Astronomers Are Using "Light Echoes" to Solve a Mystery of Protoplanetary Disks

April 28, 2016 | Joanne Kennell

Star surrounded by a protoplanetary disk
Photo credit: NASA/JPL-Caltech

A process that took just 74 seconds.

Imagine that you are in a pitch-black room and want to know what size the space is. If you shout, you can tell if the space is big or small, depending on how long it takes to hear the echo after it bounces off a wall.

Astronomers use this same principle to study objects that are too distant to see. Particularly, researchers are interested in calculating how far young stars are from the inner edge of their protoplanetary disks — disks of gas and dust surrounding a young star where planets form over the course of millions of years.

"Understanding protoplanetary disks can help us understand some of the mysteries about exoplanets, the planets in solar systems outside our own," said Huan Meng, postdoctoral research associate at the University of Arizona and lead author of the study, in a press release. "We want to know how planets form and why we find large planets called 'hot Jupiters' close to their stars."

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The new study, published in the Astrophysical Journal, used data from NASA’s Spitzer Space Telescope and four ground-based telescopes to determine the distance between a star and the inner rim of its surrounding protoplanetary disk.

When a young star surrounded by its protoplanetary disk brightens, some of the light hits the disk causing a delayed "echo." Scientists measure the time it takes for the light coming directly from the star to reach Earth, and then wait for the echo to arrive. This method is called photo-reverberation, or "light echoes."

Since light travels at a constant speed (186,282 miles per second), scientists can determine distance by multiplying the speed by the time it takes for light to get from one point to another. However, in order to be able to use this rather simple equation, astronomers have to find a star with variable emissions — a star the emits radiation unpredictably — and young stars are the best candidates.

The star used in this study is called YLW 16B. It lies about 400 light-years from Earth and is roughly the same mass as our sun at just one million years old.

Combining Spitzer data with observations from the ground-based telescopes, researchers saw consistent time lags between the star brightening and echoes in the surrounding disk. The researchers then calculated how far this light must have traveled during the time lag — about 0.08 astronomical units, or eight percent of the distance between Earth and the sun. Although this value was slightly smaller than previous estimates, but still consistent with theoretical expectations, it only took the researchers 74 seconds to measure the light echo.

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"Knowing the exact position of the inner boundary of a protoplanetary disk is important to anyone who wants to understand planet evolution," Meng said.

Most stars are born with a protoplanetary disk around them, and there is a gap between the star and its disk due to two competing processes: Close to the star, radiation ionizes gas particles in the disk, diverting them along the star’s magnetic field lines above and below the plane of the disk. The other mechanism is the star’s surface heat. Once a particle gets too close to the star, it vaporizes by either falling onto the star or getting blown out of the system.

"The predominant one of those two mechanisms plays an important role in the evolution of the disk, and right now, we don't know which it is," explained Meng.

Currently, astronomers use a technique called interferometry to locate the position of the inner edge of protoplanetary disks, however it requires assumptions about the shape of the disk which often result in controversial findings.

According to the researchers, this new and fast approach can be applied to numerous systems of stars with protoplanetary disks surrounding them.

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