Projects of the Odyssey team

Using multi-technique observations, our studies aims at characterizing the statistical properties of young stars in clusters (Initial Mass Function, multiplicity, spatial and kinematic distribution...) in order to constrain their formation process and investigate the effect of environment. In particular, we have shown indication for a top-heavy IMF for the youngest clusters of the massive star forming region Cygnus X (Maia et al. 2016). We have developed a hierarchical Bayesian code to infer cluster membership probability based on photometry and proper motion (Olivares et al. 2018; PhD thesis 2015-2017). A special effort has also been made to develop statistical tools to identify and quantify stellar substructures at various scales, from the binary regime to the loose groups. Application of these techniques to the Taurus star forming region has revealed a population of of ultra-wide systems (5,000-60,000 AU separation) whose properties favor a cascade fragmentation scenario (Joncour et al. 2017), and led to the identification of stellar groupings, which we interpreted as imprint of the star formation process in this cloud (Joncour et al. 2018, Fig. 1). This work has been done in the framework of the “StarFormMapper” H2020 European project.

Moreover, our group is deeply involved in the preparation and exploitation of the Gaia data, which provide parallaxes and proper motions for the brightest/less embedded cluster stars with an unprecedented accuracy. This led in particular to the publication of the HR diagram as seen by Gaia (Babusiaux et al. 2018, Fig. 2). While C. Babusiaux is part of the Gaia DPAC, other team members were involved in complementary ground-based kinematic survey (Gaia-ESO-Survey, DANCe). Combining photometry, spectroscopy and astrometric data, cluster membership can be refined, 3D extinction map reconstructed, and stellar stuctures analyzed in the 6D phase-space.

The early phase of star & planet formation

Our studies also aims at characterising the earliest phases of star and planet formation. We use the NOEMA and ALMA observatories to study the chemical composition of the protostellar envelopes, in particular properties of snowlines (Anderl, Maret et al. 2016), and the formation of disks at early stages (Maury et al. 2019, Maret et al. 2020). This work was funded by the ANR Chemodyn (2012-2015) that was lead by S. Maret. A by-product is the publicly-available Astrochem code (S. Maret & E. Bergin 2015) that we now use to study the chemistry in disks in the framework of the ANR PFD.

High-angular observations of proto-planetary disks

ALMA and SPHERE are two workhorse instruments, providing us with truly transformational data. The team is extremely active in the exploitation of these facilities. The scientific production was very significant in terms of sheer number of papers (100+). Among the most important discoveries is the realisation that disks, probably all of them, are not at all smooth symmetric. They all exhibit rings and gaps, and/or spirals, and/or vortices that may be just as many indications for the presence of planets (e.g. Benisty et al. 2017, Fig. 3 Left). We clearly demonstrated with ALMA that strong vertical dust sedimentation is already taking place (Pinte, Dent, Ménard et al. 2016). Signatures of slow molecular disk winds, probably magnetic, have been also revealed with ALMA (Louvet, Dougados et al. 2018). These winds may be major actors in disk angular momentum transport and evolution.

First detections of young proto-planets

More recently, as progress was made with tailored data processing, ODYSSEY also demonstrated its skills by discovering the first few planets still embedded in their young, gas-rich, disks at an epoch when they are still growing. We used SPHERE to discover PDS 70b (Keppler, Benisty et al. 2018, Fig. 3 Right). We used ALMA to discover kinematical signatures of Jupiter mass planets in HD163296 and HD97048 (Pinte, Price, Ménard et al. 2018). The ALMA discoveries made clear predictions about the exact location of the planets, these predictions are now verified unambiguously with SPHERE in HD97048 (Pinte, van der Plas, Ménard et al. 2019). Catching planets embedded at their birth sites, during their formation, was one of our long term goals and is now triggering new directions of researches.

Another of our strengths is the detailed analysis capacity we have developed in parallel. Our radiative transfer code (MCFOST) was expanded to include molecular rotational lines and is now being further developed, as part of the SPIDI ERC project, to include atomic lines to model the accretion process onto the star. Regarding the molecular line treatment, the coupling of MCFOST with Astrochem allows careful calculations of molecular abundance and proper level population and ray tracing calculations. In the context of the ANR PFD, MCFOST has also been coupled with results from (M)HD simulations, in particular performed by the Sherpas team with Pluto.

Magnetic fields and star-disk interactions

ODYSSEY members are actively involved in the scientific exploitation of optical and near-infrared spectropolarimeters (CFHT/ESPADONS and SPIROU, TBL/NARVAL, ESO/HARPS-Pol). Large scale observing campaigns are organized, including interferometry, high resolution spectroscopy and spectropolarimetry, multi color photometry to investigate the star-disk interaction region in young stellar systems. These campaigns involve a number of collaborators world-wide, such as Jean-Francois Donati (IRAP, France), Silvia Alencar (UFMG, Brazil), Gaitee Hussain (ESA, The Netherlands), Konstantin Grankin (CrAO, Crimea), Gaspard Duchêne (U. California, USA) Agnès Kospal (Konkoly Obs., Hungary) Oleg Kochukhov (Uppsala U., Sweden), and Ann-Marie Cody (NASA-Ames, USA) among others. We characterize the magnetic intensity and topology in various classes of pre-main sequence stars, from class I protostars and low-mass T Tauri stars to Herbig Ae-Be stars (Fig. 4, Villebrun et al. 2019, Pouilly et al. 2020, 2021). We are part of the SPIRou Legacy Survey (2019-2024) which will allow us to unveil the magnetic properties of deeply embedded protostars. We combine these measurements with time domain analysis of spectral and photometric variability (K2, TESS) to derive the structure and dynamics of the magnetospheric accretion region extending from the inner disk edge to the stellar surface (Alencar et al. 2018; Donati et al. 2019, 2020, Bouvier et al. 2020, Sousa et al. 2021). We are developping magnetospheric accretion models, and NLTE radiative transfer in spectral lines, in order to predict the structure of the accreting flows and their spectral signatures that we compare to our observations (Fig. 5, Pantolmos et al. 2020, Tessore et al. 2021). Ultimately, we have three aims:

• detecting nascent planets embedded in the inner disk of these systems, orbiting at a distance of only a fraction of au from the central star. This is the goal of the ERC-funded SPIDI project.
• understand the origin of stellar magnetic fields and follow their protostellar evolution. This is one of the objectives of the ANR-funded PROMETHEE project.
• predict the impact of the magnetic field on the protoplanetary disk evolution and on the inner planet formation. This will be addressed in both SPIDI and PROMETHEE projects.

Near-IR interferometric studies of young stars/disks

We conducted the first extended near-infrared interferometric survey of young pre-main sequence stars with PIONIER at VLTI (Lazareff et al. 2017) and GRAVITY (GRAVITY Collaboration: Perraut et al. 2019). The study, which contains more than 50 objects, provides the first statistical evidence that the sublimation transition is radially wide and vertically thick which puts strong constraints on dust properties. Moreover, it offers a unique database to relate inner disk properties with the larger scales observed with ALMA/SPHERE and/or with the mid-infrared interferometric instrument MATISSE. For the best resolved objects, image reconstruction using dedicated algorithms where used to map morphological distorsions in the inner disks that could betray planet formation processes (Fig. 5, Kluska et al. 2020). Members of the ODYSSEY team are strongly involved in the scientific exploitation of the GRAVITY VLTI instrument installed in 2016 (GRAVITY collaboration 2017). In particular we are co-leading the Young stellar objects GTO Large Program, which includes about a hundred young stars (Herbig and T Tauri stars) to be observed in the K-band continuum and across emission lines such as the Hydrogen Brγ and the CO bandheads (GRAVITY Collaboration: Caratti o Garatti et al. 2020). These targets span a wide range of properties as we will search for correlations with central star properties (spectral type, mass, luminosity, age, mass accretion rate) and disk properties (reprocessed flux, presence of gaps, flared/flat morphology). First GRAVITY observations of T Tauri stars across the Brγ line have been published in GRAVITY Collaboration: Garcia-Lopez et al. (2017) and in Bouvier et al. (2020) where the magnetospheric accretion region of the young pre-transitional disk system DoAr44 is probed. The latter study is part of a spectro-polarimetric and time monitoring campaign as described above.