Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 486, Issue 4, Pages 4829-4844Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stz1190
Keywords
accretion, accretion discs; planets and satellites: formation; protoplanetary discs; circumstellar matter; submillimetre: planetary systems
Categories
Funding
- DISCSIM project - European Research Council under ERC-2013-ADG [341137]
- National Science Foundation [PHY-1607611]
- Simons Foundation
- European Union [823823]
- Munich Institute for Astro-and Particle Physics (MIAPP) of the DFG cluster of excellence 'Origin and Structure of the Universe'
- Netherlands Organisation for Scientific Research (NWO) [016.Veni.192.233]
- UK Science and Technology Research Council (STFC)
- project PRIN-INAF 2016 The Cradle of Life - GENESIS-SKA (General Conditions in Early Planetary Systems for the rise of life with SKA)
- STFC [ST/S000623/1] Funding Source: UKRI
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Protoplanetary disc surveys conducted with Atacama Large Millimetre Array (ALMA) are measuring disc radii in multiple star-forming regions. The disc radius is a fundamental quantity to diagnose whether discs undergo viscous spreading, discriminating between viscosity or angular momentum removal by winds as drivers of disc evolution. Observationally, however, the sub-mm continuum emission is dominated by the dust, which also drifts inwards, complicating, the picture. In this paper we investigate, using theoretical models of dust grain growth and radial drift, how the radii of dusty viscous protoplanetary discs evolve with time. Despite the existence of a sharp outer edge in the dust distribution, we rind that the radius enclosing most of the dust mass increases with time, closely following the evolution of the gas radius. This behaviour arises because, although dust: initially grows and drifts rapidly on to the star, the residual dust retained on-Myr time-scales is relatively well coupled to the gas. Observing the expansion of the dust disc requires using definitions based on high fractions of the disc:flux (e.g. 95 percent) and very long integrations with ALMA, because the dust grains in the outer part of the disc are small and have a low sub-mm opacity. We show that existing surveys lack the sensitivity to detect viscous spreading. The disc radii they measure do not trace the mass radius or the sharp outer edge in the dust distribution, but the outer limit of where the grains have significant sub-mm opacity. We predict that these observed radii should shrink with time.
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