Closer look at star finds nearby 'dwarf'

UA team revisits spot using special optics, finds overlooked object
2010-09-27T00:00:00Z 2010-09-28T16:11:39Z Closer look at star finds nearby 'dwarf'Victoria Blute Arizona Daily Star Arizona Daily Star
September 27, 2010 12:00 am  • 

University of Arizona astronomer Laird Close understands the value of double-checking your work.

Close, part of a UA-linked astronomy team, found a brown dwarf orbiting a young star even though several other surveys - including his own - originally found nothing around it.

The discovery may help scientists learn how solar systems form in their earliest stages.

Close and the team - led by UA graduate Beth Biller and including UA grad students Eric Nielsen, Jared Males and Andy Skemer - made the discovery with the 8-meter-diameter Gemini South telescope in Chile and a new camera called the Near-Infrared Coronagraphic Imager, or NICI, which uses adaptive optics to create clear images of stars.

They made the discovery in 2009 and confirmed it in May. A paper was published last month.

Adaptive optics work by eliminating the distortion created by turbulence around images of stars. The turbulence is created by hot and cold air mixing in the atmosphere. It's the reason we see stars "twinkling" in the night sky, Close said.

He said viewing sharp images of a star without adaptive optics can be difficult.

"It looks like a nasty, shaking blob," he said. "You've got little things flying all over the place. It doesn't look pointlike at all."

Adaptive optics use a deformable glass mirror that accounts for the tiny focus defects. Close compared the telescope's adaptive optics with the Hubble Space Telescope's and said NICI is more efficient in some ways.

While the Hubble Space Telescope is above the Earth's atmosphere - one way to reduce distortion - its size is a mere 2.4 meters, compared with Gemini South's 8.

Because size and resolution are directly linked, the expense of getting an 8-meter telescope into space to take clear pictures would be huge, he said. Adaptive optics bypass that problem.

The group also used "coronagraph" technology to remove excess starlight, making the brown dwarf easier to see.

The group didn't just stumble on the discovery. In fact, the area involved was highly targeted, Close said. Despite previous studies that revealed PZ Tel A as a single star, a second look with NICI revealed the brown dwarf in its orbit.

"No one ever found that it had anything in orbit around it," he said. "In fact, even I personally have used a telescope in Chile to investigate if it had a companion, and we came up empty."

The group chose to resurvey PZ Tel A because at only 12 million years old, it's a young star - one that would be a good place to find hot, young planets, Close said. The group wanted to look again with NICI to make sure astronomers hadn't missed something.

Brown dwarfs, often referred to as "failed stars," aren't massive enough to go through the process of nuclear fusion as stars do. But they're still hot enough to be seen with infrared cameras, Close said, making them good subjects for NICI.

Close said the telescope was able to detect the pair easily. Not only that, but the brown dwarf, called PZ Tel B, was extremely close to the star when astronomers viewed it from 150 light-years away. (A single light-year is roughly 6 trillion miles.) One astronomical unit is roughly 93 million miles, the distance between the Earth and the sun, and the system's brown dwarf was seen 18 times that distance from its host star.

Close said it's like seeing two dimes side by side from a distance of seven miles.

Another reason the brown dwarf wasn't discovered until now is that it has a highly elliptical orbit, Close said. Instead of a nice, round orbit like Jupiter has around the Sun, the elliptical, oval-shaped nature of the brown dwarf's orbit sometimes obscures it from view as it moves behind the star, he said.

The brown dwarf's odd orbit means that an Earth-type planet in the system is impossible, Close said, because it will pull smaller, inner planets into elongated orbits.

"The PZ Tel system tells us that sometimes nature can form massive brown dwarfs inside young solar systems with very eccentric orbits," he said. Because these systems are rare, finding PZ Tel B tells scientists about different possible solar system formations.

The project was a great way to test the telescope's capabilities, Close said. Additionally, he said, this is the first generation of astronomers to begin understanding something about planets around other stars.

"I think we can all agree it's kind of a rare find," he said.

Close said a takeaway point for astronomers is this: It's good to look at things that have been "done to death."

Astronomy graduate student Andy Skemer agreed. He said objects that appear to be in a system together sometimes aren't what they seem. Some turn out to be background objects rather than one orbiting another, which can be disappointing.

But every once in a while, they find one that turns out to be the real thing.

"If we ever find something, we double-, triple- and quadruple-check it," he said. "It takes a while to gain confidence that it's real. Hopefully, we'll find many more."

Definitions

• Stars create light by fusing hydrogen into helium. They form as a cloud of dust and gas that gets hotter as it contracts. Extreme temperatures and mass are required to create the nuclear fusion seen in stars.

• Planets are made of tiny dust particles left over from stars, and they shine by reflecting light. Because they're made of colliding particles that stick together, they're not hot or massive enough to be stars.

• Brown dwarfs are somewhere between a giant planet and a small star in size, meaning they're not massive enough to fuse hydrogen into helium like a regular star. That's why astronomers often call them "failed stars."

Source: NASA

Contact NASA Space Grant intern Victoria Blute at vblute@azstarnet.com Correction: Jupiter revolves around the Sun, and adaptive optics use a deformable glass mirror that accounts for tiny focus defects. The original article, "Closer look at star finds nearby 'dwarf'" incorrectly stated that Jupiter revolves around the Earth and that adaptive optics use a deformable rubber mirror.

Copyright 2014 Arizona Daily Star. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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