Constraining the detectability of water ice in debris disks

Abstract

Context. Water ice is important for the evolution and preservation of life. Identifying the distribution of water ice in debris disks is therefore of great interest in the field of astrobiology. Furthermore, icy dust grains are expected to play important roles throughout the entire planet formation process. However, currently available observations only allow deriving weak conclusions about the existence of water ice in debris disks. Aims. We investigate whether it is feasible to detect water ice in typical debris disk systems. We take the following ice destruction mechanisms into account: sublimation of ice, dust production through planetesimal collisions, and photosputtering by UV-bright central stars. We consider icy dust mixture particles with various shapes consisting of amorphous ice, crystalline ice, astrosilicate, and vacuum inclusions (i.e., porous ice grains). Methods. We calculated optical properties of inhomogeneous icy dust mixtures using effective medium theories, that is, Maxwell-Garnett rules. Subsequently, we generated synthetic debris disk observables, such as spectral energy distributions and spatially resolved thermal reemission and scattered light intensity and polarization maps with our code DMS. Results. We find that the prominent ∼ 3 µm and 44 µm water ice features can be potentially detected in future observations of debris disks with the James Webb Space Telescope (JWST) and the Space Infrared telescope for Cosmology and Astrophysics (SPICA). We show that the sublimation of ice, collisions between planetesimals, and photosputtering caused by UV sources clearly affect the observational appearance of debris disk systems. In addition, highly porous ice (or ice-rich aggregates) tends to produce highly polarized radiation at around 3 µm. Finally, the location of the ice survival line is determined by various dust properties such as a fractional ratio of ice versus dust, physical states of ice (amorphous or crystalline), and the porosity of icy grains.

Scattering polarization of 3-$μ$m water-ice feature by large icy grains

“…The results shown in Figure 6 are qualitatively consistent with those of Kim et al (2019). In addition, the results obtained by Kim et al (2019) show characteristics of the polarization feature in the reflection regime. The discussion presented in Section 3 may provide a physical explanation for these previous results (e.g., Figure 14).…”

“…To our knowledge, a distinct water-ice feature has not yet been detected in debris disks (see also Hughes et al 2018), although Chen et al (2008) reported a broad peak at 60 − 75 µm that may be due to crystalline water ice. Future space missions, such as JWST, and ground-based facilities, such as E-ELT and TMT, will advance our understanding of water ice in debris disks (Kim et al 2019).…”

“…Recently, Kim et al (2019) reported that the scattering-polarization feature at 3 µm is a useful tracer of ice abundance and of different ice destruction processes in debris disks. The results shown in Figure 6 are qualitatively consistent with those of Kim et al (2019). In addition, the results obtained by Kim et al (2019) show characteristics of the polarization feature in the reflection regime.…”

“…Although it has not yet been confirmed, the polarization enhancement might be seen in protoplanetary disks as disk scattered light of the ice feature has been detected (Honda et al 2009(Honda et al , 2016. In addition, debris disks are also anticipated to exhibit the polarization enhancement (Kim et al 2019). Therefore, scattering polarization of the ice feature might be potentially observable for various star-and planet-forming regions and useful for constraining ice properties.…”

Debris Disks

“…The volatile content of the planetesimals is harder to probe through spectral features, though there are mid-and far-IR features due to ice that could be detected by JWST and Spica [45]. However, ALMA is providing a growing body of observations of gas in debris disks which is thought to have a secondary origin, i.e., to have been released from volatile-rich planetesimals, rather than being a remnant of the protoplanetary disk [21,61,60].…”