October 3, 2008
A team of University of Hawaii, California Institute of Technology (Caltech) and Massachusetts Institute of Technology astronomers using one of the Keck telescopes on Mauna Kea found evidence for cloudy weather on failed stars.
The star in question, 2M1404B, has a mass of about 3 percent of our Sun and lives with its slightly more massive sibling, 2M1404A, 75 light-years away in the constellation of Centaurus. While thick clouds surround 2M1404A, the cloud layer in 2M1404B seems to be breaking up into patches.
Both failed stars are "brown dwarfs," objects whose mass is between that of large gaseous planets, such as Jupiter, and ordinary stars. These are not normal stars because they are not massive enough to fuse hydrogen, so they cool and fade as they age.
Normally, the more massive a star or brown dwarf is, the more radiation it emits, so the team was surprised to find that 2M1404B emits 60 percent more near-infrared radiation than its higher-mass sibling.
"While smaller discrepancies have been seen in three other brown dwarf pairs, the size of this anomaly is most easily explained by clouds breaking up," says Dagny Looper, a University of Hawaii graduate student and lead author of the study, which has been published in the October 1 issue of the Astrophysical Journal.
After a stable childhood during which it exhausts its deuterium (heavy hydrogen) fuel, a brown dwarf steadily cools until materials such as enstatite (a common mineral) and iron condense to form thick globe-covering clouds. Like smog on Earth, the dust absorbs light, causing a brown dwarf to appear dimmer in the near infrared (at a wavelength of 1.2 micrometers) than it would without the clouds.
But when brown dwarfs cool even further, to temperatures comparable to those observed in 2M1404B (about 1700° Fahrenheit or 900° Celsius), the clouds suddenly disappear. The resulting clear skies cause the brown dwarf to appear brighter at certain wavelengths.
"The process by which the clouds suddenly disappear is still unknown,"
says team member Adam Burgasser, an assistant professor at MIT. "But similar brightenings have been seen in clear patches in Jupiter's cloud decks. There appears to be an interesting connection between the clouds on planets and the clouds on brown dwarfs."
"Like many other brown-dwarf binaries, the separation in the sky between the two components is very small," comments team member Chris Gelino, an astronomer at the Spitzer Science Center at Caltech. "Without the help of adaptive optics systems on large telescopes to lessen the blurring effect from Earth's atmosphere, most of these binaries would go unnoticed, appearing as one fuzzy blob. The adaptive optics system on Keck II is playing a crucial role in our understanding of brown dwarf physics, especially with discoveries like this one."
The twin 33-foot (10-meter) Keck telescopes are the largest optical and near-infrared telescopes in the world. The team used Keck II, which is equipped with an adaptive optics system that uses a laser to create an artificial star by exciting sodium atoms 55 miles (90 km) high, in Earth's mesosphere. Turbulence in Earth's atmosphere distorts the images of both real stars and the artificial star. The amount of distortion for the artificial star is measured several hundred times a second and sent to a flexible mirror that changes its shape to cancel out the effects of the turbulence in the real star. This provides astronomers with clear, sharp images down to a limit of about 0.04 arcseconds, or the diameter of a United States quarter seen at a distance of 78 miles (126 km). This new binary has a separation of only three times this limit. Its physical separation is estimated to be 4 astronomical units, or 4 times the distance between Earth and the Sun.