguously
find them in their earliest stages of formation until now.
Looking through cosmic dust
In this study, an international team of astronomers found a
so-called "proto brown dwarf" while it was still hidden in its natal star-forming
region. Guided by Spitzer data collected in 2005, they focused their search in
the dark cloud Barnard 213, a region of the Taurus-Auriga complex well known to
astronomers as a hunting ground for young cosmic objects.
"We decided to go several steps back in the process
when (brown dwarfs) are really hidden," said David Barrado of the Centro
de Astrobiologia in Madrid, Spain, lead author of the paper detailing the
discovery in the Astronomy & Astrophysics journal. "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."
Spitzer's longer-wavelength infrared camera penetrated the
dusty natal cloud to observe a baby brown dwarf named SSTB213 J041757. The
data, confirmed with near-infrared imaging from Calar Alto Observatory in Spain, revealed not one but two of what would potentially prove to be the faintest and
coolest brown dwarfs ever observed. They're also the youngest, the astronomers
think.
To pin down that the dusty envelope was in fact a celestial
womb and that these dwarfs were in their infancy, Barrado and his team had them
observed from around the world. The information gleaned from these observations
helped the astronomers characterize the brown dwarfs and the environment around
them.
"We were able to estimate that these two objects are
the faintest and coolest discovered so far," Barrado said.
The observations suggest brown dwarfs form like low-mass
stars. This theory is bolstered because the change in brightness of the objects
at various wavelengths of light 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,
Barrado said.
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