From circumstellar disks to planetary systems: observation and modeling of protoplanetary disks

Abstract

The existence of exoplanetary systems was first predicted after the discovery of accretion disks around young stars. Nowadays, with nearly 3500 exoplanets discovered, and almost 5000 more candidates identified by the Kepler space mission, planetary systems are now known to be ubiquitous around low-mass stars. The formation of these systems takes place during the stellar formation itself, from the dust and gas orbiting around the star in the protoplanetary disks. However, the process that leads to this formation is still not well understood. Studying the physical and chemical conditions of circumstellar disks is, thus, key to understand the planetary formation process.

Planets can interact with the disk, creating structures such as spiral density waves, gaps, cavities or lopsided asymmetries. Studying disks showing these features could provide us with critical information about the planetary formation process itself. In particular, transitional disks, which are protoplanetary disks with central gaps or cavities in the dust distribution, appear as excellent candidates to study the first stages of planetary formation. Cavities in transitional disks were first identified through modeling of their spectral energy distributions (SEDs). This modeling also lead to the discovery of a subfamily of transitional disks, the so-called pre-transitional disks, which are thought to present a residual inner disk inside the cavity. (Sub-)mm and polarimetric IR observations have been able to image several of these disks and confirm the presence of the central cavities or gaps.

Several mechanisms have been proposed to explain the clearing of these dust cavities: grain growth could reduce the emissivity of the dust grains, photoevaporative winds due to the high energy radiation from the star could remove material from the inner regions of the disk, and dynamical interactions with low-mass companions or planets could clear a gap or cavity. Even though some transitional disks have cavities that might be consistent with a photoevaporation origin, observations seem to indicate that most cavities in transitional disks are created by dynamical interactions with orbiting substellar or planetary companions.

Nevertheless, the sample of observed transitional disks is not complete, and it is possible that, due to an observational bias, most of the transitional disks that have been studied belong to a family of denser disks that can form giant gas planets responsible for opening their cavities. More observations are needed to completely understand the impact of all the physical processes that take place during the last stages of the evolution of protoplanetary disks.