Transformational models for Transformational Data – PLANET-FORMING-DISKS

Protoplanetary Disks around young stars are modern day alchemists, they transform dust into astronomical gold: the planets! But just how exactly they achieve this remains a mystery. Giant planets have to form in the protoplanetary disks early, when they are still rich in gas and dust. Evidence from direct imaging is slowly growing, but the details of the physics at play remains largely unconstrained by observations. The disk lifetimes are short (<10 Myr). In the so-called core-accretion model, to form a planet during that short a period dust grains must settle toward the disk midplane as they are slowed down by gas drag, rapidly forming a thin, dust-rich subdisk. In parallel, dust also must grow by collisions and sticking because of enhanced dust density, supposedly leading to the formation of pebbles and boulders. The rapid inward drift also caused by gas drag acts as a barrier for planet formation when the dust grains reach mm to cm sizes because gas drag becomes so efficient that grains drift inward onto the star on short timescales, a few 10 000yr. Collisions between mm-grains also lead to fragmentation rather than growth because of the fast collisions speeds resulting from the rapid inward drifting. This is the so-called "radial-drift barrier". Despite these severe obstacles, planet formation seems to be a very effective process in view of the numerous planets detected today. Clearly, our current understanding of planet formation is limited by a severe lack of direct measurements of the processes involved. The recent discovery of dust traps in disks (allowing particles to stop and grow for a longer time, if the traps are long-lived) and the direct measurement of an extremely flat dust sub-disk of mm-grains in HL Tau , both done recently by our team, offer tantalizing hints.

In this project we will bring together observers and theorists to scrutinise the new data, identify whether or not the observed disk features are caused by planets, and use these results to improve, calibrate and fine-tune the disk evolution models. The new generations of instruments like SPHERE at the VLT and the ALMA interferometer (both probing the disks at au scales, relevant for planet formation) should rapidly provide many more examples as well, making this project very timely.

We plan to achieve our goals by following three distinct paths. Work Package 1 (WP1) will focus on the physics of Dust Settling and Dust Trapping by combining the state-of-the-art radiative transfer codes, thermo-chemical models and (magneto-)hydrodynamical tools we already have available. WP2 will look for the best imprints of planets in disks, in particular whether spiral arms and gaps, now detected in a few disks, really trace planets or not. This is also important because the number of disks detected with gaps and rings is rapidly growing. WP3 will build on the results of WP1 and WP2 to explore the capacity of older and/or less massive disks to form planets.

For the numerical hydro/MHD models, we will use the codes Pluto and AMRVAC. Pluto is a massively parrallel finite volume code which we have used and developed in the context of protoplanetary discs. It includes modules we developed to treat low ionisation plasmas to compute state-of-the-art numerical models of protoplanetary discs. The SPH-based bi-fluid hydro code of Lyon is already pipelined with the radiative transfer code MCFOST. MCFOST is also pipelined with the thermo-chemical code ProDIMo and has been used as well with AMRVAC in the past.

In France, the expertise is excellent but the forces and efforts are dispersed. To efficiently face competition we have built a group of experts with complementary expertise. We also work together well. Our specific needs are twofold. First we need the financial support to meet regularly and facilitate exchange among us. Second, we need man-power to produce results faster, within a competitive time frame. This is required to face the fierce competition.