Abstract
The thermodynamic structure of protoplanetary discs is determined by dust opacities, which depend on the size of the dust grains and their chemical composition. In the inner regions, the grain sizes are regulated by the level of turbulence (e.g. α viscosity) and by the dust fragmentation velocity that represents the maximal velocity that grains can have at a collision before they fragment. Here, we perform self-consistently calculated 2D hydrodynamical simulations that consider a full grain size distribution of dust grains with a transition in the dust fragmentation velocity at the water-ice line. This approach accounts for the results of previous particle collision laboratory experiments, in which silicate particles typically have a lower dust fragmentation velocity than water-ice particles. Furthermore, we probe the effects of variations in the water abundance, the dust-to-gas ratio, and the turbulence parameter on the disc structure. For the discs with a transition in the dust fragmentation velocity at the water-ice line, we find a narrow but striking zone of planetary outward migration, including for low viscosities. In addition, we find a bump in the radial pressure gradient profile that tends to be located slightly inside the ice line. Both of these features are present for all tested disc parameters. Thus, we conclude that the ice line can function both as a migration trap, which can extend the growth times of planets before they migrate to the inner edge of the protoplanetary disc, and as a pressure trap, where planetesimal formation can be initiated or enhanced.
Highlights
Planets are born in the protoplanetary disc that surrounds young stars for the first couple million years of their lifetimes
We chose a water abundance of 50% and a dust-togas ratio of 1% and compared the disc featuring a transition in the dust fragmentation velocity from 1 to 5 m s−1 (Fig. 2) to discs that have a constant dust fragmentation velocity profile
This is due to the dust grains being able to grow to larger sizes for higher dust fragmentation velocity values (Eq (5)), leading to colder discs, in which the ice line is located at smaller orbital distances from the star
Summary
Planets are born in the protoplanetary disc that surrounds young stars for the first couple million years of their lifetimes. The growth of particles into pebbles was found to be facilitated at the water-ice line, where the local solid surface density is increased (Weidenschilling 1977; Hayashi 1981) and water vapour condenses on grains (Ros & Johansen 2013), preferably those that already have icy surfaces (Ros et al 2019), enabling them to grow to larger sizes than via pure coagulation This increases the local solid-to-gas ratio and enhances planetesimal formation through the streaming instability (Ros & Johansen 2013; Schoonenberg & Ormel 2017; Drazkowska & Alibert 2017; Yang et al 2017). The resulting pileup of small grains inside the water-ice line could be a trigger of planetesimal formation (Ida & Guillot 2016; Armitage et al 2016)
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