Eclipsing brown dwarfs provide new key to the star formation process

Pity the brown dwarf. It’s too large to be a planet, but too small to be a star.

Although these “failed stars” are neither fish nor fowl, they play an important role in the cosmic scheme of things. Many astronomers think that they may actually be the most common product of the stellar formation process. So information about brown dwarfs can provide valuable new insights into the dynamic processes that produce stars out of collapsing whirlpools of interstellar dust and gas.

Because brown dwarfs are smaller and dimmer than true stars, it is only in recent years that improvements in telescope technology have allowed astronomers to catalog hundreds of faint objects that they think may be brown dwarfs. But to pick out the brown dwarfs from other types of faint objects, they need a way to estimate their masses, because mass is destiny for stars and star-like objects.

That is the reason why the discovery of an eclipsing pair of brown dwarfs in the Orion Nebula – reported in the March 16 issue of the scientific journal Nature – is important: It provides the first direct measurement of the mass, size and surface temperature of this type of object, information that will help astronomers better estimate the masses of the faint objects that they have found.

Moreover, the observations provide this critical information about a pair of brown dwarfs that are only a few million years old. The new observations confirm the theoretical prediction that brown dwarfs start out as star-like objects but shrink and cool and become increasingly planet-like as they age. Before now, the only brown dwarf whose mass has been directly measured was much older, dimmer and planet-like.

“This binary pair is a ‘Rosetta stone’ that will help unlock many of the mysteries regarding brown dwarfs,” says Keivan Stassun, assistant professor of astronomy at Vanderbilt University, who led the team of astronomers who made the new observations. “We understand how stars form in the crudest sense: They are formed when clouds of dust and gas collapse. But many of the details of the process remain a mystery, particularly the factors that determine what a star will weigh.”

The researchers made the observations with two sets of telescopes located in the Chilean Andes, about 100 miles north of Santiago: the Small and Moderate Aperture Research Telescope System (SMARTS) at Cerro Tololo Inter-American Observatory, operated by a consortium including Vanderbilt, and the International Gemini Observatory operated by the National Science Foundation.

As a category the brown dwarf is quite new. The existence of such failed stars was first proposed in the 1980’s, but it wasn’t until 2000 that a brown dwarf was detected unambiguously. While brown dwarfs were hypothetical objects, astronomers differentiated them from planets by the manner in which they formed: Brown dwarfs are formed the way a star is, from a collapsing cloud of interstellar dust and gas whereas planets are built up from the disks of dust and gas that surround forming stars. Once the first candidate brown dwarf was found, however, astronomers realized that they are very difficult to tell from planets, particularly when they have stellar companions. So a growing group of astronomers favor defining brown dwarfs as objects that range from 13 to 80 times the mass of Jupiter.

While participating in a survey of the Orion nebula, a stellar nursery only 1,500 light-years from earth, Stassun and his collaborators – professor Robert Mathieu, University of Wisconsin; and astronomer Jeff Valenti, Space Telescope Science Institute – found something that had never been seen before: a pair of brown dwarfs orbiting each other around an axis perpendicular to the line of sight to Earth.

The pair orbit each other so closely that they look like a single object when viewed from Earth. Because of their special orientation, however, the two objects periodically eclipse each other. These eclipses cause regular dips in the brightness of the light coming from their joint image. By precisely timing these occultations the astronomers were able to determine the orbits of the two objects. This information, along with Newton’s laws of motion, allowed Stassun’s team to calculate the mass of the two dwarfs.

“One is 55 times the mass of Jupiter and the other is 35 times Jupiter’s mass. The margin of error is only 10 percent, so they are clearly brown dwarfs,” Stassun reports.

In addition, the astronomers also were able to measure the size of the two dwarfs by measuring the width of the dips in their light curve. They prove to be remarkably large for their mass: about the same diameter as the sun. Because the pair are located in the Orion stellar nursery, the astronomers know that they are very young, less than 10 million years old. So their large sizes support the theoretical contention that brown dwarfs are quite star-like when they are created.

According to Stassun, an analysis of the light coming from the dwarf pair indicates that they have a reddish cast. Current models also predict that brown dwarfs should have “weather” – cloud-like bands and spots similar to those visible on Jupiter and Saturn.

By measuring variations in the light spectrum coming from the pair, the astronomers were also able to determine their surface temperatures. Theory predicts that the more massive member of a pair of brown dwarfs should have a higher surface temperature. But they found just the opposite. The heavier of the two has a temperature of 2,650 degrees Kelvin (4,310 degrees Fahrenheit) and the smaller is 2,790 degrees K (4,562 degrees F). These compare to the sun’s surface temperature of 5,900 degrees K (9,980 degrees F).

“One possible explanation is that the two objects have different origins and ages,” says Stassun. If that is the case, then it supports one of the outcomes of the latest efforts to simulate the star formation process. These simulations predict that brown dwarfs are created so close together that they are likely to disrupt each other’s formation.

This research was funded by the National Science Foundation.