Grain size distribution in protoplanetary disks

Sammanfattning: Protoplanetary disks consist of matter in both gas and solid form. The sizes of the solid particles, dust grains, although in minority compared to the gas, are important in the formation of larger bodies such as planetesimals. This is a topic which has been widely explored. However, the impact of the dust grain sizes on the underlying disk structure has not yet been as extensively modeled. Different sizes of dust grains are often disregarded. Dust particles are the main contributors to the opacity of a protoplanetary disk and thus have a great impact on the disk temperature. This is due to the cooling rate dependence on opacity. The aims of this work is to, first model a dust size distribution in a protoplanetary disk, and second, calculate the new mean opacity of the disk due to the size distribution. Birnstiel et al. (2011) provides a recipe for simulating dust size distributions in protoplanetary disks numerically. The input parameters for the recipe are taken from the disk structures provided by Bitsch et al. (2015). These disks use only micrometer sized grains for opacity calculations (Bell & Lin 1994). The result is grain size distributions for the considered disk structures. The RADMC 3D code provided by Dullemond is used to calculate the mean opacity of the grain size distributions. The resulting mean opacity is compared to the Bell & Lin (1994) opacity used in the Bitsch et al. (2015) disk structures. The temperature, surface density and scale height parameters are fully determined by the disk structure. However, the fragmentation velocity of grains, the metallicity and the turbulent strength of the disk, have certain ranges of allowed values in a protoplanetary disk. The mean opacity is comparable to the Bell & Lin (1994) opacity only for a very specific parameter space. This indicates that the micrometer grain approximation is, in most cases, not very good.