The circumstellar disc of FS Tau B-a self-consistent model based on observations in the mid-infrared with NACO

Abstract

Protoplanetary discs are a byproduct of the star formation process. In the dense mid-plane of these discs, planetesimals and planets are expected to form. The first step in planet formation is the growth of dust particles from submicrometre-sized grains to macroscopic mm-sized aggregates. The grain growth is accompanied by radial drift and vertical segregation of the particles within the disc. To understand this essential evolutionary step, spatially resolved multi-wavelength observations as well as photometric data are necessary which reflect the properties of both disc and dust. We present the first spatially resolved image obtained with NACO at the VLT in the Lp band of the near edge-on protoplanetary disc FS Tau B. Based on this new image, a previously published Hubble image in H band and the spectral energy distribution from optical to millimetre wavelengths, we derive constraints on the spatial dust distribution and the progress of grain growth. For this purpose we perform a disc modelling using the radiative transfer code MC3D. Radial drift and vertical sedimentation of the dust are not considered. We find a best-fitting model which features a disc extending from 2 au to several hundreds au with a moderately decreasing surface density and Mdisc = 2.8 ×10-2M⊙. The inclination amounts to i = 80o. Our findings indicate that substantial dust grain growth has taken place and that grains of a size equal to or larger than 1mm are present in the disc. In conclusion, the parameters describing the vertical density distribution are better constrained than those describing the radial disc structure.