Brown Dwarf Found around Nearby, Sun-Like Star

by Michael Liu

Stars derive their energy from nuclear fusion in their cores, stably burning lighter elements into heavier ones. In the process, energy is released and emitted as radiation, that is, light. However, below a minimum mass, about 8% of the Sun's mass, the center of an object does not become hot enough to ignite this process. Such very low-mass objects are known as brown dwarfs. Without a stable internal energy source, brown dwarfs contain only the energy stored from their formation, presumably as collapsing masses of interstellar gas. This energy supply is steadily released as heat, and in the process these "failed stars" grow ever cooler and fainter. With masses from 15 to 80 times that of the planet Jupiter, brown dwarfs are objects intermediate between stars and planets.

Brown dwarfs were confined to the realm of theoretical speculation until their discovery in 1995. Now, many have been found as isolated objects, free-floating in interstellar space. But very few are found orbiting other stars. While many planets have been found around other stars by radial velocity studies (which search for the very weak wobbling of stars due to an unseen planet), the same studies find almost no brown dwarf companions. However, such studies probe only the inner regions around other stars, inside of 4 AU (1 AU = astronomical unit = distance from the Earth to the Sun = 93 million miles). Little is known about the region outside of 4 AU, the domain of giant planets in our own solar system.

Recently, my colleagues and I discovered a brown dwarf companion orbiting a nearby star similar to the Sun. We used two of the largest telescopes on Mauna Kea to take pictures of this very faint companion. Separated from its parent star by less than the distance between the Sun and the planet Uranus, it is the smallest separation brown dwarf companion seen with direct imaging. This work shows that brown dwarf companions to typical, Sun-like stars exist at a separation comparable to the distance between the Sun and the outer planets in our solar system. The discovery raises puzzling questions about how the brown dwarf formed, and it adds to the surprising diversity of extrasolar planetary systems being found with cutting-edge observational techniques.

The newly found companion, dubbed "15 Sge B," is located in the summer constellation Sagitta (The Arrow), around a star commonly known as 15 Sge. This star is very similar to our Sun (both are G-type stars) and is about 58 light-years from Earth. The projected physical separation of the brown dwarf from its parent star is only 14 AU. For comparison, Saturn orbits the Sun at 10 AU, with Uranus the next planet outward at 19 AU. We estimate the companion's mass at 55 to 78 times the mass of Jupiter. It is probably too massive to have formed the way we believe that planets do, namely from a circumstellar disk of gas and dust when the parent star was young. This finding suggests that a diversity of processes acts to populate the outer regions of other solar systems: the parent star is very similar to our Sun, yet it has a brown dwarf companion whose mass is dozens of times the combined mass of all the planets in our solar system.

Starting in summer 2001, we took very high-resolution pictures of 15 Sge. We first used the 8.1-meter Gemini North Telescope, equipped with a state-of-the-art adaptive optics system designed and built by IfA astronomers. Astronomical images taken from the ground are blurred due to atmospheric turbulence, limiting their resolution. Adaptive optics technology corrects for this blurring in real time. This results in a factor of 10 times improvement in resolution, producing images sharper than even the Hubble Space Telescope. We spotted a very faint object next to 15 Sge, akin to distinguishing a firefly next to a bright searchlight. However, the object could have been a distant star in the background, merely appearing to be close to 15 Sge when projected on the sky.

Over the course of six months, we continued to monitor the star with one of the Keck 10-meter telescopes and found that the faint object moved on the sky along with the primary star, proving the two objects were physically associated. A spectrum of the companion indicated a very cool temperature, characteristic of brown dwarfs. Only by using adaptive optics to produce very sharp images could we have found this companion. It is too faint and too close to its parent star to be seen otherwise.

We are continuing to search for such objects. Now that we know brown dwarfs exist in the region of giant planet formation, we would like to understand how often these oddball pairings occur, and what that can tell us about the alternate and divergent ways in which solar systems form around Sun-like stars.

More images of the 15 Sge system are available on the Web at http://www.ifa.hawaii.edu/~mliu/aas2002.