11/29/1995 08:00:00

Astronomers Announce Discovery of an Old Brown Dwarf

PASADENA—Astronomers at the California Institute of Technology and the Johns Hopkins University today announce the discovery of what they believe is a brown dwarf, and release the first image and spectrum ever taken of this elusive type of object.

The brown dwarf, called GL 229B, lies in the southern-hemisphere constellation Lepus, near Orion, where it orbits a small, dim star called GL 229. This is the first detection of such a cool object outside the solar system.

The scientists will discuss their results on Wednesday, November 29, at noon EST during a Space Astronomy Update at NASA Headquarters in Washington, D.C. The results will also appear in the November 30 issue of the journal Nature and the December 1 issue of the journal Science. The discovery was discussed at a Caltech colloquium and in October at the Florence Cool Stars meeting in Florence, Italy, but this marks the first publication of the results and the first release of the brown dwarf's image and spectrum.

Brown dwarfs are objects that astronomers have long theorized must exist, but for which proof has been indirect and never 100 percent convincing. They are formed from the same gaseous material as stars, but are much less massive. Current stellar models agree that the upper limit to the mass of brown dwarfs is about one-twelfth the mass of the sun.

When objects are more massive than this limit, the energy released by the contracting gas generates enough heat to ignite and sustain nuclear fusion. Fusion combines hydrogen atoms to form helium and releases tremendous amounts of energy as light, heat, and other types of radiation. Objects powered by fusion are called stars.

But when objects are less massive than about one-twelfth the mass of the sun, their cores never get hot enough to sustain fusion. These objects are called brown dwarfs. While young brown dwarfs can obtain enough energy from gravitational contraction to be quite bright, this source of energy isn't nearly as long lasting or as powerful as fusion. So brown dwarfs fade rapidly as they radiate away their gravitational energy. Later they radiate by means of their meager internal heat, and are much cooler, dimmer, and harder to see than stars.

While scientists concur that brown dwarfs and stars are made of the same stuff, it is not clear that they're made in the same way. Stars form by direct condensation of interstellar gas, while planets are thought to form by condensation of material within proto-planetary disks that form around stars. Brown dwarfs are intermediate in size between small stars and large planets, and could theoretically form in either manner. Astronomers want to find brown dwarfs for two reasons. First, they want to determine the smallest-mass object that can form by condensation of interstellar gas clouds, in the manner of stars, and whether enough of these hard-to-detect objects exist to comprise a dominant component mass in our galaxy. That is, could brown dwarfs solve a difficult cosmological puzzle by accounting for a significant portion of the missing "dark matter?" Second, astronomers want to study the atmospheres of brown dwarfs and learn how they are related to the atmospheres of planets. Such understanding is important to the search for other planetary systems. Because of the importance of brown dwarfs both to cosmology and to finding other planets, astronomers have made a considerable effort to find the objects, especially young brown dwarfs because they are still hot, relatively bright, and more easily seen. Young brown dwarfs are most likely to appear in star clusters, the "nurseries" where stars form. The Caltech/Johns Hopkins team has tried a different approach to finding brown dwarfs over the past few years. Caltech astronomers Shri Kulkarni, Tadashi Nakajima, Keith Matthews, and Ben Oppenheimer have been collaborating in the search with Johns Hopkins scientists Sam Durrance and David Golimowski. Instead of scouring stellar nurseries for young brown dwarfs, they looked for older, cooler brown dwarfs as companions to stars within our local neighborhood, within 45 light-years, or about 265 trillion miles, of the sun. These are not young stars, but have ages as large as 10 billion years, with an average age of 5 billion years.

There are two advantages to searching for these older, nearer brown dwarfs instead of young ones. First, scientists know the distances to nearby stars with good accuracy, so when they identify a brown dwarf candidate, they can immediately calculate its total luminosity. The lowest luminosity of any normal, hydrogen-fusing star is one ten-thousandth that of the sun. But brown dwarfs, especially those more than one billion years old, can have much lower luminosities. Searching for an object with a luminosity less than this limit is a very simple method of unambiguously detecting brown dwarfs.

Second, the minimum temperature of a star is about 1800 K, while old brown dwarfs can have much lower temperatures. This makes old brown dwarfs interesting to planetary scientists, since their cool atmospheres are similar to those of the giant gaseous planets in the solar system. For instance, it is well known that prominent absorption features are seen in the spectrum of Jupiter that are never seen in the spectrum of any star. Takashi Tsuji of the University of Tokyo in Japan has found that below 1000 K, carbon prefers to attach to hydrogen and form methane, CH4, not the more usual carbon monoxide, CO, seen in cool stars. So methane absorption lines in a spectrum is a sure sign of low temperature.

Last year the Caltech/Hopkins team started the "Byr survey," a survey of stars with ages of near a billion years. As the first step, the astronomers made an image of each of the stars they wanted to study with a "coronagraph," a camera with the ability to see faint objects in the glare of bright stars. The coronagraph blocks light from the star so that dimmer nearby objects become visible, much like a solar coronagraph blocks out the sun to allow astronomers to see the relatively faint flares and explosions on the sun's surface.

This coronagraph, used at optical wavelengths, was made by the Johns Hopkins team and has been used extensively at the 60-inch telescope at Caltech's Palomar Observatory. A similar device built by Matthews, but which detects infrared wavelengths, (1 to 2.5 microns), has been commissioned recently at the 200-inch Hale Telescope at Palomar.

The astronomers looked at each star twice, at an interval of one year. All stars move relatively quickly across the sky, so true companion stars will move the same distance and remain together after a year. But unrelated objects that line up by chance and only seem to be companions will have different motions and will drift apart over a year. The astronomers identified brown dwarf candidates on the 60-inch telescope and examined them more closely with the 200-inch Hale Telescope.

This method paid off when the scientists found a faint companion to the star GL 229, an M1-type star at a distance of 17 light-years, or about 100 trillion miles. They found that the companion had moved the same amount as GL 229 in a year, and concluded that the two must be in orbit around each other. Using the known distance to GL 229, they calculated the luminosity of the companion to be only 7 millionths that of the sun, almost 10 times less than the faintest known star. The presence of water, seen in the absorption lines of the spectrum, shows that the surface temperature is very cool, less than 1000 K, or 800 K lower than the coolest known star. They also detected methane absorption lines in its spectrum, confirming Tsuji's prediction that methane is found only in cold objects.

This discovery, the first of a cool brown dwarf, is an important first step in the search for planetary systems beyond the solar system. The strange colors of the object, extremely red in the optical wavelengths and blue in the near-infrared, and the presence of methane suggest new strategies to search for brown dwarfs and massive Jupiter-like planets. The spectra of faint objects could be screened for these unusual characteristics, allowing astronomers to concentrate on the most likely brown dwarf candidates.

The brown dwarf is 44 astronomical units, or about 4 billion miles, from the main star and has a mass roughly 20 times that of Jupiter. It is unclear whether it was formed like a star, by direct condensation of interstellar gas, or like a planet, by condensation of material within a protoplanetary disk that formed around the star. As brown dwarfs get less massive, it is increasingly difficult to explain how they could form like stars, since that process requires smaller and smaller condensations caused by ever larger pressures.

But explaining the formation of larger, Jupiter-type planets is also difficult. Thus there is a new threshold between small brown dwarfs and large gaseous planets, where one method of formation stops and the other takes over. The theories of the Caltech/Johns Hopkins team are rudimentary at this point, but the proximity of the companion to the parent star suggests that it was formed in a planetary disk rather than directly from the interstellar medium. They are continuing to observe this system and have obtained images and spectra of GL 229B using the 200-inch Hale Telescope at the Palomar Observatory, the 10-meter telescope at the Keck Observatory atop Mauna Kea, Hawaii, and the Hubble Space Telescope.

Contact: Jay Aller (818) 395-3631 aller@caltech.edu

Written by John Avery