A large population study of protoplanetary disks: Explaining the millimeter size-luminosity relation with or without sub-structure

Abstract: Recent sub-arcsecond resolution surveys of the dust continuum emission from nearby protoplanetary disks showed a strong correlation between the sizes and luminosities of the disks. We aim to explain the origin of the (sub-)millimeter size-luminosity relation (SLR) between the $68\%$ effective radius ($r_{eff}$) of disks with their continuum luminosity ($L_{mm}$), with models of gas and dust evolution in a simple viscous accretion disk and radiative transfer calculations. We use a large grid of models ($10^{5}$ simulations) with and without planetary gaps, varying the initial conditions of the key parameters. We calculate the disk continuum emission and the effective radius for all models as a function of time. By selecting those simulations that continuously follow the SLR, we can derive constraints on the input parameters of the models. We confirm previous results that models of smooth disks in the radial drift regime are compatible with the observed SLR ($L_{mm}\propto r_{eff}^{2}$) but only smooth disks cannot be the reality. We show that the SLR is more widely populated if planets are present. However they tend to follow a different relation than smooth disks, potentially implying that a mixture of smooth and sub-structured disks are present in the observed sample. We derive a SLR ($L_{mm}\propto r_{eff}^{5/4}$) for disks with strong sub-structure. To be compatible with the SLR, models need to have an initially high disk mass ($\geq 2.5 \cdot 10^{-2}M_{\star}$) and low turbulence-parameter $\alpha$ values ($\leq 10^{-3}$). Furthermore, we find that the grain composition and porosity drastically affects the evolution of disks on the size-luminosity diagram where relatively compact grains that include amorphous carbon are favoured. Moreover, a uniformly optically thick disk with high albedo ($0.9$) that follows the SLR cannot be formed from an evolutionary procedure.