The grazing-angle icy protoplanetary disk PDS 453

Context. Observations of highly inclined protoplanetary disks provide a different point of view, in particular, they provide a more direct access to the vertical disk structure when compared to less steeply inclined more pole-on disks. Aims. PDS 453 is a rare highly inclined disk where the stellar photosphere is seen at grazing incidence on the disk surface. Our goal is take advantage of this geometry to constrain the structure and composition of this protoplanetary disk. In particular, it shows a 3.1 µm water-ice band in absorption that can be uniquely related to the disk. Methods. We observed the system in polarized intensity with the VLT/SPHERE instrument, as well as in polarized light and total intensity using the HST/NICMOS camera. Infrared archival photometry and a spectrum showing the water-ice band were used to model the spectral energy distribution under the Mie scattering theory. Based on these data, we fit a model using the radiative transfer code MCFOST to retrieve the geometry and dust and ice content of the disk. Results. PDS 453 has the typical morphology of a highly inclined system with two reflection nebulae in which the disk partially attenuates the stellar light. The upper nebula is brighter than the lower nebula and shows a curved surface brightness profile in polarized intensity. This indicates a ring-like structure. With an inclination of 80° estimated from models, the line of sight crosses the disk surface, and a combination of absorption and scattering by ice-rich dust grains produces the water-ice band. Conclusions. PDS 453 is seen at high inclination and is composed of a mixture of silicate dust and water ice. The radial structure of the disk includes a significant jump in density and scale height at a radius of 70 au that produces a ring-like image. The depth of the 3.1 µm water-ice band depends on the amount of water ice, until it saturates when the optical thickness along the line of sight becomes too large. Therefore, quantifying the exact amount of water from absorption bands in edge-on disks requires a detailed analysis of the disk structure and tailored radiative transfer modeling. Further observations with JWST and ALMA will allow us to refine our understanding of the structure and content of this interesting system.