Dust in protoplanetary disks: observations

Abstract. Solid particles, usually referred to as dust, are a crucial component of inter-stellar matter and of planet forming disks surrounding young stars. Despite the relatively small mass fraction of ≈ 1 % (in the solar neighborhood of our galaxy; this number may differ substantially in other galaxies) that interstellar grains represent of the total mass budget of interstellar matter, dust grains play an important role in the physics and chem-istry of interstellar matter. This is because of the opacity dust grains at short (optical, UV) wavelengths, and the surface they provide for chemical reactions. In addition, dust grains play a pivotal role in the planet formation process: in the core accretion model of planet formation, the growth of dust grains from the microscopic size range to large, cm-sized or larger grains is the first step in planet formation. Not only the grain size distribution is affected by planet formation. Chemical and physical processes alter the structure and chemical composition of dust grains as they enter the protoplanetary disk and move closer to the forming star. Therefore, a lot can be learned about the way stars and planets are formed by observations of dust in protoplanetary disks. Ideally, one would like to mea-sure the dust mass, the grain size distribution, grain structure (porosity, fluffiness), th