Wednesday, June 4, 2008

What do pizza and brown dwarfs have in common?

If you brought a pizza to the surface of the sun, it would of course burn to a crisp.

But a brown dwarf could cook it perfectly, browning the edges of the crust, melting the cheese.

(Photo: Infrared image of the binary methane brown dwarfs known as 2MASS 1534-2952AB. Credit: Dr. Michael Liu, Institute for Astronomy, University of Hawaii.)

Brown dwarfs are that class in the continuum of objects that falls somewhere between stars like our sun and the gas giant planets like Jupiter.

And in a remarkable piece of work, researchers at the University of Hawai'i Institute for Astronomy and the University of Sydney have managed to calculate the mass of two pairs of brown dwarfs. (For details, see

They did it using 17th Century astronomy Johannes Kepler's determination that the mass of a paired, or binary, object could be determined if you knew the sized of the orbit and the precise time it takes for the objects to complete an orbit of each other.

That, plus incredibly detailed observations using both the Keck Telescope on Mauna Kea and the Hubble Space Telescope.

The team included Michael Liu and Trent J. Dupuy of the University of Hawai'i and Michael J. Ireland of the University of Sydney.

Brown dwarfs are generally bigger than planets and smaller than suns. They're hot, but not nearly as hot as the roughly 10,000-degrees of the sun. They're closer to the range of 800 degrees Fahrenheit—like a nice, hot pizza oven.

Technically, brown dwarfs are big objects smaller than 7 percent the mass of the sun. Like gas giant planets, they lack the necessary mass to launch nuclear fusion, and thus can't generate their own heat.

The Liu team studied two separate brown dwarf pairs, and was able to calculate that the mass of one pair (2MASS 1534-2952AB) is about 6 percent of the mass of the sun, and the mass of the other (HD 130948BC) is about 11 percent that of the sun.

That's still big. Jupiter is our solar system's biggest planet, and each of the planets in the smaller of the two pairs is still 30 times the mass of Jupiter.

Astronomers have long been able to calculate temperatures, distance and brightness of distant objects. A key value of this work, Liu said, is that it helps correct theoretical calculations of the mass of objects in space.

"Mass is the fundamental parameter that governs the life-history of a free-floating object...After weighing these tiny, dim, cold objects, we have confirmed that the theoretical predictions are mostly correct, but not entirely so,” he said.

One of the things that made the calculations possible is the adaptive optics on the Keck II telescope, which allows astronomers to correct for distortions of the Earth's atmosphere, and to obtain amazingly clear images of very distant objects.

"These are very challenging measurements, because brown dwarf binaries have tiny separations on the sky and orbit each other very slowly. We needed to obtain the sharpest measurements that are possible with current telescopes to precisely monitor their motion," Dupuy said.

The researchers unearthed a couple of new mysteries. Among them: temperature and energy output calculations for the two binaries don't match the theories of what they should be.

"These findings will be a challenge for the theorists, and we are inspired to measure the masses of more brown dwarfs in the coming years to better understand the problem."

The team's research was supported by the National Science Foundation and the Alfred P. Sloan Foundation.

© 2008 Jan W. TenBruggencate