Study of disk features: We are interested in studying young disks around a sample of Herbig AeBe stars and T Tauri stars with SPHERE/VLT and ALMA, VLA. With SPHERE, we will trace the small grains in the upper layers of the disk at scales of tens of AUs. The combination of optical and near-IR images, unique to SPHERE, allows us to constrain the properties of the dust grains at the surface layers (size, porosity). The images provide constraints on the distribution of small grains by resolving features (such as gaps, warps, spiral arms, dips) in the surface layers at typical scales of few tens of AU. We measure the contrast of these features with the background disk, and measure their location. We can relate these constraints to the mechanisms of disk evolution and look at the time variability of the features, which, if observed, would indirectly indicate a perturbation in the inner disk. For the objects observable with SPHERE, complementary observations with ALMA and NOEMA are already available in the archive, albeit at moderate spatial resolution of 0.2" and more. Successful proposals have been led jointly between French and Chilean researchers to look at the brightest objects with the extended configuration that enable to reach a similar resolution to SPHERE (PI : Benisty, Pérez). Such observations enable to probe the distribution of large grains and of gas at similar spatial scales (but in colder layers close to the disk mid-plane), and to measure any surface density and temperature perturbation deep in the disk.

Tracing grain growth, the first step of planetesimal growth: To form planets, dust in disks around young stars must grow in size by several orders of magnitudes. One of the pressing issues is to measure the growth timescale and identify the physical mechanisms involved. With this programme, we therefore look for the effects of dust accumulations in pressure maxima (or particle traps) and search for a spatial segregation of small and large grains which may constrain the properties of embedded planets, and variations of the spectral index. A spatial discrepancy between small/large grains (traced by complementary sub-mm observations) would indicate dust filtration, induced by a planetary companion, or by hydrodynamical instabilities.

Chemical composition and evolution of protoplanetary disks: Planetary systems are formed inside proto-planetary disks. To understand the formation of planetary systems it is of prime importance to determine their chemical composition. One of the most important questions is the heritage: is the chemical composition of the original gas kept during the stellar and planetary formation? How does the composition of planetary disks differ from the one of their parents, the protostars and protostellar cores? Are the most primitive objects of our solar system, namely the comets and some meteorites, formed from unaltered protostellar or interstellar material? Some signs make one think this idea is correct. With ALMA, we are carrying out dedicated observations with unprecedented sensitivity and spatial resolution. The spatial resolution of this radio-interferometer is of the order of 0.01 arcseconds, i.e. several AUs at a distance of 100 pc. At the same spatial resolution (1 arcsecond) ALMA is about 10 times more sensitive than today’s instruments allowing the observation of many more chemical species that can be analyzed with the help of our radiative transfer and chemical models, to determine the chemical composition of proto-planetary disks and their evolution..