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Solar-stellar connection : a new light on the solar magnetic mystery

1er juillet 2008

By measuring the magnetic field of solar twins, an international team of scientists including astronomers from Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA : INSU-CNRS, Observatoire de Paris, Universités Paris VI et VII) has shed a new light on the solar magnetic mystery by replacing this question within the wider perspective of Sun-like stars. Using the NARVAL(1) spectropolarimeter at Télescope Bernard Lyot (2) (Pic du Midi Observatory), they have measured the magnetic geometry of a strict twin of our Sun. Observing 3 other stars also resembling the Sun, except from their faster rotation, they have unveiled a dramatic change in their magnetic structure, with the fast rotation winding up field lines in a tore surrounding the star. This work brings observational confirmation to the most recent theoretical models of solar magnetism. The results are published in Monthly Notices of the Royal Astronomical Society.

Despite being the star closest to us, the Sun has not yet disclosed all its secrets to astronomers. For instance, what is the exact origin of its magnetic field, responsible for its dark spots and its violent eruptive phases ? The multiple manifestations of the magnetic Sun are able to affect the terrestrial environment by their influence on aurorae or by their role in massive electrical perturbations.

Considering such open questions, observing other stars can bring observational constraints that the Sun alone cannot offer. The vast stellar laboratory brings the opportunity to use other stars in order to investigate the magnetic behaviour of astrophysical objects very similar to the Sun, and simultaneously offer the possibility to vary some of their fundamental parameters (mass, age, rotation period) to test their influence on the resulting magnetic field. This new option is now available to astronomers thanks to the NARVAL spectropolarimeter, operated at Télescope Bernard Lyot (Pic du Midi Observatory).

Figure : Prédiction théorique de la configuration magnétique d’une "toupie stellaire", tournant trois fois plus vite que le Soleil. La rotation a pour effet d’enrouler les lignes de champ, créant ainsi des tores magnétiques. © Benjamin Brown. Cliquer sur l’image pour l’agrandir

Using this new instrumental facility, a team of scientist has studied the magnetic field of the Sun-like star 18 Sco. This star is known to be the best solar twin among nearby stars. It is located 46 light-years away from us, in the Scorpio constellation. Its mass is identical to the Sun.s, as well as its luminosity and age (of about 4 giga years). The two objects are also similar in their rotation period (23 days for 18 Sco and 25 days for the Sun). The magnetic field of 18 Sco, unveiled by NARVAL observations, confirms its status of best solar twin with a magnetic geometry very similar to that of the Sun around the peak of its magnetic cycle (last observed around the year 2000).

Similar observations were then obtained for 3 other stars, almost identical to the Sun except their faster rotation (up to 3 times the solar value). It was therefore possible to use other stars to test the impact of a specific parameter (the rotation) on their magnetic field. These new observations have confirmed very recent theoretical studies, by demonstrating that stellar magnetic fields are deeply modified by fast rotation, with the magnetic poles (as observed on the Sun) progressively transformed into magnetic tores surrounding the star. These fast rotating stars look like magnetic spinning tops !

This first success tells us that various theories of the magnegtic Sun can now be confronted to the observation of solar twins. This new connection between theory and observation offers a new method to test magnetic models in a way that the Sun alone cannot offer.

NOTES (1) NARVAL was cofunded by France (Région Midi-Pyrénées, Ministère de la Recherche, Conseil Général des Hautes Pyrénées, INSU-CNRS) and the European Union (FEDER). (2) The Telescope Bernard Lyot (TBL) is financed by INSU-CNRS. See also : On the site of Observatoire Midi-Pyrénées : Connecter le Soleil aux étoiles : vers une solution àl’énigme magnétique solaire ? Press-release INSU-CNRS

Reference

Toroidal vs. poloidal magnetic fields in Sun-like stars : a rotation threshold P. Petit, B. Dintrans, S.K. Solanki, J.-F. Donati, M. Aurière, F. Lignières, J. Morin, F. Paletou, J. Ramirez, C. Catala, R. Fares : MNRAS, in press

Contact

Claude Catala (Observatoire de Paris, LESIA, et CNRS)

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