March 17, 2008
Finally, the ‘planet’ in planetary
nebulae?
jonathan.sherwood@rochester.edu
Astronomers at the University, home to one of the
world’s largest groups of planetary nebulae specialists, have
announced that low-mass stars and possibly even super-Jupiter-sized planets
may be responsible for creating some of the most breathtaking objects in
the sky.
Twin Jets Nebula, also called the Butterfly Nebula, photographed by NASA
The news is ironic because the name
“planetary” nebula has always been a misnomer. When these
objects were discovered 300 years ago, astronomers couldn’t tell what
they were and named them for their resemblance to the planet Uranus. But as
early as the mid-19th century, astronomers realized these formations are
really great clouds of dust emitted by dying stars.
Now Rochester researchers have found that planets or
low-mass stars orbiting these aged stars may indeed be pivotal to the
creation of the nebulae’s fantastic appearance.
In a new paper in Astrophysical
Journal Letters, and in recent papers in Monthly Notices of the Royal Astronomical Society, a team of astronomers anchored by Eric Blackman, professor
of physics and astronomy at the University, has studied the consequences of
a dying star that possesses an orbiting companion.
“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,” says 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.”
Most medium-sized stars, such as our Sun, will end
their lives as planetary nebulae, says Blackman. The stage lasts only
several tens of thousands of years—a blink of an eye for stars that
typically live 10 billion—so it is a relatively rare sight. Of the
200 billion stars in our own galaxy, only about 1,500 have so far been
identified in the planetary nebula stage.
As the star begins to deplete 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. Blackman says one
time in five, this envelope keeps its roughly spherical shape as it
expands, but much more often this envelope contorts and elongates into new
and fantastic shapes.
The Rochester team’s work explored the role of
low-mass companions in shaping planetary nebulae stars, both when the
companion is in a large orbit and interacts with only the very outer edges
of the envelope, and when the companion is in a very tight orbit and so
close to the evolved star that the companion is fully engulfed by the
envelope.
Blackman, along with post-doctoral fellow Richard
Edgar, graduate student Jason Nordhaus, and professor of astrophysics Adam
Frank, showed that in the case when the planet or companion star is in a
very wide orbit, the planet’s gravity begins to drag some of the
envelope material around with it. The envelope material—essentially a
thin mixture of gas and dust—becomes compressed in spiral waves
radiating out from the central star like a twisted wagon wheel, says
Blackman. The dust and gas compresses more and more in these spiral waves
until they crest, much like waves breaking on a beach. Eventually, a torus
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,
such constrained expansion can lead to striking shapes, such as seen in the
appropriately named Dumbbell Nebula.
“Originally, we set out just to model the
geometry of the envelope under the influence of a binary companion”
says Blackman, “but Richard Edgar discovered that 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.” The globules cool
slowly enough to give the molecules within time to align into crystal
lattices. Blackman says the team’s work show’s how a
waist-cinching torus could originate to produce certain types of planetary
nebula patterns, but it also suggests an answer for why astronomers
have detected the puzzling signature of crystallized dust around
evolved stars before the nebulae is formed.
This research was funded by NASA and the National
Science Foundation.
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