Astronomers have been puzzled for decades as to how the rings of hot gas
surrounding certain types of star are formed. Now a team of scientists from
the Universities of Glasgow and Wisconsin believe they have found the
answer. The team studied a type of young, hot star, known as a “Be star”,
that has a disk of glowing gas around it, similar to the rings surrounding
Saturn. Until now, no one has been able to account for how these rings form
but in a paper published this month*, the team suggest an answer.

The gas ring surrounding a Be Star may appear and then disappear, possibly
reforming at a later time. Material in the disk is attracted back towards
the star by the pull of gravity, but if it has enough energy it can escape
into space, contributing to the stellar wind.

The new theory reveals why this material is held in a disk at some distance
from the star instead of either being pulled closer or flying away into
space. Deborah Telfer of Glasgow University explains “Our model relies on
the existence of a magnetic field around Be stars producing a ‘Magnetically
Torqued Disk’. Magnetic field lines channel stellar wind material leaving
the surface of the star down towards the equatorial plane. A disk then forms
in the region where particles have sufficient angular velocity to balance
gravity. In the outer regions, the weaker magnetic field lines should burst
open allowing particles to form part of the general stellar wind.”

Previously, the Wind Compressed Disk Model (Bjorkman and Cassinelli, 1993)
was regarded as one of the most successful explanations of circumstellar
disks. However, it predicts disks that are out-flowing (i.e. the material
moves from the star to the disk and then away into space) and expanding.
Yet Be stars are observed to have circumstellar Keplerian disks, meaning
that the disks are supported against gravity by rotation rather than gas or
radiation pressure. Deborah has been working with Joseph Cassinelli of
Wisconsin on the new model for Be star disks and they are delighted at the
success of their results.

These suggest that only a narrow range of types of star would form a
detectable Magnetically Torqued Disk and be seen as Be stars. Heavier stars
would require an unreasonably large magnetic field while lighter stars would
produce disks too small to be detected. More work is needed to explain every
aspect of observational evidence but we may finally be reaching an
understanding of what produces these Saturn-like stars.

Notes for Editors

* This work appeared in the Astrophysical Journal, October 20th 2002, “A
Magnetically Torqued Disk Model for Be Stars”

Authors: J. P. Cassinelli (University of Wisconsin-Madison), J. C. Brown
(University of Glasgow), M. Maheswaran(University of Wisconsin), N. A.
Miller((University of Wisconsin) and D. C. Telfer (University of Glasgow)

Images available from Julia Maddock (contact details below)

Artist’s impression

Taken from the perspective of one of the Hubble Space Telescope observations
of Phi Persei, this artist’s depiction provides a taste of the double- star
system’s unstable existence. The bright “Be” star – a type of hot star with
a broad, flattened disk – is the white, semicircular object looming in the
upper right of the illustration. The red, pancake-shaped object surrounding
the star is a gas disk. The small, hot subdwarf is in the lower left of the
illustration. The blasts of white light represent particles of material –
called a stellar wind – being released by the star. The red ring of material
surrounding the subdwarf was probably formed from the “Be” star’s outflow of
gas. The subdwarf is moving toward the right in its 126-day orbit around the
“Be” star.

Illustration copyright of Bill Pounds

Deborah Telfer

Deborah is in the final year of her Ph.D. in stellar physics at Glasgow
University. She started contributing to research by undertaking an RSE
Cormack vacation scholarship while in her second year of undergraduate study
at Glasgow.

Contact Details

Deborah Telfer
Dept of Physics & Astronomy
University of Glasgow
deborah@astro.gla.ac.uk
Tel: + 44 (0) 141 330 5182

Joe Cassinelli
Dept of Astronomy, University of Wisconsin-Madison,
475 N. Charter St.
Madison WI 53706
USA
Email: cassinelli@astro.wisc.edu
Tel: +1 608 263 4622

PPARC Press Office
Julia Maddock
Email: Julia.maddock@pparc.ac.uk
Tel: +44 (0) 1793 442094

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