25 Aug 2011

 

Y dwarfs discovered: as cold as us!

 

The discovery of type Y brown dwarfs, the coldest of all the failed stars, has finally been made official in two articles to be published in The Astrophysical Journal Supplement Series. Brown dwarfs are bodies with dimensions intermediate between those of planets and normal stars, not massive enough for nuclear fusion to be triggered in their nucleus, and so are destined to simply cool slowly over time.
The discovery was made possible by the excellent sensitivity of the orbiting WISE telescope to infrared light. WISE has found 100 brown dwarfs to date, and of these 6 have been confirmed as Y dwarfs, thanks to spectroscopic follow-up observations with the most powerful ground-based telescopes, as well as the Hubble Space Telescope.
The coolest stars known up to now were the T dwarfs, with surface temperatures of a few hundred degrees Celsius, the Y dwarfs can have surface temperatures as cold as a few tens of degrees, with the record now held by WISE 1828+2650, at 25°C, colder than the human body! (This object is the green point indicated by the arrow in the image on the right, while on the left is an artist's impression of what the dwarf may look like).
These objects are distinguished from giant planets only in that they are not in orbit about a star but rather isolated in space, almost certainly created by accretion within a protostellar cloud. Apart from this they shouldn't be too unlike giant planets, so that their atmospheres may well look much like that of Jupiter.
In the infrared these cold objects are about 5000 times brighter than in the optical light that our eyes can detect, making detection with optical, ground-based telescopes almost impossible. The first 6 Y dwarfs recognised (the list will surely lengthen) have distances between 9 and 40 light years, with the closest, WISE 1541-2250, actually becoming the seventh closest star to Earth, even though some will argue that it isn't really a star...

 

by Michele Ferrara & Marcel Clemens

credit: NASA/JPL-Caltech/UCLA