April 11, 2008 -- A dim, lonely, weakling star with the lowest stellar temperature yet recorded has been found just 40 light-years from Earth.
The brown dwarf star is between 15 and 30 times the mass of Jupiter and has a surface temperature of a mild 660 degrees Fahrenheit (350 Celsius) -- about the surface temperature of the planet Mercury at the equator and much cooler than the surface of Venus.
The spectacularly unspectacular object is of special interest because it falls right smack in the middle of the final frontier that divides mega-planets from the puniest stars. Stars in that realm theoretically qualify as an entirely new stellar type -- what's called a Y class dwarf.
"This would be the last spectral type between stars and planets," said stellar researcher Loic Albert of the Canada France Hawaii Telescope in Hawaii.
Albert is part of the team which identified the newfound coolest star, which is called CFBDS0059. Their paper on the discovery is accepted for publication in the journal Astronomy and Astrophysics.
Because they are so dim by nature, Y class dwarfs are very hard to find and identify. In fact no one has succeeded in positively identifying a Y class dwarf, Albert said.
"To the human eye it would be invisible, even in a telescope," Albert said.
That's because it's not quite hot enough to glow red -- like an electric stove element. Rather, it's just below the "red-hot" temperature, glowing in infrared light instead.
For that reason, the astronomers identified and studied CFBDS0059 with near-infrared and infrared instruments of the Canada France Hawaii and Gemini North telescopes in Hawaii and the European Southern Observatory's NTT telescope in Chile.
The key to confirming that CFBDS0059 or any other cool brown dwarf is a Type Y would be the presence of ammonia in its atmosphere, Albert explained. To find that requires gathering lots of infrared light, splitting it into its spectrum and looking for telltale dark lines where ammonia gas is absorbing light.
"We are starting to see a little hint of ammonia absorption," Albert told Discovery News.
Other brown dwarf researchers are confident that ammonia is showing up, but are less sure it warrants a new class of dwarfs.
"It really depends on what is decided to define this class," said Adam Burgasser of the Massachusetts Institute of Technology.
The two verified classes of brown dwarfs, L and T dwarfs, are not subtle about their differences. L Dwarfs are much hotter, reaching between 2,200 and 3,600 degrees F (1,200 and 2,000 degrees C). T dwarfs are cooler than 2,190 degrees F (1,200 degrees C) and rich in methane.
It's likely, says Burgasser, that as more of these even cooler dwarfs are found, there likely will be some that are a couple of hundred degrees cooler than CFBDS0059. That means any water in there atmospheres will condense into droplets of water vapor, which would make these dwarfs dramatically different than their L and T dwarf brothers.
In brown dwarf atmospheres, water is generally in gaseous state, while in giant planets it condenses into water ice. So an even cooler dwarf would truly be on the verge of being more of a hot, giant Jupiter than a star.
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