The magnetic nature of a mysterious naked-eye cosmic X-ray emitter
Our Sun has its flares and spots and wind, but it's a placid star
compared to some. Stars that are much more massive live fast and die
young, with blue-white, intensely hot surfaces that emit energy at a
rate millions of times greater than that of the Sun. These stars are
so bright that their light alone propels outflowing stellar winds - up
to a billion times stronger than the solar wind - at speeds of up to
one per cent of the speed of light.
An international team of astronomers has discovered that one such
star, the naked-eye tau Scorpii, unexpectedly hosts a complex network
of magnetic field lines over its surface. Tau Sco has been known for some time to
emit X-rays at an unusually high rate and to rotate slower than
most otherwise similar stars. The newly-discovered magnetic field, presumably a relic
from the star's formation stage, goes some way to explaining both characteristics,
although the mechanism by which the magnetic field slowed down tau Scorpii's rotation so
strongly remains mysterious.
These results will be published in the Monthly Notices of the Royal Astronomical Society.
The processes by which hot, massive stars expel their surface layers
through their strong outflowing winds have a major impact on a star's
long-term fate; the cast-off material can also interact with other
nearby stars, contribute matter and energy to the surrounding
interstellar medium, and even induce bursts of new star formation.
Hot massive stars are thus key actors in the life of a galaxy.
One such hot star is tau Scorpii, whose intrinsic brightness is so
great that it is easily visible with the naked eye despite its
distance of over 400 light-years. Weighting as much as 15 suns, tau
Scorpii is both 5 to 6 times bigger and hotter than our own star. Such
massive stars are relatively few compared to stars like the Sun, and
tau Scorpii is actually one of our closest massive neighbours.
Massive stars are thought to emit X-rays because of supersonic shocks
occuring within their winds. However, tau Scorpii is an unusually
strong X-ray source compared to otherwise similar stars, and the
reason for this enhanced activity was a puzzle until the present
discovery, which revealed that the star hosts a
complex network of magnetic field lines over its surface (see image).
According to the discovery team, this field is most probably
a relic from the star's formation stage.
The most interesting aspect, though, is how the
field interacts with the wind, forcing it to flow along magnetic field
lines, like beads along wires. Wind streams along 'open'
magnetic-field lines (shown in blue) freely escape the star, something
that wind streams in magnetic 'arcades' (shown in white) cannot
achieve. The result is that, within each magnetic arcade, wind flows
from both footpoints collide with each other at the loop summits,
producing tremendously energetic shocks and
turning the wind material into blobs of million-degree, X-ray emitting
plasma tied to the magnetic loops.
This model provides a natural explanation of why tau Scorpii is such
an intense X-ray emitter. However, it is not yet clear how the
magnetic field succeeded in slowing down the rotation rate of the star
to less than one-tenth that of otherwise similar, non-magnetic,
massive stars. Sun-like stars can be slowed down through their
magnetic wind, just as ice-skaters are spun down when outstretching
their arms; tau Scorpii does not, however, lose material fast enough
to have its rotation modified within its very short lifetime of 'only'
a few million years.
The researchers discovered and examined the magnetic field of the star
by looking at the tiny, very specific polarisation signals that
magnetic fields induce in the light of magnetic stars; to do this,
they used ESPaDOnS, by far the most powerful
instrument in the world
for carrying out this kind of research. This new instrument,
currently attached to the Canada-France-Hawaii Telescope on Hawaii,
was especially designed at the Observatoire Midi-Pyrénées in France
for observing and studying magnetic fields in stars other than the Sun.
Press contacts are:
Jean-François Donati, Laboratoire d'Astrophysique de Toulouse-Tarbes,
Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France. Tel: +33 561332917, Fax: +33 561332840,
Ian Howarth, Department of Physics and Astronomy, University College London, Gower Street, London WC1 E6BT, UK, Tel: +44 20-7679-3491, email: idh[AT]star.ucl.ac.uk
This team includes
JF Donati (Observatoire Midi-Pyrenees/LATT, CNRS/UPS, France),
ID Howarth (University College London, UK),
MM Jardine (University of StAndrews, UK),
P Petit (Observatoire Midi-Pyrenees/LATT, CNRS/UPS, France),
C Catala (Observatoire Paris-Meudon/LESIA, CNRS/UP7, France),
JD Lanstreet (University of Western Ontario, Canada),
JC Bouret (Observatoire de Marseille/LAM, CNRS/UdP, France),
E Alecian (Observatoire Paris-Meudon/LESIA, CNRS/UP7, France),
JR Barnes (University of StAndrews, UK),
T Forveille (Canada-France-Hawaii Telescope Corporation, USA),
F Paletou (Observatoire Midi-Pyrenees/LATT, CNRS/UPS, France) and
N Manset (Canada-France-Hawaii Telescope Corporation, USA)
ESPaDOnS was cofunded by France (CNRS/INSU, Ministère de la Recherche, LATT, Observatoire
Midi-Pyrénées, Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire
de Paris-Meudon), Canada (NSERC), CFHT and ESA (ESTEC/RSSD). First light occured at CFHT on
2004 Sept 2.
CFHT operation is funded by Canada (NSERC), France (CNRS/INSU) and the University of Hawaii.