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Astronomers can use light echoes to measure the distance from a star to
its surrounding protoplanetary disk. This diagram illustrates how the
time delay of the light echo is proportional to the distance between the
star and the inner edge of the disk. Image credit: NASA/JPL-Caltech.› Larger image
Imagine you want to measure the size of a room, but it's completely
dark. If you shout, you can tell if the space you're in is relatively
big or small, depending on how long it takes to hear the echo after it
bounces off the wall.
Astronomers use this principle to study objects so distant they
can't be seen as more than points. In particular, researchers are
interested in calculating how far young stars are from the inner edge
of their surrounding protoplanetary disks. These disks of gas and dust
are sites 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, Tucson. "We want to know how planets form and
why we find large planets called 'hot Jupiters' close to their stars."
Meng is the first author on a new study
published in the Astrophysical Journal using data from NASA's Spitzer
Space Telescope and four ground-based telescopes to determine the
distance from a star to the inner rim of its surrounding protoplanetary
Making the measurement wasn't as simple as laying a ruler on top of a
photograph. Doing so would be as impossible as using a satellite photo
of your computer screen to measure the width of the period at the end
of this sentence.
Instead, researchers used a method called "photo-reverberation,"
also known as "light echoes." When the central star brightens, some of
the light hits the surrounding disk, causing a delayed "echo."
Scientists measured the time it took for light coming directly from the
star to reach Earth, then waited for its echo to arrive.
Thanks to Albert Einstein's theory of special relativity, we know
that light travels at a constant speed. To determine a given distance,
astronomers can multiply the speed of light by the time light takes to
get from one point to another.
To take advantage of this formula, scientists needed to find a star
with variable emission -- that is, a star that emits radiation in an
unpredictable, uneven manner. Our own sun has a fairly stable emission,
but a variable star would have unique, detectable changes in radiation
that could be used for picking up corresponding light echoes. Young
stars, which have variable emission, are the best candidates.
The star used in this study is called YLW 16B and lies about 400
light-years from Earth. YLW 16B has about the same mass as our sun, but
at one million years old, it's just a baby compared to our
4.6-billion-year-old home star.
Astronomers combined Spitzer data with observations from
ground-based telescopes: the Mayall telescope at Kitt Peak National
Observatory in Arizona; the SOAR and SMARTS telescopes in Chile; and
the Harold L. Johnson telescope in Mexico. During two nights of
observation, researchers saw consistent time lags between the stellar
emissions and their echoes in the surrounding disk. The ground-based
observatories detected the shorter-wavelength infrared light emitted
directly from the star, and Spitzer observed the longer-wavelength
infrared light from the disk's echo. Because of thick interstellar
clouds that block the view from Earth, astronomers could not use
visible light to monitor the star.
Researchers then calculated how far this light must have traveled
during that time lag: about 0.08 astronomical units, which is
approximately 8 percent of the distance between Earth and its sun, or
one-quarter the diameter of Mercury's orbit. This was slightly smaller
than previous estimates with indirect techniques, but consistent with
Although this method did not directly measure the height of the
disk, researchers were able to determine that the inner edge is
Previously, astronomers had used the light echo technique to measure
the size of accretion disks of material around supermassive black
holes. Since no light escapes from a black hole, researchers compare
light from the inner edge of the accretion disk to light from the outer
edge to determine the disk size. This technique is also used to
measure the distance to other features near the accretion disk, such as
dust and the surrounding fast-moving gas.
While light echoes from supermassive black holes represent delays of
days to weeks, scientists measured the light echo from the
protoplanetary disk in this study to be a mere 74 seconds.
The Spitzer study marks the first time the light echo method was used in the context of protoplanetary disks.
"This new approach can be used for other young stars with planets in
the process of forming in a disk around them," said Peter Plavchan,
co-author of the study and assistant professor at Missouri State
University in Springfield.
NASA's Jet Propulsion Laboratory in Pasadena, California, manages
the Spitzer Space Telescope mission for NASA's Science Mission
Directorate in Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Meng was a visiting researcher at Caltech during this
research. Spacecraft operations are based at Lockheed Martin Space
Systems Company in Littleton, Colorado. Data are archived at the
Infrared Science Archive housed at the Infrared Processing and Analysis
Center at Caltech. Caltech manages JPL for NASA.