Simulating Observations of Ices in Protoplanetary Disks

Abstract

Abstract Ices are an important constituent of protoplanetary disks. New observational facilities, notably the James Webb Space Telescope (JWST), will greatly enhance our view of disk ices by measuring their infrared spectral features. We present a suite of models to complement these upcoming observations. Our models use a kinetics-based gas–grain chemical evolution code to simulate the distribution of ices in a disk, followed by a radiative transfer code using a subset of key ice species to simulate the observations. We present models reflecting both molecular inheritance and chemical reset initial conditions. We find that near-to-mid-IR absorption features of H2O, CO2, and CH3OH are readily observable in disk-integrated spectra of highly inclined disks while CO, NH3, and CH4 ice do not show prominent features. CH3OH ice has low abundance and is not observable in the reset model, making this species an excellent diagnostic of initial chemical conditions. CO2 ice features exhibit the greatest change over disk lifetime, decreasing and increasing for the inheritance and reset models, respectively. Spatially resolved spectra of edge-on disks, possible with JWST's integral field unit observing modes, are ideal for constraining the vertical distribution of ices and may be able to isolate features from ices closer to the midplane (e.g., CO) given sufficient sensitivity. Spatially resolved spectra of face-on disks can trace scattered-light features from H2O, CO2, and CH3OH, plus CO and CH4 from the outermost regions. We additionally simulate far-IR H2O ice emission features and find they are strongest for disks viewed face-on.