Protoplanetary disks and planet formation

The planet formation around low-mass and intermediate-mass stars occurs over a few-Myr timescale in dust- and gas-rich protoplanetary disks. The recent onset of high-resolution and high-contrast instruments such as the Atacama Large Millimeter Array (ALMA) and the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) at the Very Large Telescope has allowed us to spatially resolve protoplanetary disks, so as to map the disk substructures that are possibly due to the interaction with forming planets.

Substructures of protoplanetary disks

ALMA and VLT/SPHERE surveys have revealed the high occurrence of substructures in protoplanetary disks, such as large cavities, concentric rings, local over-densities. Surprisingly, these features have also been found in disks younger than 1 Myr, suggesting that the planet formation occurs earlier than what standard models predict. A prototypical case is the disc around the low-mass star AS 209 (Fedele et al. 2018, see Fig. 1) which is characterized by a dense central dusty core and two thin dusty rings that can be fit by numerical simulations of disk interaction with yet unseen giant planets.

Figure 1. ALMA image of the large dust grains in the protoplanetary disk around the low-mass star AS 209, revealing the presence of concentric rings that could be generated by the gravitational force exerted by giant planets.

The presence of disk substructures in such young protoplanetary disks has motivated the employment of VLT/SPHERE to systematically characterize the small dust grain distribution in young sources. The DARTTS survey (Disks Around TTSs) has imaged as many as 29 young protoplanetary disks revealing that disks younger than 3 Myr show a lower occurrence of disk substructures and that these are less pronounced than in older disks (Garufi et al. 2020a, see Fig. 2). Furthermore, some of these disks are still surrounded by extended filaments through which the accretion from the surrounding medium is carried out.

Figure 2. VLT/SPHERE images of the scattered light from young protoplanetary disks. Unlike the majority of old protoplanetary disks, disk substructures are rare and shallow revealing that the interaction with forming planets may be less advanced than later in the disk lifetime.

Dust vs molecules in young protoplanetary disks

Structural changes across the protoplanetary disk extent can also be seen by mapping the millimeter polarized light from protoplanetary disks. This has been shown by Bacciotti et al. (2018), who revealed a change in the direction of the polarization vector in the disk of DG Tau (see left panel of Fig. 3) which is indicative of an abruptly different optical depth between the inner and outer part of the disk and/or in a change of the dust properties. Intriguingly, Podio et al. (2019) discovered a bright ring of formaldehyde (H₂CO) emission in correspondence of this structural change (see right panel of Fig. 3) suggesting an intimate connection between the dust and the gas properties.

Characterizing the molecular distribution in protoplanetary disks of the aforementioned formaldehyde, of other simple organic molecules such the methanol (CH₃OH), or of other sulfur-bearing molecules such as sulfur monoxide (CS) or thioformaldehyde (H₂CS) is fundamental to understand the chemical assembly of planetary atmospheres and is the main focus of the ALMA-DOT campaign (ALMA chemical characterization of Disk-Outflow sources in Taurus, see Garufi et al. 2020b). This survey is providing the most spectrally varied maps of very young protoplanetary disks, which are still embedded in their natal envelope and are thus challenging to be observed because of extinction and contamination by the surrounding medium.

Figure 3. ALMA observations of the disk of the low-mass star DG Tau Left: the polarization pattern superimposed on the continuum map at 0.87 mm after the operation of unsharp masking, performed to highlight faint substructures like the visible ring. Right: the molecular distribution of CS (green) and H₂CO (blue) is compared with the dust distribution at 1.25 mm after the operation of unsharp masking is applied.

Dust properties derived from polarization

Polarization at mm wavelengths observed in protoplanetary disks arises in most cases because of self-scattering of the dust thermal radiation. In this case, one can constrain with polarization the grain size population and the geometrical distribution of the dust in the disk.

The Arcetri group investigates with ALMA the polarization in disks from young T Tauri stars (Bacciotti et al., 2018). From the polarization at 0.87 mm toward the disk around DG Tau (Fig 3, left panel), a maximum size of dust grains in the range 50 - 150 μm is derived. The polarization maps give constraints on the scale height of such grains in the disk, and highlight the presence of possible substructures not recognized before.