Astronomers find magnetic Slinky in Orion (Forwarded)

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University of California-Berkeley

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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)