The first stages of planet formation in binary systems: how far can dust coagulation proceed?

Abstract

Context. On the basis of current theoretical models, giant planets can form via gravitational instability or core accretion. The terrestrial planets are "failed cores" in the core accretion paradigm that do not evolve into gas giants. Planets residing in binary systems place strong constraints on planet formation theory as any model must work in binary systems too, in spite of the strong perturbations from the secondary star. Aims. We examine the first phase of the core accretion model, namely the dust growth/fragmentation in binary systems. In our model, a gas and dust disk exists around the primary star and is perturbed by the secondary. We study the effects of a secondary with/without eccentricity on the dust population to determine the sizes the aggregates can reach and how that compares to the dust population in disks around single stars. Methods. We solve the equation of motion of dust aggregates including gas drag and gravitational forces from both stars, as well as the hydrodynamical equations for the gas disk. We determine the velocity of these particles relative to the gas (or relative to micronsized, well-coupled dust particles) and construct a collision model with growth and erosion. Particles below the critical fragmentation velocity increase in mass by sweeping up the small dust population; particles above this critical speed are eroded by collisions with the small dust population. We use this model to determine the sizes that aggregates can reach in the disk, in addition to the equilibrium mass and stopping time distributions between growth and erosion. Results. We find that the secondary star has two effects on the dust population: 1. the disk is truncated because of the presence of the secondary star, and the maximum mass of the particles is lower in lower gas densities. This effect is predominant in the outer disk; 2. the perturbation of the secondary increases the eccentricity of the gas disk, which in turn increases the relative velocity between the dust and the gas. Hence the maximum particle sizes are further decreased. The second effect of the secondary influences the entire disk. Coagulation is efficiently reduced even at the very inner parts of the disk. The average mass of the particles is lower by four orders of magnitude (as a consequence, the stopping time is shorter by one order of magnitude) in disks around binary systems than for dust in disks around single stars.

Formation of dust-rich planetesimals from sublimated pebbles inside of the snow line

“…Assuming a sound speed c s ∼ 270(r/100 au) −3/14 m/s implies v col ∼ 50(α GI /0.1) 1/2 (τ s /0.1) 1/2 (r/100 au) −3/14 m/s. The threshold velocity for fragmentation of icy particles is predicted to be around 20−100 m/s (e.g., Blum & Wurm 2000;Zsom et al 2011;Wada et al 2011). We therefore assume that pebbles may form only in the stable parts of the disks, that is, for r < r GI , implying that the location of the pebble formation front is given by min(r peb , r GI ).…”

Understanding the water emission in the mid- and far-IR from protoplanetary disks around T Tauri stars

“…Fragmentation can explain the presence of small grains in protoplanetary disks (a < 100 μm) after 10 4 yrs (Dullemond & Dominik 2005;Zsom et al 2011). Small grains experience a complex interplay between collisions and coagulation (Dullemond & Dominik 2008).…”

Secular dynamics of planetesimals in tight binary systems: application toγ-Cephei

“…Duquennoy & Mayor 1991), ∼90% of exoplanets are associated with single stars (Zsom et al 2011). It is not yet clear whether this discrepancy is solely due to observational bias, or if the process of planetary formation may be seriously impaired even in very wide binary systems.…”