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New Mexico State University

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NMSU astronomy student gets time on world's largest optical telescope

Denise Stephens, a doctoral student in astronomy at New Mexico State University, will use one of the world's largest telescopes to examine a number of recently discovered astronomical objects known as brown dwarfs.

Stephens and NMSU astronomer Mark Marley, her adviser, will visit the Keck Observatory in Hawaii in January for three nights of observing on one of Keck's two 10-meter telescopes, the biggest optical telescopes in the world.

They will observe and collect data on about 20 recently discovered brown dwarfs -- objects that are too massive to be planets but too cool to be stars. Then, with collaborator Keith Noll of the Space Telescope Science Institute, they will take a closer look at those that appear to be in a particular temperature range.

The project will not only shed light on the nature of brown dwarfs, said Marley, it also will help scientists prepare for observing planets beyond our solar system. "Her technique of looking at brown dwarfs is a new one that will give insight into what extra-solar planets are going to look like when we start seeing them," he said.

Because NASA is interested in detecting planets around stars other than our sun, the NMSU project won a portion of NASA's time on the Keck telescope in a highly competitive process. NASA controls less than 20 percent of the telescopes' time and the rest belongs to astronomers at the California universities that are majority owners of the observatory.

Brown dwarfs are about the same size as Jupiter, the largest planet in our solar system, but they are much denser in mass. They are sometimes referred to as "failed stars."

"Stars are created when a cloud composed of gas and dust collapses," Stephens said. "If the resulting star contains enough mass, the release of gravitational potential energy during the collapse will heat the central core of the star until it is hot enough to produce fusion. A brown dwarf is formed in the same way as a star except that it does not have enough mass to fuse hydrogen in its core like our sun and other stars do."

The existence of brown dwarfs was theorized for years, but the first actual specimen wasn't discovered until 1994. Since then, with the advent of new observing methods and two all-sky surveys in the infrared spectrum where brown dwarfs are most readily detected, nearly 100 objects believed to be brown dwarfs have been discovered, Stephens said.

Extra-solar planets are an even more recently detected class of object, and none of them has actually been observed. "Even though we know there are planets around other stars, no one has ever seen one or taken a picture of one," Marley said.

From what astronomers have been able to learn about them so far, it appears that brown dwarfs share many characteristics with the so-called giant planets, like Jupiter in our solar system and the planets orbiting other stars. Learning more about brown dwarfs "can tell us more about the formation and evolution of these planets," Stephens said.

The brown dwarf study that Stephens and her collaborators will undertake in January is the first of its kind. They will observe the objects in the near-infrared part of the spectrum -- invisible to the eye -- to determine their temperatures. Analyzing spectrums of light given off by distant objects is one way astronomers learn what they are made of and what their temperatures are.

"Brown dwarfs have a large amount of dust in their atmospheres and that causes difficulty when trying to determine their temperatures using the visible part of the spectrum," Stephens explained. "We are looking in the near infrared, where dust has a minimal impact, to determine temperatures."

Although brown dwarfs are cooler than stars, their temperatures vary greatly. The first two nights of observing at Keck will allow the researchers to determine which specimens are in the cool range. Then with collaborator Noll, they will use spectroscopy to look for methane in the atmospheres of those specimens. The presence of methane will help the researchers pin down the temperatures more exactly.

"We are building atmospheric models for these objects and if we know the temperature we can adjust our current models," Stephens said.