Can massive stars regenerate their magnetism?

Despite their small number, massive stars are the main architects of a galaxy - the Sun and the multitude of its dwarf analogs are pretty dull in comparison. However, these monster stars are apparently unable to achieve one task that the Sun is very good at performing : regenerating endlessly its magnetic field. An international team of astrophysicists1 conducted by researchers from CNRS just discovered an atypical magnetic field on a massive star in the constellation of Orion; this discovery may shake our understanding of magnetism in massive stars. These results are in press in the Monthly Notices of the Royal Astronomical Society.

Massive stars are rare - typically 1 in 1000. However, they largely compensate by being extreme in many ways - gigantic, extra hot and super luminous. But they pay their utter oddness the highest price : by living fast, they die young. During the few million years of their short life, they strip off massive amounts of gas from their surfaces under their own radiation, and propel it in the interstellar medium at an incredible speed of almost 100,000 km/s! Thanks to this, they are the main galactic sources of atoms heavier than oxygen; rocks, trees and animals all started their life in massive stars. For this reason, they control the chemical life of a galaxy. They also trigger bursts of star formation, such as that occuring at the heart of the famous Orion nebula. They end their life as an apocalypse firework - a supernova - and give birth to the well known black holes.

The blue supergiant zeta Orionis (top left) is the brighest star in the immediate neighbourhood of the Horsehead and flame nebula. (© Valère Perroud).
However, the Sun and its cool cousins (as well as some planets including the Earth) have one property that massive stars do not - the ability to generate their own magnetic field, a kind of a star-size electromagnet powered by the giant convection cells that cool dwarf stars upkeep in their external layers. Admittedly, some massive stars host magnetic fields as well; these may even by the progenitors of the intriguing magnetars, the most magnetic stars of the Universe. But they differ from the Sun in a very important aspect: magnetic fields of massive stars are likely birth imprints - fossil remnants that only degrade with time - rather than magnetic fields in continuous rebirth like that of the Sun, which starts afresh every 11 years.

Located in the constellation of Orion, in the immediate neighbourhood of the Horsehead and flame nebula, zeta Orionis is a blue supergiant known from astronomers since a long time. At a distance of about 1200 light years from us, it is a naked-eye star, actually the brighest of all massive stars in the sky. Zeta Orionis is 40 times more massive, 25 times larger, 5 times hotter and 500,000 more luminous than the Sun. An international team of astrophysicists1 just discovered that zeta Orionis apparently hosts a very special magnetic field. "This field is about 10 times weaker than that detected in all other massive stars up to now. This peculiarity may suggest that the field of zeta Orionis is not a fossil", as Jean-Claude Bouret explains. "Could it be of the same nature as that of the Sun? And if so, how can it be generated - since zeta Orionis is obviously lacking the necessary ingredients (the outer convection zone) to produce a field like that of the Sun?"

The magnetic field of zeta Orionis is thus a true mistery, that Jean-Claude Bouret and his colleagues will try to unravel in the forthcoming years. In this aim, they will use the new NARVAL2 spectropolarimeter, recently installed at the 2m Télescope Bernard-Lyot3 atop Pic du Midi in the southern Pyrenees (France). NARVAL is the instrument with which the magnetic field of zeta Orionis was discovered - it is the only instrument worldwide fully dedicated to the study of stellar magnetic fields, and detects them through the polarisation they generate in stellar light.

Press contacts in France:
Marseille: Jean-Claude Bouret, Lab d'Astrophysique de Marseille, Obs Astronomique Marseille Provence. Tel: +33 491056902, email: jean-claude.bouret[AT]
Toulouse: Jean-François Donati, Lab d'Astrophysique de Toulouse-Tarbes, Obs Midi-Pyrénées. Tel: +33 561332917, email: jean-francois.donati[AT]
Montpellier: Fabrice Martins, Groupement d'Astronomie et Astrophysique du Languedoc, Université Montpellier II Tel: +33 467144042, email: fabrice.martins[AT]
[1] This team includes JC Bouret (CNRS/Université de Provence), JF Donati (CNRS/Université de Toulouse), F Martins (CNRS/Univesité Montpellier II), C Escolano (CNRS/Université de Provence), W Marcolino (CNRS/Université de Provence), T Lanz (University of Maryland, USA), I Howarth (University College London, UK)
[2] NARVAL was cofunded by the Région Midi-Pyrénées, the Ministère de la Recherche, the conseil Général des Hautes Pyrénées, the European Union (FEDER funds) and CNRS/INSU. First light occured on 2006 Nov 13.
[3] The operation of the 2m Télescope Bernard Lyot (TBL) is funded by the CNRS Institut National des Sciences de l'Univers (INSU).