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First Detection of a Planet-Forming Disk Near Environment of a Dying Star
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false-color image of the Mira star system. Blue represents data
obtained with the Hubble Space Telescope and red and green represent
ground-based data obtained at the W. M. Keck Observatory in Hawaii and
the Gemini South Observatory in Chile. Mira B glows blue. The dust
outflow from the bright star Mira A (right) has a green, nearly
transparent color from silicate dust. The red color near the companion
star, Mira B (left) is caused by heating of the opaque edge of the disk
from Mira A. Image: Michael Ireland, Caltech |
Astronomers generally assume that the dusty disks where planets form
are found around young stars in stellar nurseries. Now, for the first
time, a planet-forming disk has been found in the environment of a
dying star.
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.
"Observing Mira in the infrared is like staring straight down the
barrel of one of the brightest searchlights in the galaxy. It came as a
real revelation to see this faint mote of dust, harboring all the
possibilities of new worlds in formation, against the hostile
environment of the Red Giant," said Tuthill.
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.
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