First Detection of a Planet-Forming Disk Near Environment of a Dying Star
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A team of astronomers is reporting today at the winter meeting of
the American Astronomical Society that material from the dying star
Mira A is being captured into a disk around Mira B, its companion.
Michael Ireland of the California Institute of Technology and his
coauthors, John Monnier from the University of Michigan, Peter Tuthill
from the University of Sydney, and Richard Cohen from the W. M. Keck
Observatory, say that the finding implies that there should be many
similar undiscovered systems in the solar neighborhood, providing a
myriad of new places to look for young planets orbiting stars other
than our Sun.
Located 350 light years away in the constellation of Cetus, Mira
(christened the “Miracle star”) first shook the foundations of the
astronomy world 400 years ago with its changing brightness: visible to
the naked eye for about 1 month at a time, becoming 1,000 times fainter
and disappearing from view, only to re-appear again on an 11 month
cycle.
“When looking at one of the most celebrated and well-studied stars
in the galaxy, I was amazed to find something new and unexpected!” said
Ireland. “The discovery not only changes the way we think about a star
that’s important historically, but also how we’ll look at similar stars
in the future.”
Although Mira was once a star very similar to the sun, it is now in
its death throes as it loses its dusty outer layers at a rate of one
Earth-mass every seven years. If Mira were a single star, all this
material would travel into outer space. However, like two out of every
three star systems, Mira has a companion star that orbits around it, in
this case with a period of about 1,000 years. This companion, Mira B,
has a gravitational field that catches nearly one percent of the
material lost from Mira A.
By using specialized high-contrast techniques at the 10-meter Keck
I telescope in Hawaii and the 8-meter Gemini South telescope in Chile,
Ireland's team discovered heat radiation coming not from Mira B itself,
but also from a location offset from Mira B by a distance equivalent to
Saturn's orbit.
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Monnier agreed, saying "Our new imaging method at Keck is revealing new details that were thought to be impossible to detect due to the blurring by atmospheric turbulence. In this case -- the "detail" we discovered is potentially a whole new class of planetary system in formation!"
The intense radiation from Mira A, 5,000 times brighter than the sun, heats the edge of the disk to about Earth's temperature and causes it to glow in the infrared. The researchers were able to show that the material was indeed the edge of a disk and not just a "clump" in the wind from Mira A. By modeling the way that this system captures the outflow from Mira A, the researchers were also able to confirm that Mira B is simply an ordinary star like the sun, although about half as massive.
The key part of this result is what will happen when Mira A finishes its death throes and becomes a white dwarf in about one million years. The disk-creating process will have finished and the disk itself will be capable of forming new planets. “The expected abundance of this kind of system means a new way to find planets that we know are young around stars like our sun, ” Ireland says.
Astronomers associate the death of a star with the death of its planetary system. Here, the opposite is happening. “An aging star is laying the foundation for a new generation of planets, ” says Ireland. “This is Greek tragedy on a cosmic scale.”
Similar systems could be discovered and studied by future telescopes such as the Thirty Meter Telescope.
Source: W. M. Keck Observatory
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