Simulating Observations of Ices in Protoplanetary Disks

Abstract

Ices are an important constituent of protoplanetary disks. New observational facilities, notably 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 kineticsbased 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 H 2 O, CO 2 , and CH 3 OH near-to-mid-IR absorption features are readily observable in disk-integrated spectra of highly-inclined disks while CO, NH 3 , and CH 4 ice do not show prominent features. CH 3 OH ice has low abundance and is not observable in the reset model, making this species an excellent diagnostic of initial chemical conditions. CO 2 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 H 2 O, CO 2 , and CH 3 OH, plus CO and CH 4 from the outermost regions. We additionally simulate far-IR H 2 O ice emission features and find they are strongest for disks viewed face-on.

The water-ice feature in near-infrared disk-scattered light around HD 142527: Micron-sized icy grains lifted up to the disk surface?

“…A promising mechanism to reduce water ice at a higher disk surface is photodesorption caused by far-ultraviolet photons (Dominik et al 2005;Oka et al 2012;Furuya et al 2013;Kamp et al 2018;Ballering et al 2021). Oka et al (2012) examined the impact of photodesorption on the location of the water snowline at the disk surfaces of HD 142527, and they found that the snowline is likely located between 100 au and 300 au, which is similar to the region seen in scattered light in Honda et al (2009).…”

“…For the outer disk, we consider grain models presented in Section 2.1, and these grains are assumed to be well mixed throughout the outer disk. To clarify the influence of grain size on ice features, we also assume the same ice abundance across the outer disk, although ice abundance may vary radially and vertically in more realistic disk models (Oka et al 2012;Kamp et al 2018;Tung & Hoang 2020;Ballering et al 2021). This suggests that an ice abundance inferred by our simulations may correspond to the average ice abundance along each line of sight.…”

“…Therefore, NIRCam coronagraphic imaging is a viable tool for investigating water ice for nearly face-on disks. Combined with NIRSpec spectroscopy for edge-on disks (e.g., Ballering et al 2021), JWST will reveal ice in disks at various inclination angles. However, careful analysis would be needed when estimating the amount of ice from scattering features with NIRCam, as the feature depth depends on the scattering properties of icy grains and disk geometry.…”