Very Cool Brown Dwarf Discovered Near the Sun

Lori Stiles
March 17, 2006

Astronomers have discovered a unique "brown dwarf" right in our solar neighborhood.

If your city were the galaxy, it would be like finding someone you didn't know about living upstairs in your house, one of the discoverers said.

The rare object is only 12.7 light years from Earth, circling a primary star that itself was discovered only recently in the southern hemisphere constellation Pavo (the Peacock).

Only one other brown dwarf system has been found closer to Earth, and it's only marginally closer.

The primary star is only one-tenth the mass of our sun. This is the first time astronomers have found a cool brown dwarf companion to such a low-mass star. Until now, none has been found orbiting stars less than half the mass of our sun.

The brown dwarf is 4.5 AU from the star, or four and one-half times farther from its star than Earth is from our sun. Astronomers estimate that the brown dwarf is between nine and 65 times as massive as Jupiter.

Brown dwarfs are neither planets nor stars. They are dozens of times more massive than our solar system's largest planet, Jupiter, but too small to be self-powered by hydrogen fusion like stars.

Only about 30 similarly cool brown dwarfs have been found anywhere in the sky, and only about 10 have been discovered orbiting stars. "Besides being extremely close to Earth and in orbit around a very low-mass star, this object is a 'T dwarf ' - a very cool brown dwarf with a temperature of about 750 degrees Celsius (1,382 degrees Fahrenheit)," said Beth Biller, a graduate student at The University of Arizona.

"It is also likely the brightest known object of its temperature because it is so close," Biller said. "And it's a rare example of a brown dwarf companion within 10 astronomical units of its primary star."

Biller, along with Markus Kasper of the European Southern Observatory (ESO) and Laird Close of UA's Steward Observatory, led the team who discovered the brown dwarf, designated SCR 1845-6357B.

"What's really exciting about this is that we found the brown dwarf around one of the 25 stellar systems nearest to the sun," Close said. "Most of these nearby stars have been known for decades, and only just recently a handful of new objects have been found in our local neighborhood."

Close said, "If you think of the galaxy as being the size of Tucson, it's kind of like finding someone living in the upstairs of your house that you didn't know about before."

Close helped develop the special adaptive optics camera, the NACO Simultaneous Differential Imager(SDI), that the team used to image the brown dwarf. The camera is used on ESO's Very Large Telescope (VLT) in Chile. Another SDI camera is used at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz.

"This is also a valuable object to the scientific community because its distance is well known," said ESO's Markus Kasper. This will allow astronomers to measure the brown dwarf's luminosity accurately and, eventually, to calculate its orbital motion, Kasper said. "These properties are vital for understanding the nature of brown dwarfs."

The discovery of this brown dwarf suggests there may be more cool brown dwarfs in binary systems than single brown dwarfs floating free in the solar neighborhood, Close said. A "binary system" is where a brown dwarf revolves around a star or another brown dwarf.

Astronomers now have found five cool brown dwarfs in binary systems but only two single, isolated cool brown dwarfs within 20 light years of the sun, Close noted. They can expect to find more T dwarf companions in some newly found stellar systems within 33 light years of our solar system, he added.

Evidence that T dwarfs in binary systems outnumber single, isolated T dwarfs in the solar neighborhood has ramifications for theories that predict single brown dwarfs will form more often than binary ones, Close said.

The NACO Simultaneous Differential Imager(SDI) uses adaptive optics to remove the blurring effects of Earth's atmosphere to produce extremely sharp images. The camera enhances the ability of the VLT to detect faint companions that would otherwise be lost in the glare of their primary stars.

Close and Rainer Lenzen of the Max Planck Institute for Astronomy in Heidelberg, Germany, developed the SDI camera to search for methane-rich extrasolar planets. The SDI camera splits light from a single object into four identical images, then passes the beams through three slightly different methane-sensitive filters. When the filtered light beams hit the detector array, astronomers subtract the images so the bright star disappears and its far dimmer, methane-rich companion pops into view. The team will publish the discovery in the Astrophysical Journal Letters in the article, "Discovery of a Very Nearby Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357." In addition to Biller, Kasper and Close, team members include Wolfgang Brandner of the Max Planck Institute in Heidelberg, Germany, and Stephan Kellner of the W.M. Keck Observatory in Waimea, Hawaii.

High-res images:
Figure 1
Figure 2
Figure 3

Figure Captions:
Fig.1: 3 color image of SCR1845-6357AB generated from the SDI filterimages (blue=1.575 um, green=1.600 um, red=1.625 um). Thesubstellar companion appears blue in this image and is roughly 50 timesfainter than the primary. It is separated by an angle of 1.17 arcseconds (4.5 times the Earth-Sun distance) on the sky from its primary star.

Fig.2: A movie of the SDI camera's images of the low mass star SCR1845A and B. Note how the brown dwarf companion (B) is clearly changingin brightness in and out of the SDI methane filters. This proves that Bis metane-rich and a rare cool low-mass brown dwarf companion. Moreover,it is also clear that the "halo of speckles" around the much hotterprimary star do not change significantly in the three SDI filters. Hencethe star's halo can be removed by subtracting the images from eachother, hence detection of faint methane-rich companions (even 10 timescloser than B) is made possible with the SDI camera.

Fig.3: 3-D map of all known stellar systems within 12.7 light years from thesun. SCR 1845-6357 appears towards the bottom right hand corner of the image. This map was adapted from images by Richard Powell


Resources for the media

Beth Biller, UA
Markus Kasper
ESO Garching, Germany
Laird Close, UA