Using a technique based on the work of the 1902 Nobel Prizewinner, Pieter
Zeeman, an international team of astronomers have, for the first time,
provided conclusive proof that the magnetic field close to a number of aging
stars is 10 to 100 times stronger than that of our own Sun. These
observations suggest a solution to the long outstanding problem as to how,
at the end of their lives, a perfectly spherical star can give rise to the
complex and often far from spherical structure seen in the resulting
planetary nebula – some of the most beautiful objects in our heavens.

When stars like our Sun reach the end of their lives, they eject a large
amount of material into the space around them. This material, produced by
nuclear fusion reactions in the star, forms a thick dust shell which
eventually evolves into a planetary nebula – so called because they appear
rather like planetary discs. Due to turbulent gas flows around the star the
strong magnetic fields that have been discovered will have very different
shapes. The material which is ejected from the star “feels” this strong
magnetic field and so, as a result, the planetary nebula can have a very
complicated structure. The ejected material, containing elements such as
carbon and oxygen, in eventually recycled into new stars and planets and the
building blocks of life itself.

The group, lead by Wouter Vlemmings of Leiden Observatory, observed 4 old
stars with the U.S. National Science Foundation’s VLBA, the network of radio
telescopes operated by the American National Radio Astronomy Observatory.
They detected radio emission which originates from clouds of water vapor
ejected by the stars. In some circumstances, such a cloud can become a
maser: the equivalent of a laser for radiation with longer wavelengths. One
specific frequency of the emitted radiation, which is characteristic for the
H2O molecule, is amplified enormously, resulting in a bright, clear signal.
In this signal, the group was able to detect the Zeeman-effect for the first
time: subtle changes in the spectrum of the emission that can only be caused
by a strong magnetic field at the location of the maser.

The magnetic fields measured are as strong as the magnetic field at the
Earth’s surface, between 0.5 and 1 Gauss. As observations have shown that
the water masers occur at a large distance from the star (at about twice the
distance between the furthest planet in our solar system, Pluto, and the
Sun), the magnetic field strength at the surface of the star will be much
higher, approximately 50 to 500 Gauss, which is 10 to 100 times the magnetic
field strength of the Sun. This is sufficiently strong that the magnetic
field can play an important role in the formation of aspherical planetary
nebulae and in the process of mass-loss which creates the dusty
circumstellar envelopes.

The Zeeman-effect, which has enabled these observations to be made, was
named after the Leiden physicist Pieter Zeeman, who discovered the effect of
a magnetic field on the spectrum of a light source in 1896. As this effect
is extremely small in water molecules, the observations for the research by
Vlemmings had to be extremely precise. Because of this, the data of the
different telescopes of the VLBI-network were specially processed in the
correlator at Socorro, New Mexico, USA to give the highest possible
accuracy.

The astronomers, whose work has just been published in Astronomy &
Astrophysics, were Wouter Vlemmings of Leiden Observatory, Philip Diamond of
the University of Manchester and Huib Jan van Langevelde of the Joint
Institute for VLBI in Europe (JIVE).

An image to support this release may be found at:
http://www.jb.man.ac.uk/news/magnetism/

Image Caption:

The main image is of the “Hourglass” Planetary Nebula observed by the Hubble
Space Telescope. The inset image is that of an old “red giant” star of the
type observed in the observations, also imaged by the Hubble Space
Telescope.

Contact Details:

Wouter Vlemmings, Leiden Observatory.
e-mail: vlemming@strw.leidenuniv.nl
Phone: +31 71 5275831
FAX: +31 71 5275819

Philip Diamond, Jodrell Bank Observatory, University of Manchester.
e-mail: p.diamond@jb.man.ac.uk
Phone: +44 (0)1477 572625
FAX: +44 (0)1477 571618

Huib Jan van Langevelde, Joint Institute for VLBI in Europe.
e-mail: langeveld@jive.nl
Phone: +31 521 596 520

Background information:

The Very Long Baseline Array is a series of ten radio antennas spread across
the United States and its territories from St. Croix, the Virgin Islands, to
Mauna Kea, Hawaii. The operations center for the array is located in
Socorro, New Mexico. Astronomical data from the observations are recorded on
digital tape at each antenna site. The tapes are then shipped to the Socorro
Operations Center where they are correlated and the results sent to the
scientists. For more information, see the VLBA overview page:

http://www.aoc.nrao.edu/vlba/html/WHATIS/whatis.html

The paper relating to this work is in Astronomy and Astrophysics vol 394
pages 589 to 602.