A JWST/MIRI analysis of the ice distribution and polycyclic aromatic hydrocarbon emission in the protoplanetary disk HH 48 NE
Authors: Sturm J. A. , McClure M. K. , Harsono D. , Bergner J. B. , Dartois E. , Boogert A. C. A. , Cordiner M. A. , Drozdovskaya M. N. , Ioppolo S. , Law C. J. , Lis D. C. , McGuire B. A. , Melnick G. J. , Noble J. A. , Öberg K. I. , Palumbo M. E. , Pendleton Y. J. , Perotti G. , Rocha W. R. M. , Urso R. G. , van Dishoeck E. F. 
Published: July 2024

Context. Ice-coated dust grains provide the main reservoir of volatiles that play an important role in planet formation processes and may become incorporated into planetary atmospheres. However, due to observational challenges, the ice abundance distribution in protoplanetary disks is not well constrained. With the advent of the James Webb Space Telescope (JWST), we are in a unique position to observe these ices in the near- to mid-infrared and constrain their properties in Class II protoplanetary disks.
Aims. We present JWST Mid-InfraRed Imager (MIRI) observations of the edge-on disk HH 48 NE carried out as part of the Direc- tor’s Discretionary Early Release Science program Ice Age, completing the ice inventory of HH 48 NE by combining the MIRI data (5–28 μm) with those of NIRSpec (2.7–5 μm).
Methods. We used radiative transfer models tailored to the system, including silicates, ices, and polycyclic aromatic hydrocarbons (PAHs) to reproduce the observed spectrum of HH 48 NE with a parameterized model. The model was then used to identify ice species and constrain spatial information about the ices in the disk.
Results. The mid-infrared spectrum of HH 48 NE is relatively flat, with weak ice absorption features. We detect CO2, NH3, H2O, and tentatively CH4 and NH4+. Radiative transfer models suggest that ice absorption features are produced predominantly in the 50–100 au region of the disk. The CO2 feature at 15 μm probes a region closer to the midplane (z/r = 0.1–0.15) than the corresponding feature at 4.3 μm (z/r = 0.2–0.6), but all observations trace regions significantly above the midplane reservoirs where we expect the bulk of the ice mass to be located. Ices must reach a high scale height (z/r ~ 0.6; corresponding to a modeled dust extinction Av ~ 0.1), in order to be consistent with the observed vertical distribution of the peak ice optical depths. The weakness of the CO2 feature at 15 μm relative to the 4.3 μm feature and the red emission wing of the 4.3 μm CO2 feature are both consistent with ices being located at a high elevation in the disk. The retrieved NH3 abundance and the upper limit on the CH3OH abundance relative to H2O are significantly lower than those in the interstellar medium, but consistent with cometary observations. The contrast of the PAH emission features with the continuum is stronger than for similar face-on protoplanetary disks, which is likely a result of the edge-on system geometry. Modeling based on the relative strength of the emission features suggests that the PAH emission originates in the disk surface layer rather than the ice absorbing layer.
Conclusions. Full wavelength coverage is required to properly study the abundance distribution of ices in disks. To explain the pres- ence of ices at high disk altitudes, we propose two possible scenarios: a disk wind that entrains sufficient amounts of dust, and thus blocks part of the stellar UV radiation, or vertical mixing that cycles enough ices into the upper disk layers to balance ice photodesorption from the grains.