Brown Dwarf Found Orbiting a Young Sun-Like Star

The range of sizes of a brown dwarf compared to Jupiter and the Sun and the Earth (to scale). Brown Dwarfs are more massive than planets but less massive than stars. But they have similar diameters to planets such as Jupiter. (Source: Jon Lomberg / Gemini Observatory)

The newly imaged brown dwarf is among the youngest stars ever seen. It is located in a tight orbit around a nearby sun-like star, which is huge, with mass about 36 times larger than Jupiter’s (which is the largest planet in our solar system). The brown dwarf is dubbed PZ Tel A and its star-like neighbor is dubbed PZ Tel B. The distance between these stellar objects was measured at 18 Astronomical Units (AU), which is extremely close in comparison to the average 50 AUs. Furthermore, in just the past year the researchers observed PZ Tel B moving quickly outward from its parent star; usually, in such a short span of time stars are much more passive.

Brown dwarves are sub-stellar objects, and unlike most stars they do not maintain hydrogen-burning nuclear fusion reactions in their cores (due to smaller mass). They have fully convective surfaces and interiors, with no chemical differentiation by depth.

This recent finding – imaged using the Near-Infrared Coronagraphic Imager (NICI) on the international 8-meter Gemini-South Telescope in Chile – was part of a joint collaboration. The team was led by Beth Biller and Michael Liu from the University of Hawaii; other researchers involved include Laird Close, Eric Nielsen, Jared Males, and Andy Skemer from the University of Arizona (UA).

In their research, the astronomers compared current state information to data retrieved from an older image, taken seven years ago. The older image was reanalyzed by Laird Close, a professor at UA’s Steward Observatory and the department of astronomy. His results show that PZ Tel B was obscured by the glare from its parent star as recently as 2003, indicating its orbit is more elliptical than circular.

According to the study’s lead author, Beth Biller, "PZ Tel B travels on a particularly eccentric orbit – in the last 10 years, we have literally watched it careen through its inner solar system. This can best be explained by a highly eccentric, or oval-shaped, orbit." Close adds, "Because PZ Tel A is a rare star being both close and very young, it had been imaged several times in the past. So we were quite surprised to see a new companion around what was thought to be a single star."

Although the host star (PZ Tel A) has a similar mass to our sun, it is 400 times younger, with an age of only 12 million years. In fact, the PZ Tel system is young enough to still possess significant amounts of cold circumstellar dust, which may have been sculpted by the gravitational interaction with the young brown dwarf companion.

These unique attributes make the PZ Tel system the focus of many studies, since astronomers wish to understand the early stages of solar system formation. Due to the massive form of PZ Tel B, its orbital motion has significant implications for what type of planets can form – and whether planets can form at

The sun-like star, PZ Tel A and its brown dwarf companion, PZ Tel B. The vast majority of light from PZ Tel A has been removed from this image using specialized image analysis techniques. For size comparison, the size of Neptune’s orbit is shown; PZ Tel B is one of few brown dwarfs imaged at a distance closer than 30 Astronomical Units from its parent star. It travels around its star at a closer distance than Uranus revolves around our Sun. (Source: Beth Biller / Gemini NICI Planet-Finding Campaign)

all – in the PZ Tel system. The short distance between PZ Tel B and its parent star requires special techniques to distinguish the faint light of the companion from the light of the primary star. PZ Tel B is separated by 0.33 arcseconds from PZ Tel A, equivalent to a dime seen at a distance of 7 miles (11 km).

The team used an adaptive optics system coupled to a coronagraph in order to take pictures so close to the star. Their effort was to block out excess starlight; afterwards, they applied specialized analysis techniques to the images to detect PZ Tel B and measure its orbital motion.

The discovery of PZ Tel B is described in a paper being published by Astrophysical Journal Letters, and it wasn’t possible without the instruments NICI offers. "We are just beginning to glean the many configurations of solar systems around stars like the sun. The unique capabilities of NICI provide us with a powerful tool for studying their constituents using direct imaging," said NICI campaign leader Michael Liu.

TFOT has also covered the new images of a baby brown dwarf captured by NASA’s Spitzer Space Telescope, and the “youthful” appearance of stars known as blue stragglers, explained by astronomers from the University of Wisconsin-Madison.

For more information about the discovery of the brown dwarf and its young companion, see the University of Arizona’s press release.