Evolution of Protoplanetary Dust-Disk Sizes

Abstract

The planets of our Solar System formed within a disk of dust and gas 4.5 billion years ago. While the Solar System’s primordial protoplanetary disk is now gone, we can begin to understand its possible properties by looking out to the disks within nearby star-forming regions. To understand planet formation, dust observations are used to constrain the mechanics and evolution of planet formation. The properties of disks around brown dwarfs and very-low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. Radiative transfer models are fit to the spectral energy distributions of 11 VLMOs with known disks and find that these VLMOs do not follow previously derived disk temperature- stellar luminosity relationships if the disk outer radius scales with stellar mass. The 3 mm continuum observation of the highly inclined transition disk around the star T Cha reveals multiple dust structures: an inner disk, a spatially resolved dust gap, and an outer ring. When compared with previously published 1.6µm VLT/SPHERE imagery, it is found that the location of the outer ring is wavelength dependent. More specifically, the peak emission of the 3 mm ring is at a larger radial distance than that of the 1.6 µm ring, suggesting that millimeter-sized grains in the outer disk are located further away from the central star than micron-sized grains. The most likely origin of the dust gap is from an embedded planet or planets. The dust disk size of 152 protoplanetary disks is estimated from archival ALMA observations. This sample is combined with 47 disks from Tau/Aur and Oph whose dust disk radii were estimated, as here, through fitting radial profile models to visibility data. These 199 homogeneously derived disk sizes are used to identify empirical disk-disk and disk- host property relations as well as to search for evolutionary trends. In agreement with previous studies, we find that dust disk sizes and millimeter luminosities are correlated, but show for the first time that the relationship is not universal between regions. We find that disks in the 2-3 Myr-old are not smaller than disks in other regions of similar age, and confirm the Barenfeld et al. (2017) finding that the 5- 10 Myr USco disks are smaller than disks belonging to younger regions. Finally, we find that the outer edge of the Solar System, as defined by the Kuiper Belt, is consistent with a population of dust disk sizes which have not experienced significant truncation.