Zooming in on an infant
solar system
Like a raindrop forming in a cloud, a
star forms in a diffuse gas cloud in deep space. As the star grows,
its gravitational pull draws in dust and gas from the surrounding
molecular cloud to form a swirling disk called a "protoplanetary
disk." This disk eventually further consolidates to form planets,
moons, asteroids and comets.
Friday, June 11,
2010
A team led by University of Arizona astronomer Joshua Eisner has
observed in unprecedented detail the processes giving rise to stars
and planets in nascent solar systems.
The discoveries, published in the Astrophysical Journal,
provide a better understanding of the way hydrogen gas from the
protoplanetary disk is incorporated into the star.
By coupling both Keck telescopes on Mauna Kea in Hawaii with a
specifically engineered instrument named ASTRA (ASTrometric and
phase-Referenced Astronomy), Eisner and his colleagues were able to
peer deeply into protoplanetary disks – swirling clouds of gas and
dust that feed the growing star in its center and eventually
coalesce into planets and asteroids to form a solar system.
The big challenge facing Eisner's team lies in obtaining the
extremely fine resolution necessary to observe the processes that
happen at the boundary between the star and its surrounding disk –
500 light years from Earth. It's like standing on a rooftop in
Tucson trying to observe an ant nibbling on a grain of rice in New
York's Central Park.
"The angular resolution you can achieve with the Hubble Space
Telescope is about 100 times too coarse to be able to see what is
going on just outside of a nascent star not much bigger than our
sun," said Eisner, an assistant professor at UA's Steward
Observatory. In other words, even a protoplanetary disk close
enough to be considered in the neighborhood of our solar system
would appear as a featureless blob.
Combining the light from the two Keck telescopes provides an
angular resolution finer than Hubble's. Eisner and his team used a
technique called spectro-astrometry to boost resolution even more.
By measuring the light emanating from the protoplanetary disks at
different wavelengths with both Keck telescope mirrors and
manipulating it further with ASTRA, the researchers achieved the
resolution needed to observe processes in the centers of the
nascent solar systems.
Protoplanetary disks form in stellar nurseries when clouds of gas
molecules and dust particles begin to collapse under the influence
of gravity.
Initially rotating slowly, the cloud's growing mass and gravity
cause it to become more dense and more compact. The preservation of
rotational momentum speeds up the cloud as it shrinks, much like a
figure skater spins faster as she tugs in her arms. The centrifugal
force flattens the cloud into a spinning disk of swirling gas and
dust, eventually giving rise to planets orbiting their star in
roughly the same plane.
Combining the Keck interferometer with the spectro-astrometry
technique, Eisner and his collaborators were able to distinguish
between the distributions of gas, mostly made up of hydrogen, and
dust, thereby resolving the disk's features.
"We were able to get really, really close to the star and look
right at the interface between the gas-rich protoplanetary disk and
the star," said Eisner, who serves as project scientist on the
ASTRA team.
Astronomers know that stars acquire mass by incorporating some of
the hydrogen gas in the disk that surrounds them, in a process
called accretion.
"We want to understand how material accretes onto the star," Eisner
said. "This process has never been measured directly."
Accretion can happen in one of two ways.
In one scenario, gas is swallowed as it washes up right to the
fiery surface of the star.
In the second, much more violent scenario, the magnetic fields
sweeping from the star push back the approaching gas and cause it
to bunch up, creating a gap between the star and its surrounding
disk. Rather than lapping at the star's surface, the hydrogen atoms
travel along the magnetic field lines as if on a highway, becoming
super-heated and ionized in this process.
"Once trapped in the star's magnetic field, the gas is being
funneled along the field lines arching out high above and below the
disk's plane," Eisner explained. "The material then crashes into
the star's polar regions at high velocities."
In this inferno, which releases the energy of millions of
Hiroshima-sized atomic bombs every second, some of the arching gas
flow is ejected from the disk and spews out far into space as
interstellar wind.
Eisner's team pointed the telescopes at 15 protoplanetary disks
with young stars varying in mass between one half and 10 times that
of our sun. This sample of disks, all located in our own galaxy,
the Milky Way, represents by far the largest of its kind.
"We could successfully discern that in most cases, the gas converts
some of its kinetic energy into light very close to the stars" he
said, a tell-tale sign of the more violent accretion
scenario.
"In other cases, we saw evidence of winds launched into space
together with material accreting on the star," Eisner added. "We
even found an example – around a very high-mass star – in which the
disk may reach all the way to the stellar surface."
The solar systems the astronomers chose for this study are still
young, probably a few million years old.
"These disks will be around for a few million years more," Eisner
said. "By that time, the first planets, gas giants similar to
Jupiter and Saturn, may form, using up a lot of the disk
material."
More solid, rocky planets like the Earth, Venus or Mars, won't be
around until much later.
"But the building blocks for those could be forming now," he said,
which is why this research is important for our understanding of
how solar systems form, including those with potentially habitable
planets like Earth.
"We are going to see if we can make similar measurements of organic
molecules and water in protoplanetary disks," he said. "Those would
be the ones potentially giving rise to planets with the conditions
to harbor life."
###
Thanks to University of Arizona for this article.