Grain growth in proto-planetary discs: the role of scattering

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Planet-forming discs are the natural environment in which exoplanets are assembled. According to the core-accretion model, planet formation is a bottom up process: initially m-sized, dust grains grow several orders of magnitude to form roughly 100 km-large planetesimals. This is why assessing whether grain growth is actually taking place in Myr-old protoplanetary discs is critical to understand how the currently observed Gyr-old exoplanets are born. Furthermore, estimates of the maximum grain size in planet-forming discs posit strong constraints on the planet-formation models (e.g., how rapidly do planetesimals form? Which is the minimum amount of solids in discs at a given time for planet formation to be viable?). The grain-growth hypothesis has been tested relying on the spectral index, a (either spatially resolved or, more commonly, disc-integrated). In the optically thin regime and the Rayleigh-Jeans approximation, it can be simply related to the absorption spectral index. The latter, in turn, is a function of the maximum grain size. This was the presence of mm to cm large grains was inferred in the past years. However, it is now known that optically thick sub-structures such as rings or spirals are ubiquitous in protoplanetary discs suggesting that the previous approximation is no longer valid. Indeed, in the optically thick case a = 2 regardless of the grain size. Moreover, recent works focusing on single sources suggest that also dust self-scattering can be important in the optically thin scenario: overlooking scattering brings about an overestimation of the dust sizes for grains with high albedo. In my project I study grain growth in a self-consistent way to assess the role of dust scattering in the determination of the maximum grain size. Confronting models and data, my aim is that of providing reliable estimates of the maximum grain sizes in populations of discs in the nearby star forming regions.