Astronomers find magnetic Slinky in Orion (Forwarded)

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University of California-Berkeley

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Robert Sanders
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Tim Robishaw, UC Berkeley
(510) 643-8530

Carl Heiles, UC Berkeley
(510) 642-4510

FOR IMMEDIATE RELEASE: Thursday, January 12, 2006

Astronomers find magnetic Slinky in Orion

Astronomers announced today (Thursday, Jan. 12) what may be the first
discovery of a helical magnetic field in interstellar space, coiled like
a snake around a gas cloud in the constellation of Orion.

"You can think of this structure as a giant, magnetic Slinky wrapped
around a long, finger-like interstellar cloud," said Timothy Robishaw, a
graduate student in astronomy at the University of California, Berkeley.
"The magnetic field lines are like stretched rubber bands; the tension
squeezes the cloud into its filamentary shape."

Astronomers have long hoped to find specific cases in which magnetic
forces directly influence the shape of interstellar clouds, but
according to Robishaw, "telescopes just haven't been up to the task ...
until now."

The findings provide the first evidence of the magnetic field structure
around a filamentary-shaped interstellar cloud known as the Orion
Molecular Cloud.

Today's announcement by Robishaw and Carl Heiles, UC Berkeley professor
of astronomy, was made during a presentation at the American
Astronomical Society meeting in Washington, D.C.

Interstellar molecular clouds are the birthplaces of stars, and the
Orion Molecular Cloud contains two such stellar nurseries -- one in the
belt and another in the sword of the Orion constellation. Interstellar
clouds are dense regions embedded in a much lower-density external
medium, but the "dense" interstellar clouds are, by Earth standards, a
perfect vacuum. In combination with magnetic forces, it's the large size
of these clouds that makes enough gravity to pull them together to make
stars.

Astronomers have known for some time that many molecular clouds are
filamentary structures whose shapes are suspected to be sculpted by a
balance between the force of gravity and magnetic fields. In making
theoretical models of these clouds, most astrophysicists have treated
them as spheres rather than finger-like filaments. However, a
theoretical treatment published in 2000 by Drs. Jason Fiege and Ralph
Pudritz of McMaster University suggested that when treated properly,
filamentary molecular clouds should exhibit a helical magnetic field
around the long axis of the cloud. This is the first observational
confirmation of this theory.

"Measuring magnetic fields in space is a very difficult task," Robishaw
said, "because the field in interstellar space is very weak and because
there are systematic measurement effects that can produce erroneous
results."

The signature of a magnetic field pointing towards or away from the
Earth is known as the Zeeman effect and is observed as the splitting of
a radio frequency line.

"An analogy would be when you're scanning the radio dial and you get the
same station separated by a small blank space," Robishaw explained. "The
size of the blank space is directly proportional to the strength of the
magnetic field at the location in space where the station is being
broadcast."

The signal, in this case, is being broadcast at 1420 MHz on the radio
dial by interstellar hydrogen -- the simplest and most abundant atom in
the universe. The transmitter is located 1750 light years away in the
Orion constellation.

The antenna that received these radio transmissions is the National
Science Foundation's Green Bank Telescope (GBT), operated by the
National Radio Astronomy Observatory. The telescope, 148 meters (485
feet) tall and with a dish 100 meters (300 feet) in diameter, is located
in West Virginia where 13,000 square miles have been set aside as the
National Radio Quiet Zone. This allows radio astronomers to observe
radio waves coming from space without interference from manmade signals.

Using the GBT, Robishaw and Heiles observed radio waves along slices
across the Orion Molecular Cloud and found that the magnetic field
reversed its direction, pointing towards the Earth on the upper side of
the cloud and away from it on the bottom. They used previous
observations of starlight to inspect how the magnetic field in front of
the cloud is oriented. (There is no way to gain information about what's
happening behind the cloud since the cloud is so dense that neither
optical light nor radio waves can penetrate it.) When they combined all
available measurements, the picture emerged of a corkscrew pattern
wrapping around the cloud.

"These results were incredibly exciting to me for a number of reasons,"
Robishaw said. "There's the scientific result of a helical field
structure. Then, there's the successful measurement: This type of
observation is very difficult, and it took dozens of hours on the
telescope just to understand how this enormous dish responds to the
polarized radio waves that are the signature of a magnetic field."

The results of these investigations suggested to Robishaw and Heiles
that the GBT is not only unparalleled among large radio telescopes for
measuring magnetic fields, but it is the only one that can reliably
detect weak magnetic fields.

Heiles cautioned that there is one possible alternative explanation for
the observed magnetic field structu The field might be wrapped around
the front of the cloud.

"It's a very dense object," Heiles said. "It also happens to lie inside
the hollowed-out shell of a very large shock wave that was formed when
many stars exploded in the neighboring constellation of Eridanus."

That shock wave would have carried the magnetic field along with it, he
said, "until it reached the molecular cloud! The magnetic field lines
would get stretched across the face of the cloud and wrapped around the
sides. The signature of such a configuration would be very similar to
what we see now. What really convinces us that this is a helical field
is that there seems to be a constant pitch angle to the field lines
across the face of the cloud."

However, the situation can be clarified by further research. Robishaw
and Heiles plan to extend their measurements in this cloud and others
using the GBT. They will also collaborate with Canadian colleagues to
use starlight to measure the field across the face of this and other clouds.

"The hope is to provide enough evidence to understand what the true
structure of this magnetic field is," said Heiles. "A clear
understanding is essential in order to truly understand the processes by
which molecular clouds form stars in the Milky Way galaxy."

The research was supported by the National Science Foundation.

Web links to additional Information:
http://astron.berkeley.edu/~robishaw/press/orion.jpg (238KB)