Institut de Planétologie et d’Astrophysique de Grenoble

Magnetic fields play a tremendous role all along the formation of stars, by structuring the collapsing objects and amplifying the ejections of materials. In the Odyssey team we focus mainly on the latest stage of the stellar formation (the pre-main-sequence, PMS, phase) by addressing the following :

• the magnetic interaction between the star and its protoplanetary/accretion disk
• the genesis of fossil-and dynamo-fields in T Tauri (TTS) and Herbig Ae/Be (HAeBe) stars
• the origin of jets during the T Tauri and Herbig phases

Forming stars

Stars form by the gravitational collapse of cold molecular cores that give birth to an embedded protostar surrounded with a thick disk (class 0/I objects). Once the protostar gets rid of its envelope by accreting or expelling it, the central PMS star becomes visible (class II objects). It is surrounded with a disk from which the star continues to accrete material until it dissipates, and in which planets form. Once the disk has been dissipated, and the planets are formed, a stellar system, as our own solar system, is born, and the central star, stop its contraction, ignite nuclear reaction in its core, and starts its long journey along the main sequence (MS). Here, we focus mainly on the phases preceding the MS phase.

Magnetospheric accretion

While gravity is the main actor of star formation, the magnetic field is believed to play also a major role. In particular, it is now well accepted that the low-mass classical T Tauri stars (CTTS) are accreting from their disk via magnetic funnels. As a result gas is reaching the stellar surface at free-fall velocities, creating hot spots at the base of the funnels on the stellar surface, and emitting high-energy photons from the UV to the X-ray. The Odyssey team addresses the physics of these phenomena.

The magnetic commitment of the star to its disk

Stellar magnetic fields are anchored both in the stellar surface and in the inner rim of the accretion disk, forcing the star to rotate at the same rate as the inner part of the disk. As a result, the star, still contracting under gravity is not free to accelerate and looses angular momentum. The Odyssey team try to understand the role of the magnetic fields on the angular momentum evolution of PMS stars.

Supersonic jets driven by magnetic stars

The interaction between the magnetic field of the star and its accretion disk is also believed to be at the origin of the launch of collimated and rapid jets, carrying away mass and angular momentum. The processes at the origin of the jets are however not well understood and constitute one of the research studies of the Odyssey team.

What if the star is bigger than the Sun ?

The global picture of the magnetospheric accretion/ejection processes is now well accepted for the low-mass TTS, but it is not yet clear if this picture can be applied at high-mass (above 1.5 solar masses), i.e. among the intermediate-mass T Tauri stars or the Herbig Ae/Be stars, which evolve much faster and radiate at higher temperature. In the Odyssey team we are also studying the rotation evolution and the accretion/ejection processes in these objects.

How are generated the magnetic fields ?

To be able to understand how magnetic fields affect the formation and evolution of these PMS objects, it is important to understand their origin. It is believed that in low-mass objects, dynamo processes operating in the upper convective envelope are the main actor in the generation of magnetic fields. On the other side, in the higher-mass Herbig Ae/Be stars, without convective envelope, the magnetic fields are probably of fossil origin, i.e. remnants of star formation. The Odyssey team is therefore addressing the origin of stellar dynamo- and fossil-fields.

From stars to observers

To address these problems, the Odyssey team is using the most sensitive instruments in the world for :

• measuring magnetic fields of stars : the high-resolution spectropolarimeters ESPaDOnS (at the Canada-France-Hawaii Telescope), Narval (at the Téléscope Bernard Lyot in France), HARPSpol (at La Silla Observatory, ESO, Chile)
• measuring photometric variability : Corot, Kepler, K2
• studying the close and hot environments of young objects : SPHERE, GRAVITY and CHARA
• detecting and characterising the jets : ALMA ?

Contact : E. Alecian, J. Bouvier, C. Dougados