Failing as a Cool Star
|Click here for larger image. Brown dwarfs, such as Gliese 229B, lack sufficient mass (at least 75-80 Jupiters) to ignite core hydrogen fusion.
Credit: American Scientist/Linda Huff
At the 13th Cambridge Workshop on "Cool Stars, Stellar Systems, and the Sun," Dr. Kevin L. Luhman (Harvard-Smithsonian Center for Astrophysics) announced the discovery of a unique pair of newborn brown dwarfs in orbit around each other. Brown dwarfs are a relatively new class of objects discovered in the mid-1990s that are too small to ignite hydrogen fusion and shine as stars, yet too big to be considered planets.
"Are brown dwarfs miniature failed stars, or super-sized planets, or are they altogether different from either stars or planets?" asks Luhman. The unique nature of this new brown dwarf pair has brought astronomers a step closer to the answer.
One possible explanation for the origin of brown dwarfs is that they are born in the same way as stars. Stars form in huge interstellar clouds in which gravity causes clumps of gas and dust to collapse into "seeds," which then steadily pull in more and more material until they grow to become stars. However, when this process is studied in detail by computer, many simulations fail to produce brown dwarfs. Instead, all the seeds grow into full-fledged stars. This result led some astronomers to wonder if brown dwarfs and stars are created in different ways.
|The brown dwarf LP 944-20 (Digital Sky Survey).
Credit: European Southern Observatory
"In one alternative that has been proposed recently," explains Luhman, "the seeds in an interstellar cloud pull on each other through their gravity, causing a slingshot effect and ejecting some of the seeds from the cloud before they have a chance to grow into stars. These small bodies are what we see as brown dwarfs, according to that hypothesis."
Testing these ideas for the birth of brown dwarfs is hampered by the fact that brown dwarfs are normally extremely faint and hard to detect in the sky. For most of their lives, they are not hot enough to ignite hydrogen fusion, so they do not shine brightly like stars, and instead are relatively dim like planets. However, for a short time immediately following their birth, brown dwarfs are relatively bright due to the leftover heat from their formation.
As a result, brown dwarfs are easiest to find and study at an age of around 1 million years, which is newborn compared to the 4.5 billion year age of our Sun. Taking advantage of this fact, Luhman searched for newborn brown dwarfs in a cluster of young stars located 540 light-years away in the southern constellation of Chamaeleon. Luhman conducted his search using one of the two 6.5-meter-diameter Magellan telescopes at Las Campanas Observatory in Chile, which are among the largest telescopes in the world.
Of the two dozen new brown dwarfs found, most were isolated and floating in space by themselves. However, Luhman discovered one pair of brown dwarfs orbiting each other at a remarkably wide separation. All previously known pairs of brown dwarfs are relatively close to each other, less than half the distance of Pluto from the Sun. But the brown dwarfs in this new pair are much farther apart, about six times the distance of Pluto from the Sun.
Because these brown dwarfs are so far apart, they are very weakly bound to each other by gravity, and the slightest tug would permanently separate them. Therefore, Luhman concludes, "The mere existence of this extremely fragile pair indicates that these brown dwarfs were never subjected to the kind of violent gravitational pulls that they would undergo if they had formed as ejected seeds. Instead, it is likely that these baby brown dwarfs formed in the same way as stars, in a relatively gentle and undisturbed manner."
Dr. Alan P. Boss (Carnegie Institution) agrees, stating, "Luhman’s discovery strengthens the case for the formation mechanism of brown dwarfs being similar to that of stars like the Sun, and hence for brown dwarfs being worthy of being termed ‘stars,’ even if they are too low in mass to be able to undergo sustained nuclear fusion." The discovery of this binary brown dwarf will be published in a forthcoming issue of The Astrophysical Journal. The discovery paper currently is online in PDF format .
Since the discovery of brown dwarfs less than a decade ago, astronomers have come to think that they might be more numerous than the visible stars in the sky. In broad terms, 80 percent of the nearby stars are (red) dwarfs, 10 percent are solar-type stars, and 10 percent are more massive. There are probably around the same number of brown dwarfs as stars within the immediate solar neighborhood.
The true abundance of brown dwarfs, sub-brown dwarfs and extrasolar planets is not known, and large areas of the sky still need to be explored. Most of the brown dwarfs have been located by the Two Micron All Sky Survey (2MASS), although the Sloan Digital Sky Survey (SDSS) and the planet-finding Doppler technique also have been used to find brown dwarfs. Reid is completing a census of low-mass stars and brown dwarfs in the immediate solar neighborhood.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe. The Magellan telescopes are operated by the Carnegie Institution of Washington, the University of Arizona, Harvard University, the University of Michigan, and the Massachusetts Institute of Technology. Las Campanas Observatory is operated by the Carnegie Observatories, which was founded in 1904 by George Ellery Hale. It is one of six departments of the private, nonprofit Carnegie Institution of Washington, a pioneering force in basic scientific research since 1902.