Y-Class Brown Dwarfs Warmer than Previously Thought, Say Astronomers

Astronomers using data from NASA’s WISE mission and the Spitzer Space Telescope have found that ultra-cool Y-class brown dwarfs – the coldest class of star-like bodies – are warmer than previously thought, with surface temperatures reaching 250 – 350 degrees Fahrenheit.

This is an artist's impression of a free-floating Y-dwarf. The new study shows that these objects are warmer than previously thought, with temperatures about 250-350 degrees Fahrenheit (NASA / JPL-Caltech).

This is an artist’s impression of a free-floating Y-dwarf. The new study shows that these objects are warmer than previously thought, with temperatures about 250-350 degrees Fahrenheit (NASA / JPL-Caltech).

To reach such low surface temperatures after cooling for billions of years means that Y-dwarfs can only have about 5 to 20 times the mass of Jupiter. Unlike the Sun, these objects’ only source of energy is from their gravitational contraction, which depends directly on their mass.

“If one of these objects were found orbiting a star, there is a good chance that it would be called a planet,” said Dr Trent Dupuy from the Harvard-Smithsonian Center for Astrophysics, who is a first author of the paper published in the journal Science.

Characterizing Y-class dwarfs is challenging because they emit most of their light at infrared wavelengths, and they are very faint due to their small size and low temperature. To get accurate temperatures, astronomers need to know the distances to these objects.

“We wanted to find out if they were colder, fainter, and nearby or if they were warmer, brighter, and more distant,” Dr Dupuy said.

Using NASA’s Spitzer Space Telescope, Dr Dupuy with his colleague Prof Adam Kraus from the University of Texas at Austin determined that the Y-dwarfs in question are located at distances 20 to 50 light-years away.

To determine the distances to these objects the scientists measured their parallax – the apparent change in position against background stars over time. As the Spitzer Space Telescope orbits the Sun its perspective changes and nearby objects appear to shift back and forth slightly. The same effect occurs if you hold up a finger in front of your face and close one eye and then the other. The position of your finger seems to shift when viewed against the distant background. But even for these relatively nearby brown dwarfs, the parallax motion is small.

“To be able to determine accurate distances, our measurements had to be the same precision as knowing the position of a firefly to within 1 inch from 200 miles away,” Prof Adam Kraus said.

The new data also present new puzzles to astronomers that study cool, planet-like atmospheres. Unlike warmer brown dwarfs and stars, the observable properties of these objects don’t seem to correlate as strongly with temperature.

This suggests increased roles for other factors, such as convective mixing, in driving the chemistry at the surface.

The scientists also find evidence for disappearing alkali elements that are likely getting incorporated into noxious clouds.

______

Bibliographic information: Trent J. Dupuy and Adam L. Kraus. Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects. Science, published online September 05, 2013; doi: 10.1126/science.1241917