PDS 453: a unique disk reveals a complex interpretation of ices signatures

Planets form in disks of gas and dust. Ices play a key role in the formation of planetesimals and in the delivery of water to terrestrial planets. These ices form in the cold regions of protoplanetary disks on the surface of dust grains. Infrared spectroscopy provides crucial information on the properties of ices (type of ices, mass fraction, size of dust grains) and their spatial distribution.

A recent study led by Laurine Martinien, a Ph.D. student at IPAG, has revealed that the viewing geometry plays a surprisingly important role in the interpretation of observed ice spectra. This study focused on PDS 453, a rare system with an intermediate mass star and a highly inclined disk. Past observations with the Subaru telescope have shown that the disk contains water ice. The new analysis is based on archival Hubble Space Telescope as well as new Very Large Telescope adaptive optics images revealing a ring-like sharp transition between an inner and outer disk. PDS 453 is viewed at grazing angle, a unique geometry where the disk only partially masks the starlight. As a result, the observed spectrum results from a combination of scattering and absorption, which was ignored in past studies and must instead be considered in models. Based on model tailored to reproduce the observations of PDS 453, the team showed that the shape of the water-ice bands is dependent on the viewing geometry of the system. In particular, the depth of the band reaches a saturation when the system is too inclined which precludes an accurate quantification of the amount of ice inside the disk. These results will impact future observations of disks by the James Webb Space Telescope.

© Martinien & al., A&A, DOI: 10.1051/0004-6361/202451475