Nature's recipe for building new worlds:
a zest of magnetic fields?
a zest of magnetic fields?
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Infrared view of the star nursery in the Ophiuchus constellation © ESO |
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Artist views of the main phases of star formation © Addison Wesley |
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Artist view of a Sun-like baby star weaving magnetic links to its accretion disc © Chandra |
How do stars like the Sun form? How are planetary systems born? To answer these questions, astrophysicists need to find out Nature's recipe to turn vast cosmic clouds of gas into accretion discs and into stars and planets. One of the crucial ingredients in this recipe is likely magnetic fields. An international team of astrophysicists [1] has just succeeded at mapping the large arches and funnels that magnetic fields weave between baby stars and their accretion discs. These observations should yield more accurate models of how new-born stars interact with their accretion disc to form planetary systems such as our own. These results are published in the Monthly Notices of the Royal Astronomical Society.
Where do stars come from and how are they born? This question is all about
origins: the origin of stars and their planets, the origin of life in the
universe. We know that our Sun has at least one planet with life, Earth,
but stars other than the Sun may also have the same potential.
Stars are born in vast cosmic factories known as
molecular clouds: huge volumes of gas, mostly hydrogen and helium, richly
laced with complex organic molecules and ice-covered dust grains. The
star formation
process in these clouds produces clusters of hundreds to thousands of
stars.
An international team of astrophysicists [1] led by JF Donati (from
CNRS/Observatoire Midi-Pyrenees in Toulouse, France) looked at one of the star nurseries
closest to our Earth
to try and understand better how stars like our Sun form. Located in the
constellation of
Ophiuchus,
the particular baby star they scrutinised is named V2129 Ophiuchi.
Despite being as hot as and about 2.5 times larger than our Sun (having not yet
contracted completely), it lies at a
distance of about 420 light years from us, making it roughly one million times too
faint to be visible with the naked eye. It is only 2 million years old; scaling to
human life, this is a baby star of only a few days with still 1 year to go to complete
its transformation into a full grown Sun-like star with its surrounding planets.
Stars like V2129 Oph form when a portion of the parent molecular cloud collapse under its
own weight. In this process, the collapsing globule is progressively spinning faster
as it contracts - just as a skater does by bringing his arms closer to his body. The
original cloudlet is finally changed into a flat disc, called an
accretion disc,
whose core gives rise to a new-born star and the
surrounding material
to planets.
This star-making recipe, originally proposed by Laplace a few years after the French
revolution, is however only approximate: it predicts that very young stars should spin
extremely fast, a property that observations do not confirm. We are obviously missing
some of the recipe's basic ingredients.
The ingredient we are missing is likely magnetic fields. Despite
being present in Sun-like stars and generating spectacular phenomena such as the
solar corona,
magnetic fields no more than slightly affect the life of fully grown-up stars
like the Sun. In star forming regions however, magnetic fields are likely much more
powerful. Through invisible webs threading the protostellar cloud, magnetic fields
succeed at controlling the dynamics of accretion discs, and produce light-year long
jet-like pencil beams
by deflecting some of the originally infalling material outwards and along
the accretion disc spin axis.
Magnetic fields also manage to evacuate the core regions of accretion discs in direct
contact with the baby stars, and to guide the material from the inner disc rim onto the
star through funnels like light through optical fibres. The physical details of this
operation are crucial to understand the fate of a Sun-like baby star and its surrounding
planets. By looking at the
polarisation
magnetic fields generate in the light of V2129 Oph,
JF Donati and collaborators were able to map for the first time the
large arches
that magnetic fields weave in order to link the star to its accretion disc.
Thanks to their work, theoreticians will design new models of how Sun-like stars and their
planets form.
This discovery was obtained thanks to
ESPaDOnS
[2], the new generation spectropolarimeter
recently installed on the
3.6m Canada-France-Hawaii Telescope
[3] atop Mauna-Kea, an extinct
4200m-volcano on the big island of Hawaii, in the middle of the Pacific ocean. Thanks
to its unprecedented sensitivity, ESPaDOnS is able to detect the very small polarisation
that magnetic fields induce in the light of distant stars.
Press contacts are:
Jean-François Donati,
Laboratoire d'Astrophysique de Toulouse-Tarbes, Observatoire Midi-Pyrénées, CNRS/Université Paul-Sabatier,
14 avenue E. Belin, 31400 Toulouse, France. Tel: +33 561332917, Fax: +33 561332840,email: donati[AT]ast.obs-mip.fr.
Moira Jardine, School of Physics and Astronomy, University of StAndrews, St Andrews, Fife
SCOTLAND KY16 9SS, UK, Tel: +44 (1334) 463146, email: Moira.Jardine[AT]st-and.ac.uk
Jérome Bouvier, Laboratoire d'Astrophysique de Grenoble, Observatoire de Grenoble, CNRS/Université Joseph Fourier,
31 rue de la Piscine, 38041 Grenoble, France, Tel: +33 476514790, email: Jerome.Bouvier[AT]obs.ujf-grenoble.fr
Jean-François Donati,
Laboratoire d'Astrophysique de Toulouse-Tarbes, Observatoire Midi-Pyrénées, CNRS/Université Paul-Sabatier,
14 avenue E. Belin, 31400 Toulouse, France. Tel: +33 561332917, Fax: +33 561332840,email: donati[AT]ast.obs-mip.fr.
Moira Jardine, School of Physics and Astronomy, University of StAndrews, St Andrews, Fife
SCOTLAND KY16 9SS, UK, Tel: +44 (1334) 463146, email: Moira.Jardine[AT]st-and.ac.uk
Jérome Bouvier, Laboratoire d'Astrophysique de Grenoble, Observatoire de Grenoble, CNRS/Université Joseph Fourier,
31 rue de la Piscine, 38041 Grenoble, France, Tel: +33 476514790, email: Jerome.Bouvier[AT]obs.ujf-grenoble.fr