Jets Are a Real Drag (Forwarded)

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For Release: Wednesday, December 26, 2007

Release No.: 2007-34

Jets Are a Real Drag

Cambridge, MA -- Astronomers have found the best evidence yet of matter
spiraling outward from a young, still-forming star in fountain-like jets.
Due to the spiral motion, the jets help the star to grow by drawing angular
momentum from the surrounding accretion disk.

"Theorists knew that a star has to shed angular momentum as it forms," said
astronomer Qizhou Zhang of the Harvard-Smithsonian Center for Astrophysics
(CfA). Now, we see evidence to back up the theory."

Angular momentum is the tendency for a spinning object to continue spinning.
It applies to star formation because a star forms at the center of a
rotating disk of hydrogen gas. A star grows by gathering material from the
disk. However, gas cannot fall inward toward the star until that gas sheds
its excess angular momentum.

As hydrogen nears the star, a fraction of the gas is ejected outward
perpendicular to the disk in opposite directions, like water from a fire
hose, in a bipolar jet. If the gas spirals around the axis of the jet, then
it will carry angular momentum with it away from the star.

Using the Submillimeter Array (SMA), an international team of astronomers
observed an object called Herbig-Haro (HH) 211, located about 1,000
light-years away in the constellation Perseus. HH 211 is a bipolar jet
traveling through interstellar space at supersonic speeds. The central
protostar is about 20,000 years old with a mass only six percent the mass of
our Sun. It eventually will grow into a star like the Sun.

The astronomers found clear evidence for rotation in the bipolar jet. Gas
within the jet swirls around at speeds of more than 3,000 miles per hour,
while also blasting away from the star at a velocity greater than 200,000
miles per hour.

"HH 211 essentially is a 'reverse whirlpool.' Instead of water swirling
around and down into a drain, we see gas swirling around and outward,"
explained Zhang.

In the future, the team plans to take a closer, more detailed look at HH
211. They also hope to observe additional protostar-jet systems.

"These are intrinsically difficult measurements. We need narrow jets to be
able to detect signs of rotation, and they have to be close enough for us to
observe them with high resolution," said CfA astronomer Tyler Bourke. "There
are very few jets around that meet those criteria."

The technological capabilities of the SMA were crucial in gathering these
data.

"The SMA has been in operation since the end of 2003. It has hit its
scientific stride and is producing a substantial amount of high-quality
scientific results," said SMA director Ray Blundell.

In the more distant future, new ground-based observatories will turn their
powerful gaze on this and other newborn stars.

ASIAA Director Paul Ho notes, "A much more powerful radio interferometer,
the Atacama Large Millimeter/Submillimeter Array (ALMA), is now under
construction in northern Chile, as a much more powerful version of the SMA.
It will allow us to zoom in to these stellar birthplaces with much finer
details and unravel the process of stellar birth directly."

A paper on this work was published in the December 1 issue of the
Astrophysical Journal and is available online at
http://arxiv.org/abs/0708.1365

The paper was authored by Chin-Fei Lee (Academia Sinica Institute of
Astronomy and Astrophysics, or ASIAA), Paul Ho (ASIAA and CfA), Aina Palau
(Laboratorio de Astrofisica Espacial y Fisica Fundamental), Naomi Hirano
(ASIAA), Tyler Bourke (CfA), Hsien Shang (ASIAA), and Qizhou Zhang (CfA).

The Submillimeter Array is an 8-element interferometer located atop Mauna
Kea in Hawaii. It is a collaboration between the Smithsonian Astrophysical
Observatory and the Institute of Astronomy and Astrophysics of the Academia
Sinica of Taiwan.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

[NOTE: Images supporting this release are available at
http://cfa-www.harvard.edu/press/200...34_images.html ]