A large population study of protoplanetary disks

Explaining the millimeter size-luminosity relation with or without substructure

¹ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
e-mail: zormpas@usm.lmu.de
² Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
³ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Received: 18 August 2021
Accepted: 2 February 2022

Abstract

Recent subarcsecond resolution surveys of the dust continuum emission from nearby protoplanetary disks show 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 (10⁵ simulations) with and without planetary gaps, and vary 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 ∝ r_eff²), 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 substructured disks are present in the observed sample. We derive a SLR (L_mm ∝ r_eff^5/4) for disks with strong substructure. To be compatible with the SLR, models need to have an initially high disk mass (≥2.5 × 10⁻² M_*) and low turbulence-parameter a values (≤10⁻³). Furthermore, we find that the grain composition and porosity drastically affects the evolution of disks in the size-luminosity diagram where relatively compact grains that include amorphous carbon are favored. Moreover, a uniformly optically thick disk with high albedo (0.9) that follows the SLR cannot be formed from an evolutionary procedure.