The characterisation of water ice in debris discs: implications for JWST scattered light observations

TL;DR

This study investigates how water ice in debris discs affects their observational signatures, especially in the context of JWST, highlighting the importance of scattering geometry, ice phase, and filter choices for detecting and characterizing ice features.

Contribution

It provides a detailed analysis of water ice signatures in debris discs across stellar types and offers guidance on optimal JWST filter combinations for ice detection and grain size estimation.

Findings

Ice features at 2.7 and 3.3 μm depend on ice fraction and scattering angle.

Edge-on discs with strong forward/backward scattering yield the strongest detections.

Specific JWST filter combinations can constrain grain size and ice fraction, but degeneracies exist.

Abstract

Water ice plays a crucial role throughout the different stages of planetary evolution and is abundant in the Universe. However, its presence and nature in debris discs of exoplanetary systems are not yet strongly established observationally. In this study, we quantify and discuss the impact of ice parameters such as volume fraction ${\mathcal{F}}_{\rm ice}$, blow-out grain size, size distribution, and its phase on the observational appearance of debris discs, considering the diverse nature of these systems around stellar spectral types ranging from A to M. Our findings reveal that the prominent ice features at approximately 2.7 and 3.3\,$\mu$m depend on both the water ice fraction ${\mathcal{F}}_{\rm ice}$ and the scattering angle, with backscattering geometries yielding the most prominent signatures. When the phase function is considered and data are not background limited, strong forward and backward scattering (near edge-on discs) are expected to yield the strongest detections in images/spectra for A or F-type stars, while scattering angle matters less for later type stars. The Fresnel peak at 3.1\,$\mu$m serves as a viable discriminant for the transitional phase (crystalline/amorphous), while simultaneously constraining the water ice temperature. For JWST imaging, we find that the F356W and F444W filter combination is most effective for constraining the grain size distribution, while the F356W and F277W filter combination provides better constraints on the ice fraction ${\mathcal{F}}_{\rm ice}$ in debris discs. However, degeneracy between the grain size distribution and ice fraction when using photometric flux ratios means that obtaining robust constraints will likely require more than two filters, or spectroscopic data.