Star of Extreme Colors is Cool

Astronomers discover cool stars in nearby space

These images show the cool brown dwarf SDSS1416B and its warmer companion brown dwarf SDSS1416A (the left image is from the UKIRT telescope and the right image from the Subaru telescope). Credit: RAS

An international team, led by astronomers at the University of Hertfordshire, has discovered what may be the coolest sub-stellar body ever found outside our own solar system. Using the United Kingdom Infrared Telescope (UKIRT) in Hawaii, a discovery has been made of an object which is technically known as a brown dwarf. The team’s findings have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

What has excited astronomers are its very peculiar colors, which actually make it appear either very blue or very red, depending on which part of the spectrum is used to look at it.

The object is known as SDSS 1416+13B and it is in a wide orbit around a somewhat brighter and warmer brown dwarf, SDSS 1416+13A. The brighter member of the pair was detected in visible light by the Sloan Digital Sky Survey. By contrast, SDSS 1416+13B is only seen in infrared light. The pair is located between 15 and 50 light-years from the solar system, which is quite close in astronomical terms.

Brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to ignite core hydrogen fusion.
Credit: American Scientist/Linda Huff

“This looks like being the fourth time in three years that the UKIRT has made a record breaking discovery of the coolest known brown dwarf, with an estimated temperature not far above 200 degrees Celsius,” said Dr. Philip Lucas at the University of Hertfordshire’s School of Physics, Astronomy and Mathematics.

“We have to be a bit careful about this one because its colors are so different than anything seen before that we don’t really understand it yet. Even if it turns out that the low temperature is not quite record breaking, the colors are so extreme that this object will keep a lot of physicists busy trying to explain it.”

SDSS 1416+13B was first noticed by Dr. Ben Burningham of the University of Hertfordshire as part of a dedicated search for cool brown dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS). The object appeared far bluer at near infrared wavelengths than any brown dwarf seen before. A near infrared spectrum taken with the Japanese Subaru Telescope in Hawaii showed that it is a type of brown dwarf called a T dwarf, which has a lot of methane in its atmosphere, but with peculiar features including a big gap at certain wavelengths.

The United Kingdom Infra-Red Telescope is sited in Hawaii near the summit of Mauna Kea at an altitude of 4194m above sea level.
Credit: United Kingdom Infra-Red Telescope

Dr. Burningham soon noticed that a previously observed brighter star (SDSS 1416+13A) which appears close by in the UKIDSS discovery image was also a brown dwarf. Team member Dr. Sandy Leggett, of the Gemini Observatory, then used the orbiting Spitzer Space Telescope to investigate SDSS 1416+13B at longer wavelengths. She measured its color at mid-infrared wavelengths, which are thought to be the most reliable indicator of temperature, and found that it is the reddest known brown dwarf at these wavelengths by some margin. Comparison with theoretical models of the brown dwarf atmospheres then provided a temperature estimate of about 500 Kelvin (227 degrees Celsius).

“The fact that it is a binary companion to a warmer brown dwarf that also has an unusual spectrum is helping us to fill in some gaps in our understanding”, says Dr. Burningham. “It seems likely that both brown dwarfs are somewhat poor in heavy elements. This can be explained if they are very old, which also fits with the very low temperature of the faint companion.”

Too small to be stars, brown dwarfs have masses smaller than stars but larger than gas giant planets like Jupiter. Due to their low temperature these objects are very faint in visible light, and are detected by their glow at infrared wavelengths. They were originally dubbed “brown dwarfs” long before any were actually discovered, to describe the idea of bodies that were cooler, fainter and redder than red dwarf stars, with the color brown representing the mix of red and black.