The term "planetary
nebula" has always been a misnomer, but these spectacular clouds of dust
and gas may actually have something to do with planets after all, astronomers
have found.
When
astronomers discovered these celestial objects 300 years ago, they couldn't
tell what they were and so named them for the resemblance they had to the planet
Uranus as seen through early, relatively crude telescopes. But by the mid-19th
century, it was realized that they were actually great
clouds of dust emitted by dying stars.
Now,
researchers at the University of Rochester have found that low-mass stars or
possibly even giant gas planets orbiting these aged stars could be pivotal in
creating some of the nebulae's unusual shapes.
"Few
researchers have explored how something as small as a very low-mass star, a
brown dwarf or even a massive planet can produce several flavors of nebulae and
even change the chemical composition of the dust around these evolved
stars," said study leader Eric Blackman. "If the companions can be
this small, it's important because low-mass stars and high-mass planets are
likely quite common and could go a long way toward explaining the many dusty
shapes we see surrounding these evolved stars."
The team's
research is detailed in papers in the Astrophysical Journal Letters and
the Monthly Notices of the Royal Astronomical Society and was funded by
NASA and the National Science Foundation.
End of
life
Planetary
nebulae are the last stage of life for most medium-sized stars, such as our
Sun. This stage only lasts for several tens of thousands of years—a blink of an
eye in the star's 10 billion year lifespan. Of the 200 billion stars in the Milky Way, only about 1,500 have been
found to be in the planetary nebula phase. So that makes these glowing clouds a
relatively rare sight.
The stage
begins as the star depletes its fuel near the end of its life. Its core
contracts and its envelope expands, eventually throwing off its outermost
layers millions of miles into space.
One time in
five, the envelope keeps its spherical shape as it expands, forming a glowing
orb. But more often, the envelope contorts into a dazzling array of shapes.
Spiral waves
Blackman
and his team explored the role that low-mass companion stars or
super-Jupiter-sized planets might have in sculpting the shapes of planetary
nebulae.
The
researchers explored two scenarios: when the companion was in a large orbit and
interacted only with the very edges of the envelope, and when it was in a tight
orbit, entirely engulfed by the envelope.
In a wide
orbit, the companion star or planet's gravity begins to drag some of the
envelope material—essentially a thin mixture of gas and dust—around with it.
The material becomes compressed in spiral waves that radiate out from the
central star like a wagon wheel, Blackman said. The gas and dust continue to
compress until the spiral waves break like waves on a beach.
Eventually,
a doughnut-shaped ring of dust forms around the star's mid-section, likely
blocking much of the expanding envelope, like a belt around an inflating
balloon. Over time, shapes like the aptly named Dumbbell
Nebula can form.
This
behavior also accounts for the strange signature of crystallized dust that
astronomers have observed around evolved stars before the nebulae form.
"As
the spiral waves break, they release their compressed, pent-up energy in a
burst of heat, sufficient to melt the dust into liquid globules," Black
explained. The globules cool slowly enough that the molecules within them have
time to align into crystal lattices.
Short
orbit
When a
companion's orbit is close to the main star, one of three outcomes is likely to
occur, the team found.
The companion
star or planet could spin up the envelope so quickly as it plows through that
the material is ejected, deforming into a large disk or torus around the star's
equator.
Another
possibility is that the companion spins up the material more gently, causing
the inner regions of the envelope to spin faster than the outer regions. This
difference in motion could stretch and amplify the star's magnetic fields,
which would then act like a giant spring, ejecting the envelope material out
the star's poles as jets.
The third
scenario sees the companion itself ejecting out of the star's jets, Blackman
said. This could happen if the companion star or planet was too small to eject
the envelope material before it falls prey to the grasp of the main star's
gravity. The intense pull of the star can shred the planet as its orbit
shrinks, eventually smearing it into a disk of turbulent debris around the
star. This debris would be ejected as jets with the envelope material.
The team
now plans to investigate the mixing and transport of different chemical
elements within the nebulae to see how the distinct chemical signatures
detected in planetary nebulae come about.
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