Projets de l’équipe Odyssey

The origin and evolution of stellar clusters

It is today well known that most stars form in clusters with N>100 members. The cluster environment at stellar birth may significantly impact the properties of young stars through, e.g., gravitational encounters, ionizing radiation field, etc. This is particularly true in dense and rich young clusters. A fundamental issue then is to disentangle properties that are the direct imprint of the star formation process from those that have been subsequently affected by the cluster’s environment.

What are the initial conditions of cluster formation ? Is mass segregation the consequence early dynamical evolution in the embedded cluster or is it an intrinsic property of the formation process ? What is the origin of the mass distribution of stars, brown dwarfs, and planetary mass objects in stellar clusters ? How does the environment impact the properties of young stars and brown dwarfs ?

To address these issues, we aim at :

- fully characterizing the properties of the stellar and substellar populations of young clusters (Initial Mass Function, spatial distribution and kinematics, multiplicity, disk diagnostics, etc.) from multi-wavelengths observations and tailored statistical tools
- understanding the evolution of these properties during the embedded protostellar stage, by developing numerical simulations of the dynamical evolution of stellar clusters that allow us to trace back the initial conditions of their formation.

This work prepares the exploitation of Gaia data that will revolutionize our understanding of stellar clusters. We are involved in ground-based surveys to complement Gaia results, such as the Gaia-ESO-Survey to measure radial velocities and the DANCE project to derive proper motion of faint cluster members.

Another important part of our research on this theme is related to the angular momentum evolution of young stars. Clusters are most useful in this regard as they provide us with a temporal scale to trace the evolution of the rotational properties of stars as they age. To account for the observed evolution of rotational period distributions, we develop parametric models of the angular momentum evolution of low-mass stars as well as MHD simulations of the star-disk interaction. So far, our observational studies and modeling efforts have mostly concentrated on the pre-main sequence phase, and will be extended to the protostellar stage.

Staff members : Jérome Bouvier, Isabelle Joncour, Estelle Moraux Postdocs : Francisco Maia, Colin Folsom

Origin and role of magnetic fields in star formation

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

Gas and dust evolution in protoplanetary disks

The ODYSSey team aims at investigating the dynamical processes that drive the evolution of protoplanetary disks. The variety of exoplanetary systems has to be accounted for from the initial conditions prevailing in the circumstellar disks. In particular, the assumption of axisymmetry has to be relaxed in order to fully address the complex large-scale structure of primordial disks.

The first ALMA observations of protoplanetary disks in the millimeter range (Marel 2013) have shown strongly non-axisymmetric structures that prompt a new consideration of dynamical evolutionary processes. It is often assumed that forming planets interacting with the disk may be at the origin of these large-scale structures. If correct, the characterization of the disk structures may lead to the discovery of planetary systems in the process of formation, while they are still embedded in the disk. This topics is actively pursued in the ODYSSey team, by performing high-angular resolution imaging of disks from the infrared (SPHERE) to the millimeter domain (ALMA, NOEMA), and by developing numerical models of dust and gas evolution coupled to radiative transfer tools (MCFOST).

Disk asymmetries are also present are small scales, notably close to the inner disk edge, at a few 0.1 AU from the central star. Photometric monitoring campaigns (e.g., the CoRoT/Spitzer CSI 2264 campaign ; cf. Cody et al. 2014) have revealed a rich variety of light curves. Of these, many are due to the obscuration of the central star by dusty clumps located in the inner rotating disk. The physical processes that drive the complex 3D structure of the inner disk, such as star-disk magnetospheric interaction and/or embedded inner planets, still need to be fully elucidated and modeled. This is the one of the goals pursued in the ODYSSey team, with an approach that combines spectropolarimetric measurements of surface magnetic fields in young stars (ESPADONS), and spectro-interferometric imaging of the disk’s inner regions (PIONIER/VLTI, CHARA).

Numerical models provide some clues on the origin of large-scale disk asymmetries (e.g. Turner et al. 2014). Thus, gas vortices can result from the barocline instability (Lesur & Papaloizou 2010) while self-organized turbulence may also lead to zonal flows that favors the formation of large-scale structures (Kunz 2013).