Protostars and Planets VI, Heidelberg, July 15-20, 2013
Large grains can grow in circumstellar discs
Meru, Farzana (ETH Zürich)
Galvagni, Marina (Universität Zürich)
Olczak, Christoph (Universität Heidelberg)
Garaud, Pascale (UC Santa Cruz)
We perform coagulation and fragmentation simulations to understand the growth of grains in circumstellar discs. We present a new physically motivated approach to determining the collision velocities of dust particles by combining analytical and numerical techniques. This approach uses a probability distribution function for the collision velocities (also adopted earlier by Okuzumi et al., 2011; Galvagni et al, 2011; Windmark et al., 2012) but crucially, the key development is that it separates the deterministic (i.e. directional) and stochastic (i.e. non-directional) velocities. The particle velocity in any one direction is given by the deterministic velocities (radial drift, azimuthal drift and vertical settling) while the spread in the distribution function is determined by the stochastic components (turbulence and Brownian motion). By applying this technique to T Tauri disc conditions, growth to larger sizes is possible compared to previous studies - up to a few tens of metres and metre-sizes at distances of 1AU and 30AU from the central star, respectively. We study the impact of different disc and star parameters on the underlying physical mechanisms that affect growth as well as on how large the grains can grow. A robust result of our model is the existence of two particle populations: one at small and one at large sizes. This potentially explains a long-standing problem of grain growth in T Tauri discs: growing dust aggregates while still maintaining a population of smaller-sized aggregates that are also detected using infrared wavelength observations.
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