The simple magnetic field of an ultra-cool star

Although it looks always the same, the Sun is variable in time. Since Galileo, we know that dark spots come and go on its surface. The Sun also triggers changes of its luminosity, which, although small, are nevertheless able to affect the Earth climate. For instance, scientists think that a decrease in the luminosity of the Sun is the probable cause of the Little Ice Age, the cool period that prevailed on Earth from the 15th to the 18th centuries. These changes of the Sun are attributed to secular modifications of the magnetic field that the Sun produces in its interior through a mechanism which is not yet fully deciphered.

Magnetic loops at the surface of the Sun, as seen with the TRACE solar spacecraft. (©TRACE operation team, Lockheed Martin)

It is widely believed that the Sun's magnetic field is generated and maintained by a physical process operating inside the Sun, called magnetic dynamo. The magnetic dynamo involves two main ingredients. The first ingredient is the large-scale cellular motions (convection) operating in the outer layers of the Sun's interior (called the convection zone) to evacuate the energy produced by the Sun in its core. The second ingredient is differential rotation, describing how rotation varies with latitudes and depths throughout the convective zone. The contribution of both ingredients produces a large-scale magnetic field that oscillates with time, on a period of about 22 yr. Modern theories of the solar magnetic dynamo speculate that the Sun's magnetic field is mostly produced within the thin interface layer between the outer convective zone and the inner solar regions; there are however many aspects of the solar dynamo that the current models cannot reproduce.

Internal structure of the Sun. Going from the inner to the outer regions, one can successively see, in this cutaway image, the radiative core (where the energy of the Sun is produced), the radiative zone (that radiates the core energy outwards through photons), the convective zone (that evacuates the energy outwards through cellular motions, like boiling water in a heated pan) and the surface (where dark spots are seen). A thin interface layer is present between the radiative zone and the convective zone, and is thought to be where the magnetic field of the Sun is produced. (©David Hathaway, NASA)

Studying magnetic fields of stars is a novel way of studying the magnetic field of our Sun, and investigate how it depends on the mass, age, rotation or temperature of the star. Ultra-cool stars have turbulent convection patterns extending all the way from their surfaces to their centres and thus feature no interface layer like that of the Sun. This has led scientists to predict that their magnetic fields should be more chaotic and less structured than the organised pattern of polar fields and spot belts seen in the Sun.
The problem is that most stars are far too distant for us to be able to disclose details at their surfaces, even using the most sophisticated imaging tools such as astronomical interferometers. Fortunately enough, stars that rotate fast enough offer an opportunity to overcome this problem. Using tomographic imaging techniques such as those used in medical applications (eg scanners), one can reconstruct the details of the brightness and magnetic field distributions at the surfaces of such stars from spectroscopic observations carried out over complete rotation cycles.
This technique is called Doppler imaging as it uses the Doppler effect that the rotation of the star induces in the stellar light; this is the very same effect that produces the change of an ambulance siren's pitch as the vehicle races towards, then away from you.

Basic principles of Doppler imaging. Rotation induces a strong correlation between the spatial location of features at the surface of a star, and their signatures in spectral lines. In this example, the model star (upper panel) is shown at rotational phase 0.15 and features a magnetic spot with 2 kG radial field, centred at phase 0.3 and latitude 30 deg. At rotational phase 0.15, the magnetic region appears on the approaching stellar limb and produces a line profile signature shifted to negative velocities with respect to line centre (lower panel), ie at a spectral position reflecting the spatial location of the parent magnetic region. Click on the image to see an animation (1.1 Mb) of how the line profile evolves as the star rotates. (©JF Donati)

V374 Pegasi is an ultra-cool star, with a surface temperature of only 2900 C, in contrast to the Sun's 5500 C. Its mass and radius are less than one-third the mass and radius of the Sun. Such stars are believed to host turbulent convection patterns extending all the way from their surfaces to their centres.
By applying Doppler imaging techniques to V374 Peg, an international team of astrophysicists[1] succeeded in recovering the distribution of magnetic fields emerging from its surface. They find that V374 Peg has a very simple, organised global magnetic field structure rather like that of the Earth (though much stronger), or of a bar magnet. The strength of this magnet is comparable to that of the field emerging from the sunspots, about 1000 times stronger than the global magnet of the Sun itself. They could also infer that V374 Peg rotates mostly as a solid body.

This plot shows the distribution of magnetic field reconstructed at the surface of V374 Peg (phase 0.5, upper panel) with Doppler imaging techniques; the lower panel illustrates the line profile corresponding to this distribution and rotation phase. Click on the image to see an animation (1.5 Mb) of how the line profile evolves as the star rotates. The magnetic image was derived from the observations of how the line profile evolves as the star rotates. (©JF Donati)

Although these new observations confirm the theoretical prediction that small deeply-turbulent stars such as V374 Peg should rotate as solid bodies, the simple form of the magnetic field came as a complete surprise to the team. The magnetic topology of V374 Peg is indeed very different from that of all stars observed with the same technique, and hosting relatively shallow convective zones like the Sun; these stars show a much more intricate distribution of magnetic regions than that derived for V374 Peg. Moreover, the magnetic topology of V374 Peg is very challenging for all existing theoretical models of magnetic dynamos of ultra-cool stars predicting that ultra-cool stars should produce mostly chaotic field structures, in complete contrast with the simple topology found by the team.

This image shows the magnetic field lines of V374 Peg, extending in space above the surface of the star. The simple magnet-like structure of the field is very obvious. Field lines forming loops above the surface are shown in white, while field lines open to the interstellar medium are shown in blue. Click on the image to see an animation (6 Mb) showing how the observer's view changes as the star rotates. (©MM Jardine & JF Donati)

This discovery was made possible thanks to ESPaDOnS[2], the new instrument built by the Laboratoire d'Astrophysique de Toulouse-Tarbes (Observatoire Midi-Pyrenees, France) and recently installed on the Canada-France-Hawaii Telescope[3] atop the big island of Hawaii. This instrument, especially designed for observing and studying magnetic fields in stars other than the Sun, is the most powerful instrument worldwide for carrying out this kind of research. In particular, ESPaDOnS is the only instrument that can study magnetic field topologies of small, faint stars such as V374 Peg, that are notoriously difficult to observe in detail. A copy of ESPaDOnS, nicknamed NARVAL[4], will be implemented very soon on the 2m Bernard Lyot Telescope atop Pic du Midi in France. An industrial partnership is being studied for building additional copies.

ESPaDOnS consists of two modules, a polarimeter mounted at the telescope focus (top right), fibre-feeding a bench-mounted high-resolution spectrograph (top left). The Canada-France-Hawaii Telescope is located atop the Mauna Kea volcano, in the big island of Hawaii(©OMP, CFHT)