Optical appearance of ices in planet forming discs

Abstract

This thesis aims to improve our understanding of the role of ices in planet forming discs, known as protoplanetary discs. Computer models of such discs have been used extensively by the community to understand and predict the observational data. It has always been a challenge to observe ices in the discs and their optical appearance is largely omitted in the disc models. This is due to the complexity and high computational power requirement for computing the opacities while the chemical composition and abundance change at every point in the disc. This work is focused on providing a computationally efficient way of estimating the optical properties of ices in the state-of-the-art Protoplanetary Disk Model (ProDiMo). Although observations of ices are rare, some observations particularly for the 3 micron water ice feature are available. These have been studied by means of different radiative transfer disc models, however, the effect of the position-dependent ice opacities on the physico-chemical disc structure has not been considered. In this work, it is found that the optical properties of dust depend on the ice composition, the ice abundances and how the ice accumulates on the refractory dust grains. It is shown that using a pre-calculated opacity grid and performing interpolation between these grid points provides an accurate and computationally efficient estimation of position dependent ice opacities in protoplanetary discs. Further, it is found that the inclusion of chemically consistent ice opacities calculation in the disc model does not have a significant effect on the physico-chemical structure of the disc. However, the appearance of the disc changes significantly. Dark bands appear in disc images at wavelengths corresponding to ice absorption features. These features are stronger when the thickness of the ice layer on the refractory dust grains is assumed to be constant but weaker when the ice/refractory volume ratio is assumed to be constant. The predicted disc spectra are analysed considering the spatial resolution offerred by spectrographic modes of Near-InfraRed Spectrograph (NIRSpec) onboard James Webb Space Telescope (JWST), expected to launch in 2021. This shows that the ice features can be observed, particularly when the disc is seen against a distant background source (silhouette discs). Thus, silhouette discs are promising observational targets to study ices in discs.