Ancient Brown Dwarfs
November 21, 2013

The Galaxy’s Ancient Brown Dwarf Population Revealed

John P. Millis, PhD for - Your Universe Online

The galaxy is a diverse place, containing stars, planets, nebulous gas, black holes, and all matter of other objects, some of which we still do not understand. But among this vast array sits an object that is neither a star, nor a planet. These brown dwarfs have masses only a fraction of that of our Sun, but are still at least 13 times more massive than Jupiter.

But because of the lack of substantial fusion, the surface of these objects is quite cool (250 – 600 K) compared to the surface of the Sun (around 5,600K). Coupled with their small size – nearly identical to that of Jupiter – brown dwarfs are difficult to find. Usually, intense infrared studies are undertaken to isolate these objects, but because of the plethora of other infrared objects in our galaxy and beyond, locating and measuring these “failed stars” is difficult.

Now, new research has found the two oldest brown dwarfs in the galaxy, and they have a somewhat unexpected property: they move extremely fast. A team of astronomers, led by Dr. David Pinfield from the University of Hertfordshire, found two brown dwarfs located in the constellations Pisces (WISE 0013+0634) and Hydra (WISE 0833+0052) using NASA’s Wide-field Infrared Survey Explorer (WISE). The data, which represents the mid-infrared sky, was taken during 2010 and 2011.

These two objects were found to contain almost no heavy elements. Since nearly all elements heavier than hydrogen and helium are created in the cores of massive stars and during supernovae, older objects, which predate most of the galactic population, would lack these heavier elements and have a composition resembling the early Universe. The team therefore concluded that these brown dwarfs were at least 10 billion years old.

The peculiar thing about the two new objects, however, is their speed. They are moving at nearly 200 kilometers per second, much faster than other brown dwarfs that have been observed. "Unlike in other walks of life, the Galaxy's oldest members move much faster than its younger population," said Pinfield.

Of course, the only way to know for sure is to build up the data set. With hundreds of billions of brown dwarfs thought to exist in the Milky Way galaxy alone, continued study is needed to piece together how and when these objects formed. Furthermore, it seems that brown dwarfs may contain different characteristics depending on what part of the galaxy they live.

"These two brown dwarfs may be the tip of an iceberg and are an intriguing piece of astronomical archaeology", said Pinfield. "We have only been able to find these objects by searching for the faintest and coolest things possible with WISE. And by finding more of them we will gain insight into the earliest epoch of the history of the Galaxy."